Patent Publication Number: US-11645810-B2

Title: Method for continued bounding volume hierarchy traversal on intersection without shader intervention

Description:
CROSS-REFERENCE TO RELATED PATENTS AND APPLICATIONS 
     This application is related to the following commonly-assigned US patents and patent applications, the entire contents of each of which are incorporated by reference: U.S. application Ser. No. 14/563,872 titled “Short Stack Traversal of Tree Data Structures” filed Dec. 8, 2014; U.S. Pat. No. 9,582,607 titled “Block-Based Bounding Volume Hierarchy”; U.S. Pat. No. 9,552,664 titled “Relative Encoding For A Block-Based Bounding Volume Hierarchy” as; U.S. Pat. No. 9,569,559 titled “Beam Tracing” filed Mar. 18, 2015; U.S. Pat. No. 10,025,879 titled “Tree Data Structures Based on a Plurality of Local Coordinate Systems”; U.S. application Ser. No. 14/737,343 titled “Block-Based Lossless Compression of Geometric Data” filed Jun. 11, 2015; and the following US Applications filed concurrently herewith:
         U.S. patent application Ser. No. 16/101,109 titled “Method for Efficient Grouping of Cache Requests for Datapath Scheduling” filed Aug. 10, 2018;   U.S. patent application Ser. No. 16/101,247 titled “A Robust, Efficient Multiprocessor-Coprocessor Interface” filed Aug. 10, 2018;   U.S. patent application Ser. No. 16/101,180 titled “Query-Specific Behavioral Modification of Tree Traversal” filed Aug. 10, 2018;   U.S. patent application Ser. No. 16/101,148 titled “Watertight Ray Triangle Intersection” filed Aug. 10, 2018;   U.S. patent application Ser. No. 16/101,196 titled “Method for Handling Out-of-Order Opaque and Alpha Ray/Primitive Intersections” filed Aug. 10, 2018; and   U.S. patent application Ser. No. 16/101,232 titled “Method for Forward Progress and Programmable Timeouts of Tree Traversal Mechanisms in Hardware” filed Aug. 10, 2018.       

     FIELD 
     The present technology relates to computer graphics, and more particularly to ray tracers. More particularly, the technology relates to hardware acceleration of computer graphics processing including but not limited to ray tracing. Still more particularly, the example non-limiting technology herein relates to a hardware-based traversal coprocessor that efficiently traverses an acceleration data structure e.g., for real time ray tracing. In still more detail, the technology herein provides an improved hardware-based traversal coprocessor that accelerates traversal of a data structure by omitting reporting of one or more primitives the ray is determined to intersect. The omitted one or more primitives may include primitives which are provably capable of being omitted without a functional impact on visualizing the virtual scene. 
     BACKGROUND &amp; SUMMARY 
     If you look around the visual scene before you, you will notice that some of the most interesting visual effects you see are produced by light rays interacting with surfaces. This is because light is the only thing we see. We don&#39;t see objects—we see the light that is reflected or refracted by the objects. Most of the objects we can see reflect light (the color of an object is determined by which parts of light the object reflects and which parts it absorbs). Shiny surfaces such as metallic surfaces, glossy surfaces, ceramics, the surfaces of liquids and a variety of others (even the corneas of the human eyes) act as mirrors that specularly reflect light. For example, a shiny metal surface will reflect light at the same angle as it hit the surface. An object can also cast shadows by preventing light from reaching other surfaces that are behind the object relative to a light source. If you look around, you will notice that the number and kinds of reflections and the number, kinds and lengths of shadows depend on many factors including the number and type of lights in the scene. A single point light such as a single faraway light bulb will produce single reflections and hard shadows. Area light sources such as windows or light panels produce different kinds of reflection highlights and softer shadows. Multiple lights will typically produce multiple reflections and more complex shadows (for example, three separated point light sources will produce three shadows which may overlap depending on the positions of the lights relative to an object). 
     If you move your head as you survey the scene, you will notice that the reflections change in position and shape (the shadows do the same). By changing your viewpoint, you are changing the various angles of the light rays your eyes detect. This occurs instantaneously—you move your head and the visual scene changes immediately. 
     The simple act of drinking a cup of tea is a complex visual experience. The various shiny surfaces of the glossy ceramic cup on the table before you reflect each light in the room, and the cup casts a shadow for each light. The moving surface of the tea in the cup is itself reflective. You can see small reflected images of the lights on the tea&#39;s surface, and even smaller reflections on the part of the tea&#39;s surface where the liquid curves up to meet the walls of the cup. The cup walls also cast shadows onto the surface of the liquid in the cup. Lifting the cup to your mouth causes these reflections and shadows to shift and shimmer as your viewpoint changes and as the surface of the liquid is agitated by movement. 
     We take these complexities of reflections and shadows for granted. Our brains are adept at decoding the positions, sizes and shapes of shadows and reflections and using them as visual cues. This is in part how we discern the position of objects relative to one another, how we distinguish one object from another and how we learn what objects are made of. Different object surfaces reflect differently. Specular (mirror type) reflection of hard metal creates images of reflected objects, while diffuse reflection off of rough surfaces is responsible for color and lights up objects in a softer way. Shadows can be soft and diffuse or hard and distinct depending on the type of lighting, and the lengths and directions of the shadows will depend on the angle of the light rays relative to the object and our eyes. 
     Beginning artists typically don&#39;t try to show reflection or shadows. They tend to draw flat scenes that have no shadows and no reflections or highlights. The same was true with computer graphics of the past. 
     Real time computer graphics have advanced tremendously over the last 30 years. With the development in the 1980&#39;s of powerful graphics processing units (GPUs) providing 3D hardware graphics pipelines, it became possible to produce 3D graphical displays based on texture-mapped polygon primitives in real time response to user input. Such real time graphics processors were built upon a technology called scan conversion rasterization, which is a means of determining visibility from a single point or perspective. Using this approach, three-dimensional objects are modelled from surfaces constructed of geometric primitives, typically polygons such as triangles. The scan conversion process establishes and projects primitive polygon vertices onto a view plane and fills in the points inside the edges of the primitives. See e.g., Foley, Van Dam, Hughes et al, Computer Graphics: Principles and Practice (2d Ed. Addison-Wesley 1995 &amp; 3d Ed. Addison-Wesley 2014). 
     Hardware has long been used to determine how each polygon surface should be shaded and texture-mapped and to rasterize the shaded, texture-mapped polygon surfaces for display. Typical three-dimensional scenes are often constructed from millions of polygons. Fast modem GPU hardware can efficiently process many millions of graphics primitives for each display frame (every 1/30 th  or 1/60 th  of a second) in real time response to user input. The resulting graphical displays have been used in a variety of real time graphical user interfaces including but not limited to augmented reality, virtual reality, video games and medical imaging. But traditionally, such interactive graphics hardware has not been able to accurately model and portray reflections and shadows. 
     Some have built other technologies onto this basic scan conversion rasterization approach to allow real time graphics systems to accomplish a certain amount of realism in rendering shadows and reflections. For example, texture mapping has sometimes been used to simulate reflections and shadows in a 3D scene. One way this is commonly done is to transform, project and rasterize objects from different perspectives, write the rasterized results into texture maps, and sample the texture maps to provide reflection mapping, environment mapping and shadowing. While these techniques have proven to be useful and moderately successful, they do not work well in all situations. For example, so-called “environment mapping” may often require assuming the environment is infinitely distant from the object. In addition, an environment-mapped object may typically be unable to reflect itself. See e.g., http://developer.download.nvidia.com/CgTutorial/cg_tutorial_chapter07.html. These limitations result because conventional computer graphics hardware—while sufficiently fast for excellent polygon rendering—does not perform the light visualization needed for accurate and realistic reflections and shadows. Some have likened raster/texture approximations of reflections and shadows as the visual equivalent of AM radio. 
     There is another graphics technology which does perform physically realistic visibility determinations for reflection and shadowing. It is called “ray tracing”. Ray tracing was developed at the end of the 1960&#39;s and was improved upon in the 1980&#39;s. See e.g., Apple, “Some Techniques for Shading Machine Renderings of Solids” (SJCC 1968) pp. 27-45; Whitted, “An Improved Illumination Model for Shaded Display” Pages 343-349 Communications of the ACM Volume 23 Issue 6 (June 1980); and Kajiya, “The Rendering Equation”, Computer Graphics (SIGGRAPH 1986 Proceedings, Vol. 20, pp. 143-150). Since then, ray tracing has been used in non-real time graphics applications such as design and film making. Anyone who has seen “Finding Dory” (2016) or other Pixar animated films has seen the result of the ray tracing approach to computer graphics—namely realistic shadows and reflections. See e.g., Hery et al, “Towards Bidirectional Path Tracing at Pixar” (2016). 
     Ray tracing is a primitive used in a variety of rendering algorithms including for example path tracing and Metropolis light transport. In an example algorithm, ray tracing simulates the physics of light by modeling light transport through the scene to compute all global effects (including for example reflections from shiny surfaces) using ray optics. In such uses of ray tracing, an attempt may be made to trace each of many hundreds or thousands of light rays as they travel through the three-dimensional scene from potentially multiple light sources to the viewpoint. Often, such rays are traced relative to the eye through the scene and tested against a database of all geometry in the scene. The rays can be traced forward from lights to the eye, or backwards from the eye to the lights, or they can be traced to see if paths starting from the virtual camera and starting at the eye have a clear line of sight. The testing determines either the nearest intersection (in order to determine what is visible from the eye) or traces rays from the surface of an object toward a light source to determine if there is anything intervening that would block the transmission of light to that point in space. Because the rays are similar to the rays of light in reality, they make available a number of realistic effects that are not possible using the raster based real time 3D graphics technology that has been implemented over the last thirty years. Because each illuminating ray from each light source within the scene is evaluated as it passes through each object in the scene, the resulting images can appear as if they were photographed in reality. Accordingly, these ray tracing methods have long been used in professional graphics applications such as design and film, where they have come to dominate over raster-based rendering. 
     The main challenge with ray tracing has generally been speed. Ray tracing requires the graphics system to compute and analyze, for each frame, each of many millions of light rays impinging on (and potentially reflected by) each surface making up the scene. In the past, this enormous amount of computation complexity was impossible to perform in real time. 
     One reason modem GPU 3D graphics pipelines are so fast at rendering shaded, texture-mapped surfaces is that they use coherence efficiently. In conventional scan conversion, everything is assumed to be viewed through a common window in a common image plane and projected down to a single vantage point. Each triangle or other primitive is sent through the graphics pipeline and covers some number of pixels. All related computations can be shared for all pixels rendered from that triangle. Rectangular tiles of pixels corresponding to coherent lines of sight passing through the window may thus correspond to groups of threads running in lock-step in the same streaming processor. All the pixels falling between the edges of the triangle are assumed to be the same material running the same shader and fetching adjacent groups of texels from the same textures. In ray tracing, in contrast, rays may start or end at a common point (a light source, or a virtual camera lens) but as they propagate through the scene and interact with different materials, they quickly diverge. For example, each ray performs a search to find the closest object. Some caching and sharing of results can be performed, but because each ray potentially can hit different objects, the kind of coherence that GPU&#39;s have traditionally taken advantage of in connection with texture mapped, shaded triangles is not present (e.g., a common vantage point, window and image plane are not there for ray tracing). This makes ray tracing much more computationally challenging than other graphics approaches—and therefore much more difficult to perform on an interactive basis. 
     Much research has been done on making the process of tracing rays more efficient and timely. See e.g., Glassner, An Introduction to Ray Tracing (Academic Press Inc., 1989). Because each ray in ray tracing is, by its nature, evaluated independently from the rest, ray tracing has been called “embarrassingly parallel.” See e.g., Akenine-Möller et al., Real Time Rendering at Section 9.8.2, page 412 (Third Ed. CRC Press 2008). As discussed above, ray tracing involves effectively testing each ray against all objects and surfaces in the scene. An optimization called “acceleration data structure” and associated processes allows the graphics system to use a “divide-and-conquer” approach across the acceleration data structure to establish what surfaces the ray hits and what surfaces the ray does not hit. Each ray traverses the acceleration data structure in an individualistic way. This means that dedicating more processors to ray tracing gives a nearly linear performance increase. With increasing parallelism of graphics processing systems, some began envisioning the possibility that ray tracing could be performed in real time. For example, work at Saarland University in the mid-2000&#39;s produced an early special purpose hardware system for interactive ray tracing that provided some degree of programmability for using geometry, vertex and lighting shaders. See Woop et al., “RPU: A Programmable Ray Processing Unit for Real Time Ray Tracing” (ACM 2005). As another example, Advanced Rendering Technology developed “RenderDrive” based on an array of AR250/350 rendering processors derived from ARM1 and enhanced with custom pipelines for ray/triangle intersection and STMD vector and texture math but with no fixed-function traversal logic. See e.g., http://www.graphicshardware.org/previous/www_2001/presentations/Hot3D_Daniel_Hal l.pdf 
     Then, in 2010, NVIDIA took advantage of the high degree of parallelism of NVIDIA GPUs and other highly parallel architectures to develop the OptiX™ ray tracing engine. See Parker et al., “OptiX: A General Purpose Ray Tracing Engine” (ACM Transactions on Graphics, Vol. 29, No. 4, Article 66, July 2010). In addition to improvements in API&#39;s (application programming interfaces), one of the advances provided by OptiX™ was improving the acceleration data structures used for finding an intersection between a ray and the scene geometry. Such acceleration data structures are usually spatial or object hierarchies used by the ray tracing traversal algorithm to efficiently search for primitives that potentially intersect a given ray. OptiX™ provides a number of different acceleration structure types that the application can choose from. Each acceleration structure in the node graph can be a different type, allowing combinations of high-quality static structures with dynamically updated ones. 
     The OptiX™ programmable ray tracing pipeline provided significant advances, but was still generally unable by itself to provide real time interactive response to user input on relatively inexpensive computing platforms for complex 3D scenes. Since then, NVIDIA has been developing hardware acceleration capabilities for ray tracing. See e.g., U.S. Pat. Nos. 9,582,607; 9,569,559; US20160070820; and US20160070767. 
     Given the great potential of a truly interactive real time ray tracing graphics processing system for rendering high quality images of arbitrary complexity in response for example to user input, further work is possible and desirable. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    illustrates an example non-limiting ray tracing graphics system. 
         FIG.  2 A  shows an example specular object. 
         FIG.  2 B  shows the example object within a bounding volume. 
         FIG.  2 C  shows an example volumetric subdividing of the  FIG.  2 B  bounding volume. 
         FIGS.  2 D,  2 E and  2 F  show example further levels of volumetric subdivision of the bounding volume to create a bounding volume hierarchy (BVH). 
         FIG.  2 G  shows an example portion of the object comprised of primitive surfaces, in this case triangles. 
         FIGS.  3 A- 3 C  show example simplified ray tracing tests to determine whether the ray passes through a bounding volume containing geometry and whether the ray intersects geometry. 
         FIG.  4    illustrates an example ray tracing flowchart. 
         FIGS.  5 A- 5 C  show example different ray-primitive intersection scenarios. 
         FIGS.  6 A and  6 B  show an example of how texture mapping can impact ray-primitive intersection results. 
         FIGS.  7 A and  7 B  illustrate ray instance transforms. 
         FIG.  8 A  illustrates an example non-limiting bounding volume hierarchy (BVH). 
         FIG.  8 B  shows an example acceleration data structure in the form of a graph or tree. 
         FIG.  9    shows a simplified example non-limiting traversal co-processor comprising a tree traversal unit (TTU). 
         FIG.  10 A  illustrates an example non-limiting ray tracing shading pipeline flowchart. 
         FIGS.  10 B and  10 C  illustrate more detailed ray tracing pipelines. 
         FIG.  11    illustrates an example non-limiting method for accelerated ray-primitives intersection test. 
         FIG.  12    illustrates a result queue according to an exemplary embodiment. 
         FIG.  13    illustrates a flowchart of an example non-limiting method for accelerated ray-triangle intersection test. 
         FIG.  14    illustrates an example flowchart for generating an image. 
         FIG.  15    illustrates an example parallel processing unit (PPU). 
         FIG.  16    illustrates an example memory partition unit. 
         FIG.  17    illustrates an example general processing cluster (GPC) within the parallel processing unit of  FIG.  15   . 
         FIG.  18    is a conceptual diagram of a graphics processing pipeline implemented by the GPC of  FIG.  17   . 
         FIGS.  19  and  20    illustrate an example streaming multi-processor. 
         FIG.  21    is a conceptual diagram of a processing system implemented using PPUs of  FIG.  15   . 
         FIG.  22    expands  FIG.  21    to show additional interconnected devices. 
     
    
    
     DETAILED DESCRIPTION OF NON-LIMITING EMBODIMENTS 
     The technology herein provides hardware capabilities that accelerate ray tracing to such an extent that it brings the power of ray tracing to games and other interactive real time computer graphics, initially enabling high effect quality in shadows and reflections and ultimately global illumination. In practice, this means accelerating ray tracing by a factor of up to an order of magnitude or more over what would be possible in software on the same graphics rendering system. 
     In more detail, the example non-limiting technology provides dedicated hardware to accelerate ray tracing. In non-limiting embodiments, a hardware co-processor (herein referred to as a “traversal coprocessor” or in some embodiments a “tree traversal unit” or “TTU”) accelerates certain processes supporting interactive ray tracing including ray-bounding volume intersection tests, ray-primitive intersection tests and ray “instance” transforms. 
     In some non-limiting embodiments, the traversal co-processor performs queries on an acceleration data structure for processes running on potentially massively-parallel streaming multiprocessors (SMs). The traversal co-processor traverses the acceleration data structure to discover information about how a given ray interacts with an object the acceleration data structure describes or represents. For ray tracing, the traversal coprocessors are callable as opposed to e.g., fixed function units that perform an operation once between logical pipeline stages running different types of threads (e.g., vertex threads and pixel threads). 
     In some non-limiting embodiments, the acceleration data structure comprises a hierarchy of bounding volumes (bounding volume hierarchy or BVH) that recursively encapsulates smaller and smaller bounding volume subdivisions. The largest volumetric bounding volume may be termed a “root node.” The smallest subdivisions of such hierarchy of bounding volumes (“leaf nodes”) contain items. The items could be primitives (e.g., polygons such as triangles) that define surfaces of the object. Or, an item could be a sphere that contains a whole new level of the world that exists as an item because it has not been added to the BVH (think of the collar charm on the cat from “Men in Black” which contained an entire miniature galaxy inside of it). If the item comprises primitives, the traversal co-processor tests rays against the primitives to determine which object surfaces the rays intersect and which object surfaces are visible along the ray. 
     The traversal co-processor performs a test of each ray against a wide range of bounding volumes, and can cull any bounding volumes that don&#39;t intersect with that ray. Starting at a root node that bounds everything in the scene, the traversal co-processor tests each ray against smaller (potentially overlapping) child bounding volumes which in turn bound the descendent branches of the BVH. The ray follows the child pointers for the bounding volumes the ray hits to other nodes until the leaves or terminal nodes (volumes) of the BVH are reached. Once the traversal co-processor traverses the acceleration data structure to reach a terminal or “leaf” node that contains a geometric primitive, it performs an accelerated ray-primitive intersection test that determines whether the ray intersects that primitive (and thus the object surface that primitive defines). The ray-primitive test can provide additional information about primitives the ray intersects that can be used to determine the material properties of the surface required for shading and visualization. Recursive traversal through the acceleration data structure enables the traversal co-processor to discover all object primitives the ray intersects, or the closest (from the perspective of the viewpoint) primitive the ray intersects (which in some cases is the only primitive that is visible from the viewpoint along the ray). 
     The traversal co-processor also accelerates the transform of each ray from world space into object space to obtain finer and finer bounding box encapsulations of the primitives and reduce the duplication of those primitives across the scene. Objects replicated many times in the scene at different positions, orientations and scales can be represented in the scene as instance nodes which associate a bounding box and leaf node in the world space BVH with a transformation that can be applied to the world-space ray to transform it into an object coordinate space, and a pointer to an object-space BVH. This avoids replicating the object space BVH data multiple times in world space, saving memory and associated memory accesses. The instance transform increases efficiency by transforming the ray into object space instead of requiring the geometry or the bounding volume hierarchy to be transformed into world (ray) space and is also compatible with additional, conventional rasterization processes that graphics processing performs to visualize the primitives. 
     The presently disclosed non-limiting embodiments thus provide a traversal co-processor, a new subunit of one or a group of streaming multiprocessor SMs of a 3D graphics processing pipeline. In order to understand where the traversal co-processor fits in the overall picture, it may be helpful to understand a few fundamentals of the algorithm employed by most or all modern ray tracers. But it should be pointed out that the technology herein provides a generic capability to determine, for a thread running in a GPU, what the nearest visible thing is from a given point along a specified direction, or if anything lies between two points. A common use case for such capability will be in processes that start tracing rays from points that have already been rasterized on triangles using conventional scan conversion techniques. The disclosed technology can but does not necessarily replace or substitute for scan conversion technology, and may often augment it and be used in conjunction with scan conversion techniques to enhance images with photorealistic reflections, shadows and other effects. 
     Ray Tracing Techniques 
     Generally, ray tracing is a rendering method in which rays are used to determine the visibility of various elements in the scene. Ray tracing can be used to determine if anything is visible along a ray (for example, testing for occluders between a shaded point on a geometric primitive and a point on a light source) and can also be used to evaluate reflections (which may for example involve performing a traversal to determine the nearest visible surface along a line of sight so that software running on a streaming processor can evaluate a material shading function corresponding to what was hit—which in turn can launch one or more additional rays into the scene according to the material properties of the object that was intersected) to determine the light returning along the ray back toward the eye. In classical Whitted-style ray tracing, rays are shot from the viewpoint through the pixel grid into the scene, but other path traversals are possible. Typically, for each ray, the closest object is found. This intersection point can then be determined to be illuminated or in shadow by shooting a ray from it to each light source in the scene and finding if any objects are in between. Opaque objects block the light, whereas transparent objects attenuate it. Other rays can be spawned from an intersection point. For example, if the intersecting surface is shiny or specular, rays are generated in the reflection direction. The ray may accept the color of the first object intersected, which in turn has its intersection point tested for shadows. This reflection process is recursively repeated until a recursion limit is reached or the potential contribution of subsequent bounces falls below a threshold. Rays can also be generated in the direction of refraction for transparent solid objects, and again recursively evaluated. See Akenine-Möller et al., cited above. Ray tracing technology thus allows a graphics system to develop physically correct reflections and shadows that are not subject to the limitations and artifacts of scan conversion techniques. 
     Traversal Coprocessor 
     The basic task the traversal coprocessor performs is to test a ray against all primitives (commonly triangles in one embodiment) in the scene and report either the closest hit (according to distance measured along the ray) or simply the first (not necessarily closest) hit encountered, depending upon use case. The naïve algorithm would be an O( n ) brute-force search. By pre-processing the scene geometry and building a suitable acceleration data structure in advance, however, it is possible to reduce the average-case complexity to O(log n). In ray tracing, the time for finding the closest (or for shadows, any) intersection for a ray is typically order O(log n) for n objects when an acceleration data structure is used. For example, bounding volume hierarchies (BVHs) of the type commonly used for modern ray tracing acceleration data structures typically have an O(log n) search behavior. 
     Bounding Volume Hierarchies 
     The acceleration data structure most commonly used by modern ray tracers is a bounding volume hierarchy (BVH) comprising nested axis-aligned bounding boxes (AABBs). The leaf nodes of the BVH contain the primitives (e.g., triangles) to be tested for intersection. The BVH is most often represented by a graph or tree structure data representation. In such instances, the traversal coprocessor may be called a “tree traversal unit” or “TTU”. 
     Given a BVH, ray tracing amounts to a tree search where each node in the tree visited by the ray has a bounding volume for each descendent branch or leaf, and the ray only visits the descendent branches or leaves whose corresponding bound volume it intersects. In this way, only a small number of primitives must be explicitly tested for intersection, namely those that reside in leaf nodes intersected by the ray. In the example non-limiting embodiments, the traversal coprocessor accelerates both tree traversal (including the ray-volume tests) and ray-primitive tests. As part of traversal, the traversal coprocessor can also handle “instance transforms”—transforming a ray from world-space coordinates into the coordinate system of an instanced mesh (object space) e.g., in order to avoid the computational complexity of transforming the primitive vertices into world space. It can do so in a MIMD (multiple-instruction, multiple data) fashion, meaning that the rays are handled independently once inside the traversal coprocessor. 
     Example Non-Limiting Real Time Interactive Ray Tracing System 
       FIG.  1    illustrates an example real time ray interactive tracing graphics system  100  for generating images using three dimensional (3D) data of a scene or object(s). System  100  includes an input device  110 , a processor(s)  120 , a graphics processing unit(s) (GPU(s))  130 , memory  140 , and a display(s)  150 . The system shown in  FIG.  1    can take on any form factor including but not limited to a personal computer, a smart phone or other smart device, a video game system, a wearable virtual or augmented reality system, a cloud-based computing system, a vehicle-mounted graphics system, a system-on-a-chip (SoC), etc. 
     The processor  120  may be a multicore central processing unit (CPU) operable to execute an application in real time interactive response to input device  110 , the output of which includes images for display on display  150 . Display  150  may be any kind of display such as a stationary display, a head mounted display such as display glasses or goggles, other types of wearable displays, a handheld display, a vehicle mounted display, etc. For example, the processor  120  may execute an application based on inputs received from the input device  110  (e.g., a joystick, an inertial sensor, an ambient light sensor, etc.) and instruct the GPU  130  to generate images showing application progress for display on the display  150 . 
     Based on execution of the application on processor  120 , the processor may issue instructions for the GPU  130  to generate images using 3D data stored in memory  140 . The GPU  130  includes specialized hardware for accelerating the generation of images in real time. For example, the GPU  130  is able to process information for thousands or millions of graphics primitives (polygons) in real time due to the GPU&#39;s ability to perform repetitive and highly-parallel specialized computing tasks such as polygon scan conversion much faster than conventional software-driven CPUs. For example, unlike the processor  120 , which may have multiple cores with lots of cache memory that can handle a few software threads at a time, the GPU  130  may include hundreds or thousands of processing cores or “streaming multiprocessors” (SMs)  132  running in parallel. 
     In one example embodiment, the GPU  130  includes a plurality of programmable streaming multiprocessors (SMs)  132 , and a hardware-based graphics pipeline including a graphics primitive engine  134  and a raster engine  136 . These components of the GPU  130  are configured to perform real-time image rendering using a technique called “scan conversion rasterization” to display three-dimensional scenes on a two-dimensional display  150 . In rasterization, geometric building blocks (e.g., points, lines, triangles, quads, meshes, etc.) of a 3D scene are mapped to pixels of the display (often via a frame buffer memory). 
     The GPU  130  converts the geometric building blocks (i.e., polygon primitives such as triangles) of the 3D model into pixels of the 2D image and assigns an initial color value for each pixel. The graphics pipeline may apply shading, transparency, texture and/or color effects to portions of the image by defining or adjusting the color values of the pixels. The final pixel values may be anti-aliased, filtered and provided to the display  150  for display. Many software and hardware advances over the years have improved subjective image quality using rasterization techniques at frame rates needed for real-time graphics (i.e., 30 to 60 frames per second) at high display resolutions such as 4096×2160 pixels or more on one or multiple displays  150 . 
     Traversal Coprocessor Addition to Architecture 
     To enable the GPU  130  to perform ray tracing in real time in an efficient manner, the GPU is provided with traversal coprocessor  138  coupled to one or more SMs  132 . The traversal coprocessor  138  includes hardware components configured to perform operations commonly utilized in ray tracing algorithms. A goal of the traversal coprocessor  138  is to accelerate operations used in ray tracing to such an extent that it brings the power of ray tracing to real-time graphics application (e.g., games), enabling high-quality shadows, reflections, and global illumination. As discussed in more detail below, the result of the traversal coprocessor  138  may be used together with or as an alternative to other graphics related operations performed in the GPU  130 . 
     In the example architecture shown, the new hardware component called a “traversal coprocessor”  138  is used to accelerate certain tasks including but not limited to ray tracing. Ray tracing refers to casting a ray into a scene and determining whether and where that ray intersects the scene&#39;s geometry. This basic ray tracing visibility test is the fundamental primitive underlying a variety of rendering algorithms and techniques in computer graphics. For example, ray tracing can be used together with or as an alternative to rasterization and z-buffering for sampling scene geometry. It can also be used as an alternative to (or in combination with) environment mapping and shadow texturing for producing more realistic reflection, refraction and shadowing effects than can be achieved via texturing techniques or other raster “hacks”. To overcome limitations in image quality that can be achieved with rasterization, system  100  can also generate entire images or parts of images using ray tracing techniques. Ray tracing may also be used as the basic primitive to accurately simulate light transport in physically-based rendering algorithms such as path tracing, photon mapping, Metropolis light transport, and other light transport algorithms. 
     More specifically, SMs  132  and the traversal coprocessor  138  may cooperate to cast rays into a 3D model and determine whether and where that ray intersects the model&#39;s geometry. Ray tracing directly simulates light traveling through a virtual environment or scene. The results of the ray intersections together with surface texture, viewing direction, and/or lighting conditions are used to determine pixel color values. Ray tracing performed by SMs  132  working with traversal coprocessor  138  allows for computer-generated images to capture shadows, reflections, and refractions in ways that can be indistinguishable from photographs or video of the real world. Since ray tracing techniques are even more computationally intensive than rasterization due in part to the large number of rays that need to be traced, the traversal coprocessor  138  is capable of accelerating in hardware certain of the more computationally-intensive aspects of that process. 
     In the example non-limiting technology herein, traversal coprocessor  138  accelerates both ray-box tests and ray-primitive tests. As part of traversal, it can also handle at least one level of instance transforms, transforming a ray from world-space coordinates into the coordinate system of an instanced mesh. In the example non-limiting embodiments, the traversal coprocessor  138  does all of this in MIMD fashion, meaning that rays are handled independently once inside the traversal coprocessor. 
     In the example non-limiting embodiments, the traversal coprocessor  138  operates as a servant (coprocessor) to the SMs (streaming multiprocessors)  132 . In other words, the traversal coprocessor  138  in example non-limiting embodiments does not operate independently, but instead follows the commands of the SMs  132  to perform certain computationally-intensive ray tracing related tasks much more efficiently than the SMs  132  could perform themselves. 
     In the examples shown, the traversal coprocessor  138  receives commands via SM  132  instructions and writes results back to an SM register file. For many common use cases (e.g., opaque triangles with at most one level of instancing), the traversal coprocessor  138  can service the ray tracing query without further interaction with the SM  132 . More complicated queries (e.g., involving alpha-tested triangles, primitives other than triangles, or multiple levels of instancing) may require multiple round trips. In addition to tracing rays, the traversal coprocessor  138  is capable of performing more general spatial queries where an AABB or the extruded volume between two AABBs (which we call a “beam”) takes the place of the ray. Thus, while the traversal coprocessor  138  is especially adapted to accelerate ray tracing related tasks, it can also be used to perform tasks other than ray tracing. 
     In addition to the traversal coprocessor  138 , the example non-limiting technology used to support the system  100  of  FIG.  1    provides additional accelerated ray tracing enhancements to a number of units as well as a substantial effort devoted to BVH construction. BVH construction need not be hardware accelerated (although it may be in some non-limiting embodiments) but could instead be implemented using highly-optimized software routines running on SMs  132  and/or CPU  120  and/or other development systems e.g., during development of an application. The following exposition describes, among other things, software-visible behavior of the traversal coprocessor  138 , interfaces to surrounding units (SMs  132  and the memory subsystem), and additional features that are part of a complete ray-tracing solution such as certain enhancements to the group of SMs  132  and the memory caching system. 
     As discussed above, the traversal coprocessor  138  allows for quick traversal of an acceleration data structure (e.g., a BVH) to determine which primitives (e.g., triangles used for generating a scene) in the data structure are intersected by a query data structure (e.g., a ray). For example, the traversal coprocessor  138  may determine which triangles in the acceleration data structure are intersected by the ray and return the results to the SM  132 . However, returning to the SM  132  a result on every triangle intersection is costly in terms of interface and thread synchronization. The traversal coprocessor  138  provides a hardware logic configured to hide those items or triangles which are provably capable of being hidden without a functional impact on the resulting scene. The reduction in returns of results to the SM and synchronization steps between threads greatly improves the overall performance of traversal. 
     Previous implementations, done in software only, needed to inspect all triangles or item ranges that are intersected. The example non-limiting embodiments of the traversal coprocessor  138  disclosed in this application provides for some of the intersections to be discarded within the traversal coprocessor  138  without SM  132  intervention so that less intersections are returned to the SM  132  and the SM  132  does not have to inspect all intersected triangles or item ranges. 
     Traversing an Acceleration Data Structure 
     A good way to accelerate ray tracing is to use an acceleration data structure. The acceleration data structure represents the 3D model of an object or a scene in a manner that will help assist in quickly deciding which portion of the object a particular ray is likely to intersect and quickly rejecting large portions of the scene the ray will not intersect. A bounding volume hierarchy (BVH) data structure is one type of acceleration data structure which can help reduce the number of intersections to test. The BVH data structure represents a scene or object with a bounding volume and subdivides the bounding volume into smaller and smaller bounding volumes terminating in leaf nodes containing geometric primitives. The bounding volumes are hierarchical, meaning that the topmost level encloses the level below it, that level encloses the next level below it, and so on. In one embodiment, leaf nodes can potentially overlap other leaf nodes in the bounding volume hierarchy. 
     To illustrate how a bounding volume hierarchy works,  FIGS.  2 A- 2 G  show a teapot recursively subdivided into smaller and smaller hierarchical bounding volumes.  FIG.  2 A  shows a teapot object, and  FIG.  2 B  shows a bounding volume  202  (in this case a box, cube or rectangular parallelepiped) enclosing the whole teapot. The bounding volume  202 , which can be efficiently defined by its vertices, provides an indication of the spatial location of the object and is typically dimensioned to be just slightly larger than the object. 
     The first stage in acceleration structure construction acquires the bounding boxes of the referenced geometry. This is achieved by executing for each geometric primitive in an object a bounding box procedure that returns a conservative axis-aligned bounding box for its input primitive such as box  202  shown in  FIG.  2 B . Using these bounding boxes as elementary primitives for the acceleration structures provides the necessary abstraction to trace rays against arbitrary user-defined geometry (including several types of geometry within a single structure). Because in  FIG.  2 B  the bounding volume  202  is larger than and completely contains the teapot, a ray that does not intersect bounding volume cannot intersect the teapot, although a ray that does intersect the bounding volume may or may not intersect the teapot. Because the bounding volume  202  is readily defined by the x,y,z coordinates of its vertices in 3D space and a ray is defined by its x,y,z coordinates in 3D space, the ray-bounding volume test to determine whether a ray intersects the bounding volume  202  is straightforward (although some transform may be used to adjust to different coordinate systems, as will be explained below). 
       FIG.  2 C , shows the bounding volume  202  subdivided into smaller contained bounding volumes. While the subdivision scheme shown here for purposes of illustration is a so-called 8-ary subdivision or “octree” in which each volume is subdivided into eight smaller volumes of uniform size, many other spatial hierarchies and subdivision schemes are known such as a binary tree, a four-ary tree, a k-d tree, a binary space partitioning (BSP) tree, and a bounding volume hierarchy (BVH) tree. See e.g., U.S. Pat. No. 9,582,607. 
     Each of the subdivided bounding volumes shown in  FIG.  2 C  can be still further subdivided.  FIG.  2 D  shows one of the subdivided volumes  204  of  FIG.  2 C  being further subdivided to provide additional subdivided encapsulated bounding volumes. As shown in  FIG.  2 D , some of the subdivided bounding volumes include portions of the teapot and some do not. Volumes that do not contain a portion of the teapot are not further subdivided because the further subdivisions provide no further spatial information about the teapot. Already subdivided bounding volumes that do include at least one portion of the teapot can be still further recursively subdivided—like the emergence of each of a succession of littler and littler cats from the hats of Dr. Seuss&#39;s′  The Cat In The Hat Comes Back  (1958). The portions of the space within bounding volume  202  that contain geometry are recursively subdivided to permit the traversal coprocessor  138  to use the volumetric subdivisions to efficiently discover where the geometry is located relative to any given ray. It can be noted that while a spatial or active subdivision of the volume is possible, many implementations will create the hierarchical structure defining volumes and subvolumes ahead of time. In such cases, the builder may often build the hierarchy up from individual triangles and not down from the whole scene. Building up means you do not need to determine if some subdivided volume contains anything since by definition it contains what is below it in a hierarchy of volumetric subdivisions. 
       FIG.  2 E  shows a further such subdivision of bounding volume  204  into a further smaller contained bounding volume  206  containing in this example just the spout of the teapot plus another surface on the wall of the teapot, and  FIG.  2 F  shows an additional subdivision of bounding volume  206  into still smaller contained subdivision  208  encapsulating the end of the teapot&#39;s spout. Depending on the way the BVH is constructed, bounding volume  208  can be further and further subdivided as desired—and traversal coprocessor  138  enables the  FIG.  1    system  100  to efficiently traverse the BVH down to any arbitrary subdivision level. The number and configurations of recursive subdivisions will depend on the complexity and configuration of the 3D object being modeled as well as other factors such as desired resolution, distance of the object from the viewpoint, etc. 
     At some level of subdivision (which can be different levels for different parts of the BVH), the traversal coprocessor  138  encounters geometry making up the encapsulated object being modeled. Using the analogy of a tree, the successive volumetric subdivisions are the trunk, branches, boughs and twigs, and the geometric is finally revealed at the very tips of the tree, namely the leaves. In this case,  FIG.  2 G  shows the surface of the teapot&#39;s spout defined by an example mesh of geometric primitives. The geometric primitives shown are triangles but other geometric primitives, such as quads, lines, rectangles, quadrics, patches, or other geometric primitives known to those familiar with the state of the art, may be used (in one embodiment, such other types of primitives may be expressed as or converted into triangles). The geometric primitives in the mesh represent the shape of the 3D surface of the object being modeled. The example shown here is a mesh, but bounded geometry can include discontinuous geometry such as particles that may not be connected. In the example non-limiting embodiments, the traversal coprocessor  138  also accelerates ray intersection tests with this geometry to quickly determine which triangles are hit by any given ray. Determining ray-primitive intersections involves comparing the spatial xyz coordinates of the vertices of each primitive with the xyz coordinates of the ray to determine whether the ray and the surface the primitive defines occupy the same space. The ray-primitive intersection test can be computationally intensive because there may be many triangles to test. For example, in the mesh shown in  FIG.  2 G , the spout of the teapot alone is made up of over a hundred triangles—although it may be more efficient in some implementations to further volumetrically subdivide and thereby limit the number of triangles in any such “leaf node” to something like 16 or fewer. 
     As discussed above, ray tracing procedures determine what geometric primitives of a scene are intersected by a ray. However, due to the large number of primitives in a 3D scene, it may not be efficient or feasible to test every geometric primitive for an intersection. Acceleration data structures, such as BVH, allow for quick determination as to which bounding volumes can be ignored, which bounding volumes may contain intersected geometric primitives, and which intersected geometric primitives matter for visualization and which do not. 
     Ray Intersection Testing 
       FIGS.  3 A- 3 C  illustrate ray tracing applied to the  FIG.  2 G  bounding volume  208  including triangle mesh  320 .  FIG.  3 A  shows a ray  302  in a virtual space including bounding volumes  310  and  315 . To determine whether the ray  302  intersects one or more triangles in the mesh  320 , each triangle could be directly tested against the ray  302 . But to accelerate the process (since the object could contain many thousands of triangles), the ray  302  is first tested against the bounding volumes  310  and  315 . If the ray  302  does not intersect a bounding volume, then it does not intersect any triangles inside of the bounding volume and all triangles inside the bounding volume can be ignored for purposes of that ray. Because in  FIG.  3 A  the ray  302  misses bounding volume  310 , the triangles of mesh  320  within that bounding volume need not be tested for intersection. While bounding volume  315  is intersected by the ray  302 , bounding volume  315  does not contain any geometry and so no further testing is required. 
     On the other hand, if a ray such as ray  304  shown in  FIG.  3 B  intersects a bounding volume  310  that contains geometry, then the ray may or may not intersect the geometry inside of the bounding volume so further tests need to be performed on the geometry itself to find possible intersections. Because the rays  304 ,  306  in  FIGS.  3 B and  3 C  intersect a bounding volume  310  that contains geometry, further tests need to be performed to determine whether any (and which) of the primitives inside of the bounding volume are intersected. In  FIG.  3 B , further testing of the intersections with the primitives would indicate that even though the ray  304  passes through the bounding volume  310 , it does not intersect any of the primitives the bounding volume encloses (alternatively, as mentioned above, bounding volume  310  could be further volumetrically subdivided so that a bounding volume intersection test could be used to reveal that the ray does not intersect any geometry or more specifically which primitives the ray may intersect). 
       FIG.  3 C  shows a situation in which the bounding volume  310  intersected by ray  306  and contains geometry that ray  306  intersects. Traversal coprocessor  138  tests the intersections between the ray  306  and the individual primitives to determine which primitives the ray intersects. 
     Ray Tracing Operations 
       FIG.  4    is a flowchart summarizing example ray tracing operations the traversal coprocessor  138  performs as described above in cooperation with SM(s)  132 . The  FIG.  4    operations are performed by traversal coprocessor  138  in cooperation with its interaction with an SM  132 . The traversal coprocessor  138  may thus receive the identification of a ray from the SM  132  and traversal state enumerating one or more nodes in one or more BVH&#39;s that the ray must traverse. The traversal coprocessor  138  determines which bounding volumes of a BVH data structure the ray intersects (the “ray-complet” test  512 ) and subsequently whether the ray intersects one or more primitives in the intersected bounding volumes and which triangles are intersected (the “ray-primitive test”  520 ). In example non-limiting embodiments, “complets” (compressed treelets) specify root or interior nodes (i.e., volumes) of the bounding volume hierarchy with children that are other complets or leaf nodes of a single type per complet. 
     First, the traversal coprocessor  138  inspects the traversal state of the ray. If a stack the traversal coprocessor  138  maintains for the ray is empty, then traversal is complete. If there is an entry on the top of the stack, the traversal co-processor  138  issues a request to the memory subsystem to retrieve that node. The traversal co-processor  138  then performs a bounding box test  512  to determine if a bounding volume of a BVH data structure is intersected by a particular ray the SM  132  specifies (step  512 ,  514 ). If the bounding box test determines that the bounding volume is not intersected by the ray (“No” in step  514 ), then there is no need to perform any further testing for visualization and the traversal coprocessor  138  can return this result to the requesting SM  132 . This is because if a ray misses a bounding volume (as in  FIG.  3 A  with respect to bounding volume  310 ), then the ray will miss all other smaller bounding volumes inside the bounding volume being tested and any primitives that bounding volume contains. 
     If the bounding box test performed by the traversal coprocessor  138  reveals that the bounding volume is intersected by the ray (“Yes” in Step  514 ), then the traversal coprocessor determines if the bounding volume can be subdivided into smaller bounding volumes (step  518 ). In one example embodiment, the traversal coprocessor  138  isn&#39;t necessarily performing any subdivision itself. Rather, each node in the BVH has one or more children (where each child is a leaf or a branch in the BVH). For each child, there is a bounding volume and a pointer that leads to a branch or a leaf node. When a ray processes a node using traversal coprocessor  138 , it is testing itself against the bounding volumes of the node&#39;s children. The ray only pushes stack entries onto its stack for those branches or leaves whose representative bounding volumes were hit. When a ray fetches a node in the example embodiment, it doesn&#39;t test against the bounding volume of the node—it tests against the bounding volumes of the node&#39;s children. The traversal coprocessor  138  pushes nodes whose bounding volumes are hit by a ray onto the ray&#39;s traversal stack in an order determined by ray configuration. For example, it is possible to push nodes onto the traversal stack in the order the nodes appear in memory, or in the order that they appear along the length of the ray, or in some other order. If there are further subdivisions of the bounding volume (“Yes” in step  518 ), then those further subdivisions of the bounding volume are accessed and the bounding box test is performed for each of the resulting subdivided bounding volumes to determine which subdivided bounding volumes are intersected by the ray and which are not. In this recursive process, some of the bounding volumes may be eliminated by test  514  while other bounding volumes may result in still further and further subdivisions being tested for intersection by traversal coprocessor  138  recursively applying steps  512 - 518 . 
     Once the traversal coprocessor  138  determines that the bounding volumes intersected by the ray are leaf nodes (“No” in step  518 ), the traversal coprocessor performs a primitive (e.g., triangle) intersection test  520  to determine whether the ray intersects primitives in the intersected bounding volumes and which primitives the ray intersects. The traversal coprocessor  138  thus performs a depth-first traversal of intersected descendent branch nodes until leaf nodes are reached. The traversal coprocessor  138  processes the leaf nodes. If the leaf nodes are primitive ranges, the traversal coprocessor  138  tests them against the ray. If the leaf nodes are instance nodes, the traversal coprocessor  138  applies the instance transform. If the leaf nodes are item ranges, the traversal coprocessor  138  returns them to the requesting SM  132 . In the example non-limiting embodiments, the SM  132  can command the traversal coprocessor  138  to perform different kinds of ray-primitive intersection tests and report different results depending on the operations coming from an application (or an software stack the application is running on) and relayed by the SM to the TTU. For example, the SM  132  can command the traversal coprocessor  138  to report the nearest visible primitive revealed by the intersection test, or to report all primitives the ray intersects irrespective of whether they are the nearest visible primitive. The SM  132  can use these different results for different kinds of visualization. Once the traversal coprocessor  138  is done processing the leaf nodes, there may be other branch nodes (pushed earlier onto the ray&#39;s stack) to test. 
     Multiple Intersections 
     In more detail, as shown in  FIG.  3 C , any given ray may intersect multiple primitives within a bounding volume. Whether the ray intersection within a given primitive matters for visualization depends on the properties and position of that primitive as well as the visualization procedures the SM  132  is performing. For example, primitives can be opaque, transparent or partially transparent (i.e., translucent). Opaque primitives will block a ray from passing through the primitive because the eye cannot see through the primitive&#39;s opaque surface. Transparent primitives will allow the ray to pass through (because the eye can see through the transparent primitive) but the situation may be more complex. For example, transparent primitives may have specular properties that cause some portion of the ray to reflect (think of reflection from a window pane) and the rest of the ray to pass through. Other transparent primitives are used to provide a surface onto which a texture is mapped. For example, each individual leaf of a tree may be modeled by a transparent primitive onto which an image of the leaf is texture mapped. 
       FIGS.  5 A- 5 C  illustrate some of these scenarios using an example of three triangles assumed to be in the same bounding volume and each intersected by a ray.  FIG.  5 A  illustrates a ray directed towards these three triangles, with the first triangle the ray encounters relative to the viewpoint being opaque. Because the “front” (from the standpoint of the direction of the ray from the eye) intersected triangle is opaque, that triangle will block the ray so the ray will not reach the other triangles even through it spatially intersects them. In this example, the triangles “behind” the opaque triangle from the viewpoint can be ignored (culled) after the intersection of the opaque triangle is identified because the “front”, opaque triangle hides the other triangles from the user&#39;s view along the ray. Culling is indicated by dotted lines in  FIGS.  5 A- 5 C . In this case, the traversal coprocessor  138  may only need to report the identification of the first, opaque triangle to the SM  132 . 
       FIG.  5 B  illustrates a ray directed towards the same three triangles but now the nearest visible triangle is partially transparent rather than opaque. Because the nearest visible intersected triangle is at least partially transparent, the ray may pass through it to hit the opaque triangle behind it. In this case, the opaque triangle will be visible through the partially transparent triangle but will block the user&#39;s view of the third triangle along the ray. Here, the traversal coprocessor  138  may report the identification of both front triangles to the SM  132  but not report the third, culled triangle even though the ray spatially intersects that third triangle. Order of discovery may matter here. In the case of an alpha and opaque triangle, if the opaque was found first, the traversal coprocessor  138  returns the opaque triangle to the SM  132  with traversal state that will resume testing at the alpha triangle. While there is an implication here that the alpha means transparent, it really means “return me to the SM  132  and let the SM determine how to handle it.” For example, an alpha triangle might be trimmed according to a texture or function so that portions of the triangle are cut away (i.e., absent, not transparent). The traversal coprocessor  138  does not know how the SM  132  will handle the alpha triangles (i.e., it does not handle transparent triangles differently from trimmed triangles). Thus, alpha triangles may or may not block or tint the light arriving from points beyond them along the ray, and in example embodiments, they require SM  132  intervention to handle/determine those things. 
       FIG.  5 C  illustrates a scenario in which the first two triangles the ray encounters are partially transparent. Because the first and second intersected triangles are at least partially transparent, the ray will pass through the first and second triangles to impinge upon the also-intersecting third opaque triangle. Because third intersected triangle is opaque, it will block the ray, and the ray will not impinge upon any other triangles behind the third triangle even though they may be spatially intersected by it. In this case, the traversal coprocessor  138  may report all three triangles to the SM  132  but need not report any further triangles behind the opaque triangle because the opaque triangle blocks the ray from reaching those additional triangles. 
     In some modes, however, the SM  132  may need to know the identities of all triangles the ray intersects irrespective of whether they are opaque or transparent. In those modes, the traversal coprocessor  138  can simply perform the intersection test and return the identities of all triangles the ray spatially intersects (in such modes, the traversal coprocessor will return the same intersection results for all three scenarios shown in  FIGS.  5 A- 5 C ) and allow the SM  132  to sort it out—or in some cases command the traversal coprocessor  138  to do more tests on these same triangles. 
     As will be discussed in more detail below, when a ray intersects an opaque triangle, the traversal coprocessor  138  can in certain operations be programmed to reduce the length of the ray being tested to the location of the opaque triangle intersection so it will not report any triangles “behind” the intersected triangle. When a partially transparent triangle is determined to be intersected by a ray, the traversal coprocessor  138  will return a more complete list of triangles the ray impinges upon for purposes of visualization, and the requesting SM  132  may perform further processing to determine whether, based for example any texture or other properties of the triangle, the ray will be blocked, passed or partially passed and partially reflected. In example embodiments, the traversal coprocessor  138  does not have access to texture properties of triangles and so does not attempt to determine visualization with respect to those properties. 
     Textures or Other Surface Modifications 
     For example,  FIGS.  6 A and  6 B  show a transparent triangle  610  with a texture  615  of a leaf applied to the triangle. One could think of a triangle made of Plexiglas with a decal of a leaf applied to it. As shown in  FIG.  6 A , the ray  620  intersects the transparent triangle  610  at a point that is outside the applied texture  615 . Because the ray  620  intersects the triangle outside the applied texture  615 , the texture will not block the ray  620  and the ray will pass through the transparent triangle  610  without obstruction. This is like being able to see through the parts of the Plexiglas triangle that are not covered by the leaf decal. Note that in one example embodiment, the SM  132  makes the visibility determination since the traversal coprocessor  138  does not necessarily have access to information concerning the leaf decal. The traversal coprocessor  138  helps the SM  132  by returning to the SM the identification of the triangle that the ray intersects along with information concerning the properties of that triangle. 
     In  FIG.  6 B , the ray  630  intersects the transparent triangle where the texture  615  is applied. SM  132  will determine whether subsequent traversal by the traversal coprocessor  138  is necessary or not based on whether the texture  615  will block the ray  630  or allow the ray  630  to pass through. If the ray  630  is blocked by the texture  615 , other triangles behind the transparent triangle  610 , which may have otherwise been intersected by the ray  630 , will be obstructed by the texture and not contribute to visualization along the ray. In the example non-limiting embodiments herein, the traversal coprocessor  138  does not have access to texture information and so it does not attempt to accelerate this determination. Traversal coprocessor  138  may for example return to the requesting SM  132  all intersections between the ray and the various triangles within the object, and the SM may then use the graphics primitive engine  134  to make further ray tracing visualization determinations. In other example embodiments, traversal coprocessor  138  could accelerate some or all of these tests by interacting with the texture mapping unit and other portions of the 3D graphics pipeline within graphics primitive engine  134  to make the necessary visualization determinations. 
     Coordinate Transforms 
       FIGS.  2 A- 3 C  involve only a single object, namely a teapot. Just as the room you are in right now contains multiple objects, most 3D scenes contain many objects. For example, a 3D scene containing a teapot will likely also contain a cup, a saucer, a milk pitcher, a spoon, a sugar bowl, etc. all sitting on a table. In 3D graphics, each of these objects is typically modelled independently. The graphics system  100  then uses commands from the processor  120  to put all the models together in desired positions, orientations and sizes into the common scene for purposes of visualization (just as you will set and arrange the table for serving tea). What this means is that the SM  132  may command traversal processor  138  to analyze the same ray with respect to multiple objects in the scene. However, the fact that each of these objects will be transformed in position, orientation and size when placed into the common scene is taken into account and accelerated by the traversal coprocessor  138 . In non-limiting example embodiments, the transform from world-to-object space is stored in the world space BVH along with a world-space bounding box. The traversal coprocessor  138  accelerates the transform process by transforming the ray from world (scene) space into object space for purposes of performing the tests shown in  FIG.  4   . In particular, since the transformation of the geometry from object space into world (scene) space is computationally intensive, that transformation is left to the graphics pipeline graphics primitive engine  134  and/or raster engine  136  to perform as part of rasterization. The traversal coprocessor  138  instead transforms a given ray from world space to the coordinate system of each object defined by an acceleration data structure and performs its tests in object space. 
       FIGS.  7 A and  7 B  illustrates how the traversal coprocessor  138  transforms the same ray into three different object spaces.  FIG.  7 A  shows three objects on a table: a cup, a teapot and a pitcher. These three objects and a table comprise a scene, which exists in world space. A ray that also is defined in world space emanates from the viewpoint and intersects each of the three objects. 
       FIG.  7 B  shows each of the three objects as defined in object spaces. Each of these three objects is defined by a respective model that exists in a respective object space. The traversal coprocessor  138  in example non-limiting embodiments transforms the ray into the object space of each object before performing the intersection tests for that object. This “instance transform” saves the computational effort of transforming the geometry of each object and the associated volumetric subdivisions of the acceleration data structure from object space to world space for purposes of the traversal coprocessor  138  performing intersection tests. 
     The requesting SM  132  keeps track of which objects are in front of which other objects with respect to each individual ray and resolves visibility in cases where one object hides another object, casts a shadow on another object, and/or reflects light toward another object. The requesting SM  132  can use the traversal processor  138  to accelerate each of these tests. 
     Example Tree BVH Acceleration Data Structure 
       FIGS.  8 A and  8 B  show a recursively-subdivided bounding volume of a 3D scene ( FIG.  8 A ) and a corresponding tree data structure ( FIG.  8 B ) that may be accessed by the traversal coprocessor  138  and used for hardware-accelerated operations performed by traversal coprocessor. The division of the bounding volumes may be represented in a hierarchical tree data structure with the large bounding volume shown in  FIG.  2 B  represented by a parent node of the tree and the smaller bounding volumes represented by children nodes of the tree that are contained by the parent node. The smallest bounding volumes are represented as leaf nodes in the tree and identify one or more geometric primitives contained within these smallest bounding volumes. 
     The tree data structure may be stored in memory outside of the traversal coprocessor  138  and retrieved based on queries the SMs  132  issue to the traversal coprocessor  138 . The tree data structure includes a plurality of nodes arranged in a hierarchy. The root nodes N 1  of the tree structure correspond to bounding volume N 1  enclosing all of the triangles O 1 -O 8 . The root node N 1  may identify the vertices of the bounding volume N 1  and children nodes of the root node. 
     In  FIG.  8 A , bounding volume N 1  is subdivided into bounding volumes N 2  and N 3 . Children nodes N 2  and N 3  of the tree structure of  FIG.  8 B  correspond to and represent the bounding volumes N 2  and N 3  shown in  FIG.  8 A . The children nodes N 2  and N 3  in the tree data structure identify the vertices of respective bounding volumes N 2  and N 3  in space. Each of the bounding volumes N 2  and N 3  is further subdivided in this particular example. Bounding volume N 2  is subdivided into contained bounding volumes N 4  and N 5 . Bounding volume N 3  is subdivided into contained bounding volumes N 6  and N 7 . Bounding volume N 7  include two bounding volumes N 8  and N 9 . Bounding volume N 8  includes the triangles O 7  and O 8 , and bounding volume N 9  includes leaf bounding volumes N 10  and N 11  as its child bounding volumes. Leaf bounding volume N 10  includes a primitive range (e.g., triangle range) O 10  and leaf bounding volume N 11  includes an item range O 9 . Respective children nodes N 4 , N 5 , N 6 , N 8 , N 10  and N 11  of the  FIG.  8 B  tree structure correspond to and represent the  FIG.  8 A  bounding volumes N 4 , N 5 , N 6 , N 8 , N 10  and N 11  in space. 
     The  FIG.  8 B  tree is only three to six levels deep so that volumes N 4 , N 5 , N 6 , N 8 , N 10  and N 11  constitute “leaf nodes”—that is, nodes in the tree that have no child nodes.  FIG.  8 A  shows that each of leaf node bounding volumes N 4 , N 5 , N 6 , and N 8 , contains two triangles of the geometry in the scene. For example, volumetric subdivision N 4  contains triangles O 1  &amp; O 2 ; volumetric subdivision N 5  contains triangles O 3  &amp; O 4 ; volumetric subdivision N 6  contains trials O 5  &amp; O 6 ; and volumetric subdivision N 8  contains triangles O 7  &amp; O 8 . The tree structure shown in  FIG.  8 B  represents these leaf nodes N 4 , N 5 , N 6 , and N 7  by associating them with the appropriate ones of triangles O 1 -O 8  of the scene geometry. To access this scene geometry, the traversal coprocessor  138  traverses the tree data structure of  FIG.  8 B  down to the leaf nodes. In general, different parts of the tree can and will have different depths and contain different numbers of triangles. Leaf nodes associated with volumetric subdivisions that contain no geometry need not be explicitly represented in the tree data structure (i.e., the tree is “trimmed”). 
     According to some embodiments, the subtree rooted at N 7  may represent a set of bounding volumes or BVH that is defined in a different coordinate space than the bounding volumes corresponding to nodes N 1 -N 3 . When bounding volume N 7  is in a different coordinate space from its parent bounding volume N 3 , an instance node N 7 ′ which provides the ray transformation necessary to traverse the subtree rooted at N 7 , may connect the rest of the tree to the subtree rooted at N 7 . Instance node N 7 ′ connects the bounding volume or BVH corresponding to nodes N 1 -N 3 , with the bounding volumes or BVH corresponding to nodes N 7  etc. by defining the transformation from the coordinate space of N 1 -N 3  (e.g., world space) to the coordinate space of N 7  etc. (e.g., object space). 
     The Internal Structure and Operation of Traversal Coprocessor  138   
       FIG.  9    shows an example simplified block diagram of traversal coprocessor  138  including hardware configured to perform accelerated traversal operations as described above (a still more detailed implementation of this traversal coprocessor  138  is described below). Because the traversal coprocessor  138  shown in  FIG.  9    is adapted to traverse tree-based acceleration data structures such as shown in  FIGS.  8 A,  8 B , it may also be called a “tree traversal unit” or “TTU”  700  (the 700 reference number is used to refer to the more detailed non-limiting implementation of traversal coprocessor  138  shown in  FIG.  1   ). Tree traversal operations may include, for example, determining whether a ray intersects bounding volumes and/or primitives of a tree data structure (e.g., a BVH tree), which tests may involve transforming the ray into object space. 
     The TTU  700  includes dedicated hardware to determine whether a ray intersects bounding volumes and dedicated hardware to determine whether a ray intersects primitives of the tree data structure. In some embodiments, the TTU  700  may perform a depth-first traversal of a bounding volume hierarchy using a short stack traversal with intersection testing of supported leaf node primitives and mid-traversal return of alpha primitives and unsupported leaf node primitives (items). The intersection of primitives will be discussed with reference to triangles, but other geometric primitives may also be used. 
     In more detail, TTU  700  includes an intersection management block  722 , a ray management block  730  and a stack management block  740 . Each of these blocks (and all of the other blocks in  FIG.  9   ) may constitute dedicated hardware implemented by logic gates, registers, hardware-embedded lookup tables or other combinatorial logic, etc. 
     The ray management block  730  is responsible for managing information about and performing operations concerning a ray specified by an SM  132  to the ray management block. The stack management block  740  works in conjunction with traversal logic  712  to manage information about and perform operations related to traversal of a BVH acceleration data structure. Traversal logic  712  is directed by results of a ray-complet test block  710  that tests intersections between the ray indicated by the ray management block  730  and volumetric subdivisions represented by the BVH, using instance transforms as needed. The ray-complet test block  710  retrieves additional information concerning the BVH from memory  140  via an L0 complet cache  752  that is part of the TTU  700 . The results of the ray-complet test block  710  informs the traversal logic  712  as to whether further recursive traversals are needed. The stack management block  740  maintains stacks to keep track of state information as the traversal logic  712  traverses from one level of the BVH to another, with the stack management block pushing items onto the stack as the traversal logic traverses deeper into the BVH and popping items from the stack as the traversal logic traverses upwards in the BVH. The stack management block  740  is able to provide state information (e.g., intermediate or final results) to the requesting SM  132  at any time the SM requests. 
     The intersection management block  722  manages information about and performs operations concerning intersections between rays and primitives, using instance transforms as needed. The ray-primitive test block  720  retrieves information concerning geometry from memory  140  on an as-needed basis via an L0 primitive cache  754  that is part of TTU  700 . The intersection management block  722  is informed by results of intersection tests the ray-primitive test and transform block  720  performs. Thus, the ray-primitive test and transform block  720  provides intersection results to the intersection management block  722 , which reports geometry hits and intersections to the requesting SM  132 . 
     A Stack Management Unit  740  inspects the traversal state to determine what type of data needs to be retrieved and which data path (complet or primitive) will consume it. The intersections for the bounding volumes are determined in the ray-complet test path of the TTU  700  including one or more ray-complet test blocks  710  and one or more traversal logic blocks  712 . A complet specifies root or interior nodes of a bounding volume. Thus, a complet may define one or more bounding volumes for the ray-complet test. The ray-complet test path of the TTU  700  identifies which bounding volumes are intersected by the ray. Bounding volumes intersected by the ray need to be further processed to determine if the primitives associated with the intersected bounding volumes are intersected. The intersections for the primitives are determined in the ray-primitive test path including one or more ray-primitive test and transform blocks  720  and one or more intersection management blocks  722 . 
     The TTU  700  receives queries from one or more SMs  132  to perform tree traversal operations. The query may request whether a ray intersects bounding volumes and/or primitives in a BVH data structure. The query may identify a ray (e.g., origin, direction, and length of the ray) and a BVH data structure and traversal state (short stack) which includes one or more entries referencing nodes in one or more Bounding Volume Hierarchies that the ray is to visit. The query may also include information for how the ray is to handle specific types of intersections during traversal. The ray information may be stored in the ray management block  730 . The stored ray information (e.g., ray length) may be updated based on the results of the ray-primitive test. 
     The TTU  700  may request the BVH data structure identified in the query to be retrieved from memory outside of the TTU  700 . Retrieved portions of the BVH data structure may be cached in the level-zero (L0) cache  750  within the TTU  700  so the information is available for other time-coherent TTU operations, thereby reducing memory  140  accesses. Portions of the BVH data structure needed for the ray-complet test may be stored in a L0 complet cache  752  and portions of the BVH data structure needed for the ray-primitive test may be stored in an L0 primitive cache  754 . 
     After the complet information needed for a requested traversal step is available in the complet cache  752 , the ray-complet test block  710  determines bounding volumes intersected by the ray. In performing this test, the ray may be transformed from the coordinate space of the bounding volume hierarchy to a coordinate space defined relative to a complet. The ray is tested against the bounding boxes associated with the child nodes of the complet. In the example non-limiting embodiment, the ray is not tested against the complet&#39;s own bounding box because (1) the TTU  700  previously tested the ray against a similar bounding box when it tested the parent bounding box child that referenced this complet, and (2) a purpose of the complet bounding box is to define a local coordinate system within which the child bounding boxes can be expressed in compressed form. If the ray intersects any of the child bounding boxes, the results are pushed to the traversal logic to determine the order that the corresponding child pointers will be pushed onto the traversal stack (further testing will likely require the traversal logic  712  to traverse down to the next level of the BVH). These steps are repeated recursively until intersected leaf nodes of the BVH are encountered 
     The ray-complet test block  710  may provide ray-complet intersections to the traversal logic  612 . Using the results of the ray-complet test, the traversal logic  712  creates stack entries to be pushed to the stack management block  740 . The stack entries may indicate internal nodes (i.e., a node that includes one or more child nodes) that need to be further tested for ray intersections by the ray-complet test block  710  and/or triangles identified in an intersected leaf node that need to be tested for ray intersections by the ray-primitive test and transform block  720 . The ray-complet test block  710  may repeat the traversal on internal nodes identified in the stack to determine all leaf nodes in the BVH that the ray intersects. The precise tests the ray-complet test block  710  performs will in the example non-limiting embodiment be determined by mode bits, ray operations (see below) and culling of hits, and the TTU  700  may return intermediate as well as final results to the SM  132 . 
     The intersected leaf nodes identify primitives that may or may not be intersected by the ray. One option is for the TTU  700  to provide e.g., a range of geometry identified in the intersected leaf nodes to the SM  132  for further processing. For example, the SM  132  may itself determine whether the identified primitives are intersected by the ray based on the information the TTU  700  provides as a result of the TTU traversing the BVH. To offload this processing from the SM  132  and thereby accelerate it using the hardware of the TTU  700 , the stack management block  740  may issue requests for the ray-primitive and transform block  720  to perform a ray-primitive test for the primitives within intersected leaf nodes the TTU&#39;s ray-complet test block  710  identified. In some embodiments, the SM  132  may issue a request for the ray-primitive test to test a specific range of primitives and transform block  720  irrespective of how that geometry range was identified. 
     After making sure the primitive data needed for a requested ray-primitive test is available in the primitive cache  754 , the ray-primitive and transform block  710  may determine primitives that are intersected by the ray using the ray information stored in the ray management block  730 . The ray-primitive test block  720  provides the identification of primitives determined to be intersected by the ray to the intersection management block  722 . 
     The intersection management block  722  can return the results of the ray-primitive test to the SM  132 . The results of the ray-primitive test may include identifiers of intersected primitives, the distance of intersections from the ray origin and other information concerning properties of the intersected primitives. In some embodiments, the intersection management block  722  may modify an existing ray-primitive test (e.g., by modifying the length of the ray) based on previous intersection results from the ray-primitive and transform block  710 . 
     The intersection management block  722  may also keep track of different types of primitives. For example, the different types of triangles include opaque triangles that will block a ray when intersected and alpha triangles that may or may not block the ray when intersected or may require additional handling by the SM. Whether a ray is blocked or not by a transparent triangle may for example depend on texture(s) mapped onto the triangle, area of the triangle occupied by the texture (see  FIGS.  6 A and  6 B ) and the way the texture modifies the triangle. For example, transparency (e.g., stained glass) in some embodiments requires the SM  132  to keep track of transparent object hits so they can be sorted and shaded in ray-parametric order, and typically don&#39;t actually block the ray. Meanwhile, alpha “trimming” allows the shape of the primitive to be trimmed based on the shape of a texture mapped onto the primitive—for example, cutting a leaf shape out of a triangle. (Note that in raster graphics, transparency is often called “alpha blending” and trimming is called “alpha test”). In other embodiments, the TTU  700  can push transparent hits to queues in memory for later handling by the SM  132  and directly handle trimmed triangles by sending requests to the texture unit. Each triangle may include a designator to indicate the triangle type. The intersection management block  722  is configured to maintain a result queue for tracking the different types of intersected triangles. For example, the result queue may store one or more intersected opaque triangle identifiers in one queue and one or more transparent triangle identifiers in another queue. 
     For opaque triangles, the ray intersection can be fully determined in the TTU  700  because the area of the opaque triangle blocks the ray from going past the surface of the triangle. For transparent triangles, ray intersections cannot in some embodiments be fully determined in the TTU  700  because TTU  700  performs the intersection test based on the geometry of the triangle and may not have access to the texture of the triangle and/or area of the triangle occupied by the texture (in other embodiments, the TTU may be provided with texture information by the texture mapping block of the graphics pipeline). To fully determine whether the triangle is intersected, information about transparent triangles the ray-primitive and transform block  710  determines are intersected may be sent to the SM  132 , for the SM to make the full determination as to whether the triangle affects visibility along the ray. 
     The SM  132  can resolve whether or not the ray intersects a texture associated with the transparent triangle and/or whether the ray will be blocked by the texture. The SM  132  may in some cases send a modified query to the TTU  700  (e.g., shortening the ray if the ray is blocked by the texture) based on this determination. 
     In one embodiment, the TTU  700  may be configured to return all triangles determined to intersect the ray to the SM  132  for further processing. Because returning every triangle intersection to the SM  132  for further processing is costly in terms of interface and thread synchronization, the TTU  700  may be configured to hide triangles which are intersected but are provably capable of being hidden without a functional impact on the resulting scene. For example, because the TTU  700  is provided with triangle type information (e.g., whether a triangle is opaque or transparent), the TTU  700  may use the triangle type information to determine intersected triangles that are occluded along the ray by another intersecting opaque triangle and which thus need not be included in the results because they will not affect the visibility along the ray. As discussed above with reference to  FIGS.  5 A- 5 C , if the TTU  700  knows that a triangle is occluded along the ray by an opaque triangle, the occluded triangle can be hidden from the results without impact on visualization of the resulting scene. 
     The intersection management block  722  may include a result queue for storing hits that associate a triangle ID and information about the point where the ray hit the triangle. When a ray is determined to intersect an opaque triangle, the identity of the triangle and the distance of the intersection from the ray origin can be stored in the result queue. If the ray is determined to intersect another opaque triangle, the other intersected opaque triangle can be omitted from the result if the distance of the intersection from the ray origin is greater than the distance of the intersected opaque triangle already stored in the result queue. If the distance of the intersection from the ray origin is less than the distance of the intersected opaque triangle already stored in the result queue, the other intersected opaque triangle can replace the opaque triangle stored in the result queue. After all of the triangles of a query have been tested, the opaque triangle information stored in the result queue and the intersection information may be sent to the SM  132 . 
     In some embodiments, once an opaque triangle intersection is identified, the intersection management block  722  may shorten the ray stored in the ray management block  730  so that bounding volumes (which may include triangles) behind the intersected opaque triangle (along the ray) will not be identified as intersecting the ray. 
     The intersection management block  722  may store information about intersected transparent triangles in a separate queue. The stored information about intersected transparent triangles may be sent to the SM  132  for the SM to resolve whether or not the ray intersects a texture associated with the triangle and/or whether the texture blocks the ray. The SM may return the results of this determination to the TTU  700  and/or modify the query (e.g., shorten the ray if the ray is blocked by the texture) based on this determination. 
     Example Ray Tracing Shading Pipeline 
       FIG.  10 A  shows an exemplary ray tracing shading pipeline  900  that may be performed by SM  132  and accelerated by TTU  700 . The ray tracing shading pipeline  900  starts by an SM  132  invoking ray generation  910  and issuing a corresponding ray tracing request to the TTU  700 . The ray tracing request identifies a single ray cast into the scene and asks the TTU  700  to search for intersections with an acceleration data structure the SM  132  also specifies. The TTU  700  traverses ( FIG.  10 A  block  920 ) the acceleration data structure to determine intersections or potential intersections between the ray and the volumetric subdivisions and associated triangles the acceleration data structure represents. Potential intersections can be identified by finding bounding volumes in the acceleration data structure that are intersected by the ray. Descendants of non-intersected bounding volumes need not be examined. 
     For triangles within intersected bounding volumes, the TTU  700  ray-primitive test block  720  performs an intersection  930  process to determine whether the ray intersects the primitives. The TTU  700  returns intersection information to the SM  132 , which may perform an “any hit” shading operation  940  in response to the intersection determination. For example, the SM  132  may perform (or have other hardware perform) a texture lookup for an intersected primitive and decide based on the appropriate texel&#39;s value how to shade a pixel visualizing the ray. The SM  132  keeps track of such results since the TTU  700  may return multiple intersections with different geometry in the scene in arbitrary order. 
     Alternatively, primitives that the TTU  700  determines are intersected may be further processed to determine  950  whether they should be shaded as a miss  960  or as a closest hit  970 . The SM  132  can for example instruct the TTU  700  to report a closest hit in the specified geometry, or it may instruct the TTU to report all hits in the specified geometry. For example, it may be up to the SM  132  to implement a “miss” shading operation for a primitive the TTU  700  determines is intersected based on implemented environment lookups (e.g., approximating the appearance of a reflective surface by means of a precomputed texture image) such as shown in  FIGS.  6 A &amp;  6 B . The SM  132  may perform a closest hit shading operation to determine the closest intersected primitive based on material evaluations and texture lookups in response to closest hit reports the TTU  700  provided for particular object geometry. 
     The  FIG.  10 B  more detailed diagram of a ray-tracing pipeline flowchart shows the data flow and interaction between components for a representative use case: tracing rays against a scene containing geometric primitives, with instance transformations handled in hardware. In one example non-limiting embodiment, the ray-tracing pipeline of  FIG.  10 B  is essentially software-defined (which in example embodiments means it is determined by the SMs  132 ) but makes extensive use of hardware acceleration by TTU  700 . Key components include the SM  132  (and the rest of the compute pipeline), the TTU  700  (which serves as a coprocessor to SM), and the L1 cache and downstream memory system, from which the TTU fetches BVH and triangle data. 
     The pipeline shown in  FIG.  10 B  shows that bounding volume hierarchy creation  1002  can be performed ahead of time by a development system. It also shows that ray creation and distribution  1004  are performed or controlled by the SM  132  or other software in the example embodiment, as shading (which can include lighting and texturing). The example pipeline includes a “top level” BVH tree traversal  1006 , ray transformation  1014 , “bottom level” BVH tree traversal  1018 , and a ray/triangle (or other primitive) intersection  1026  that are each performed by the TTU  700 . These do not have to be performed in the order shown, as handshaking between the TTU  700  and the SM  132  determines what the TTU  700  does and in what order. 
     The SM  132  presents one or more rays to the TTU  700  at a time. Each ray the SM  132  presents to the TTU  700  for traversal may include the ray&#39;s geometric parameters, traversal state, and the ray&#39;s ray flags, mode flags and ray operations information. In an example embodiment, a ray operation (RayOp) provides or comprises an auxiliary arithmetic and/or logical test to suppress, override, and/or allow storage of an intersection. The traversal stack may also be used by the SM  132  to communicate certain state information to the TTU  700  for use in the traversal. A new ray query may be started with an explicit traversal stack. For some queries, however, a small number of stack initializers may be provided for beginning the new query of a given type, such as, for example: traversal starting from a complet; intersection of a ray with a range of triangles; intersection of a ray with a range of triangles, followed by traversal starting from a complet; vertex fetch from a triangle buffer for a given triangle, etc. In some embodiments, using stack initializers instead of explicit stack initialization improves performance because stack initializers require fewer streaming processor registers and reduce the number of parameters that need to be transmitted from the streaming processor to the TTU. 
     In the example embodiment, a set of mode flags the SM  132  presents with each query (e.g., ray) may at least partly control how the TTU  700  will process the query when the query intersects the bounding volume of a specific type or intersects a primitive of a specific primitive type. The mode flags the SM  132  provides to the TTU  700  enable the ability by the SM and/or the application to e.g., through a RayOp, specify an auxiliary arithmetic or logical test to suppress, override, or allow storage of an intersection. The mode flags may for example enable traversal behavior to be changed in accordance with such aspects as, for example, a depth (or distance) associated with each bounding volume and/or primitive, size of a bounding volume or primitive in relation to a distance from the origin or the ray, particular instances of an object, etc. This capability can be used by applications to dynamically and/or selectively enable/disable sets of objects for intersection testing versus specific sets or groups of queries, for example, to allow for different versions of models to be used when application state changes (for example, when doors open or close) or to provide different versions of a model which are selected as a function of the length of the ray to realize a form of geometric level of detail, or to allow specific sets of objects from certain classes of rays to make some layers visible or invisible in specific views. 
     In addition to the set of mode flags which may be specified separately for the ray-complet intersection and for ray-primitive intersections, the ray data structure may specify other RayOp test related parameters, such as ray flags, ray parameters and a RayOp test. The ray flags can be used by the TTU  700  to control various aspects of traversal behavior, back-face culling, and handling of the various child node types, subject to a pass/fail status of an optional RayOp test. RayOp tests add flexibility to the capabilities of the TTU  700 , at the expense of some complexity. The TTU  700  reserves a “ray slot” for each active ray it is processing, and may store the ray flags, mode flags and/or the RayOp information in the corresponding ray slot buffer within the TTU during traversal. 
     In the example shown in  FIG.  10 B , the TTU  700  performs a top level tree traversal  1006  and a bottom level tree traversal  1018 . In the example embodiment, the two level traversal of the BVH enables fast ray tracing responses to dynamic scene changes. 
     Ray transformation  1014  provides the appropriate transition from the top level tree traversal  1006  to the bottom level tree traversal  1018  by transforming the ray, which may be used in the top level traversal in a first coordinate space (e.g., world space), to a different coordinate space (e.g., object space) of the BVH of the bottom level traversal. An example BVH traversal technique using a two level traversal is described in previous literature, see, e.g., Woop, “A Ray Tracing Hardware Architecture for Dynamic Scenes”, Universitat des Saarlandes, 2004, but embodiments are not limited thereto. 
     In some embodiments, the top level traversal (in world space) is made in a BVH that may be dynamically recalculated (e.g., by SM  132 ) in response to changes in the scene, and the bottom level traversal is made in a BVH of bounding volumes that remain static or substantially static even when changes in the scene occur. The bounding volumes in the BVH used for the bottom level tree traversal  1018  (in object space) may encompass more detailed information regarding the scene geometry than the respective bounding volumes used in the top level tree traversal  1006 , thereby avoiding or at least reducing the modification of the bottom level traversal BVH in response to scene changes. This helps to speed up ray tracing of dynamic scenes. 
     Example Top Level Tree Traversal 
     The top level tree traversal  1006  by TTU  700  receives complets from the L1 cache  1012 , and provides an instance to the ray transformation  1014  for transformation, or a miss/end output  1013  to the SM  132  for closest hit shader  1015  processing by the SM (this block can also operate recursively based on non-leaf nodes/no hit conditions). In the top level tree traversal  1006 , a next complet fetch step  1008  fetches the next complet to be tested for ray intersection in step  1010  from the memory and/or cache hierarchy and ray-bounding volume intersection testing is done on the bounding volumes in the fetched complet. 
     As described above, an instance node connects one BVH to another BVH which is in a different coordinate system. When a child of the intersected bounding volume is an instance node, the ray transformation  1014  is able to retrieve an appropriate transform matrix from the L1 cache  1016 . The TTU  700 , using the appropriate transform matrix, transforms the ray to the coordinate system of the child BVH. U.S. patent application Ser. No. 14/697,480, which is already incorporated by reference, describes transformation nodes that connect a first set of nodes in a tree to a second set of nodes where the first and second sets of nodes are in different coordinate systems. The instance nodes in example embodiments may be similar to the transformation nodes in U.S. application Ser. No. 14/697,480. In an alternative, non-instancing mode of TTU  700  shown in  FIG.  10 C , the TTU does not execute a “bottom” level tree traversal  1018  and noninstanced tree BVH traversals are performed by blocks  1008 ,  1010  e.g., using only one stack. The TTU  700  can switch between the  FIG.  10 B  instanced operations and the  FIG.  10 C  non-instanced operations based on what it reads from the BVH and/or query type. For example, a specific query type may restrict the TTU to use just the non-instanced operations. In such a query, any intersected instance nodes would be returned to the SM. 
     In some non-limiting embodiments, ray-bounding volume intersection testing in step  1010  is performed on each bounding volume in the fetched complet before the next complet is fetched. Other embodiments may use other techniques, such as, for example, traversing the top level traversal BVH in a depth-first manner. U.S. Pat. No. 9,582,607, already incorporated by reference, describes one or more complet structures and contents that may be used in example embodiments. U.S. Pat. No. 9,582,607 also describes an example traversal of complets. 
     When a bounding volume is determined to be intersected by the ray, the child bounding volumes (or references to them) of the intersected bounding volume are kept track of for subsequent testing for intersection with the ray and for traversal. In example embodiments, one or more stack data structures is used for keeping track of child bounding volumes to be subsequently tested for intersection with the ray. In some example embodiments, a traversal stack of a small size may be used to keep track of complets to be traversed by operation of the top level tree traversal  1006 , and primitives to be tested for intersection, and a larger local stack data structure can be used to keep track of the traversal state in the bottom level tree traversal  1018 . 
     Example Bottom Level Tree Traversal 
     In the bottom level tree traversal  1018 , a next complet fetch step  1022  fetches the next complet to be tested for ray intersection in step  1024  from the memory and/or cache hierarchy  1020  and ray-bounding volume intersection testing is done on the bounding volumes in the fetched complet. The bottom level tree traversal, as noted above, may include complets with bounding volumes in a different coordinate system than the bounding volumes traversed in the upper level tree traversal. The bottom level tree traversal also receives complets from the L1 cache and can operate recursively or iteratively within itself based on non-leaf/no-hit conditions and also with the top level tree traversal  1006  based on miss/end detection. Intersections of the ray with the bounding volumes in the lower level BVH may be determined with the ray transformed to the coordinate system of the lower level complet retrieved. The leaf bounding volumes found to be intersected by the ray in the lower level tree traversal are then provided to the ray/triangle intersection  1026 . 
     The leaf outputs of the bottom level tree traversal  1018  are provided to the ray/triangle intersection  1026  (which has L0 cache access as well as ability to retrieve triangles via the L1 cache  1028 ). The L0 complet and triangle caches may be small read-only caches internal to the TTU  700 . The ray/triangle intersection  1026  may also receive leaf outputs from the top level tree traversal  1006  when certain leaf nodes are reached without traversing an instanced BVH. 
     After all the primitives in the primitive range have been processed, the Intersection Management Unit inspects the state of the result Queue and crafts packets to send to the Stack Management Unit and/or Ray Management Unit to update the ray&#39;s attributes and traversal state, set up the ray&#39;s next traversal step, and/or return the ray to the SM  132  (if necessary). If the result queue contains opaque or alpha intersections found during the processing of the primitive range then the Intersection Management Unit signals the parametric length (t) of the nearest opaque intersection in the result queue to the ray management unit to record as the ray&#39;s tmax to shorten the ray. To update the traversal state to set up the ray&#39;s next traversal step the Intersection Management Unit signals to the Stack Management Unit whether an opaque intersection from the primitive range is present in the resultQueue, whether one or more alpha intersections are present in the result queue, whether the resultQueue is full, whether additional alpha intersections were found in the primitive range that have not been returned to the SM and which are not present in the resultQueue, and the index of the next alpha primitive in the primitive range for the ray to test after the SM consumes the contents of the resultQueue (the index of the next primitive in the range after the alpha primitive with the highest memory-order from the current primitive range in the result queue). 
     When the Stack Management Unit  740  receives the packet from Intersection Management Unit  722 , the Stack Management Unit  740  inspects the packet to determine the next action required to complete the traversal step and start the next one. If the packet from Intersection Management Unit  722  indicates an opaque intersection has been found in the primitive range and the ray mode bits indicate the ray is to finish traversal once any intersection has been found the Stack Management Unit  740  returns the ray and its results queue to the SM with traversal state indicating that traversal is complete (a done flag set and/or an empty top level and bottom level stack). If the packet from Intersection Management Unit  722  indicates that there opaque or alpha intersection in the result queue and that there are remaining alpha intersections in the primitive range not present in the result queue that were encountered by the ray during the processing of the primitive range that have not already been returned to the SM, the Stack Management Unit  740  returns the ray and the result queue to the SM with traversal state modified to set the cull opaque bit to prevent further processing of opaque primitives in the primitive range and the primitive range starting index advanced to the first alpha primitive after the highest alpha primitive intersection from the primitive range returned to the SM in the ray&#39;s result queue. If the packet from Intersection Management Unit  722  indicates that no opaque or alpha intersections were found when the ray processed the primitive range the Stack Management Unit  740  pops the top of stack entry (corresponding to the finished primitive range) off the active traversal stack. If the packet from Stack Management Unit  740  indicates or that either there are opaque intersections in the result queue and the ray mode bits do not indicate that the ray is to finish traversal once any intersection has been found and/or there are alpha intersections in the result queue, but there were no remaining alpha intersections found in the primitive range not present in the result queue that have not already been returned to the SM the Stack Management Unit  740  pops the top of stack entry (corresponding to the finished primitive range) off the active traversal stack and modifies the contents of the result queue to indicate that all intersections present in the result queue come from a primitive range whose processing was completed. 
     If the active stack is the bottom stack, and the bottom stack is empty the Stack Management Unit  740  sets the active stack to the top stack. If the top stack is the active stack, and the active stack is empty, then the Stack Management Unit  740  returns the ray and its result queue to the SM with traversal state indicating that traversal is complete (a done flag set and/or an empty top level and bottom level stack). If the active stack contains one or more stack entries, then the Stack Management Unit  740  inspects the top stack entry and starts the next traversal step. Testing of primitive and/or primitive ranges for intersections with a ray and returning results to the SM  132  are described in co-pending U.S. application Ser. No. 16/101,148 entitled “Watertight Ray Triangle Intersection” and U.S. application Ser. No. 16/101,196 entitled “Method for Handling Out-of-Order Opaque and Alpha Ray/Primitive Intersections,” which are hereby incorporated by reference in their entireties. 
     While the above disclosure is framed in the specific context of computer graphics and visualization, ray tracing and the disclosed traversal coprocessor could be used for a variety of applications beyond graphics and visualization. Non-limiting examples include sound propagation for realistic sound synthesis, simulation of sonar systems, design of optical elements and systems, particle transport simulation (e.g., for medical physics or experimental high-energy physics), general wave propagation simulation, comparison to LIDAR data for purposes e.g., of robot or vehicle localization, and others. OptiX™ has already been used for some of these application areas in the past. 
     Example Ray Primitive Traversal without Shader Intervention 
     As discussed above, the TTU  700  includes dedicated hardware to determine whether a ray intersects bounding volumes and dedicated hardware to determine whether a ray intersects primitives of the tree data structure. The primitives of the tree data structure may include different geometric primitives (e.g., points, lines, triangles, quads, meshes, etc.), which are identified in the leaf nodes of the tree data structure. At least some of the primitives (e.g., triangles) may include a type designator to distinguish between different types of primitives. As discussed with reference to  FIGS.  5 A- 5 C , different types of triangles may include opaque triangles and alpha triangles. The TTU  700  includes hardware that is configured to fully determine intersections for some type of primitives, while for other primitives the TTU  700  may only by itself determine possible intersections. For example, the TTU may fully determine that a ray intersects an opaque triangle. For alpha triangles, the TTU may only be able to determine that an intersection is possible and will need for the SM to resolve (e.g., based on texture information) whether or not the ray intersects a texture applied to the transparent triangle surface and/or whether the texture will block the ray. 
     In one embodiment, the TTU  700  may be configured to return all primitives determined to intersect the ray to the SM  132  for further processing. However, returning each intersected primitive and possibly intersected primitive to the SM  132  is costly in terms of interface and thread synchronization. In addition, the SM  132  will need resources to process at least some of the returned primitives to determine, for example based on texture mapping information, whether or not the ray intersects a texture applied to the transparent triangle and/or whether the ray will be blocked by the texture. 
     The TTU  700  provides a hardware solution to determine which intersected primitives can be omitted from the results provided to the SM  132  without a functional impact on visualization of the resulting scene. This allows for the TTU  700  to continue traversal of the BVH without intervention of a shader implemented in the SM with or without other associated hardware. For example, the TTU  700  may be configured to omit those primitives which are positioned behind an opaque primitive relative to a ray origin because, as discussed with reference to  FIGS.  5 A- 5 C , a ray in a scene will be blocked by an opaque primitive and will not reach other primitives even through the ray spatially intersects with them. 
     To handle the different primitive hit types, the Intersection Management Unit  722  of the TTU  700  may maintain a resultQueue data structure which contains storage for each ray for one opaque primitive intersection, or one alpha primitive intersection, and zero or more additional entries for additional alpha intersections. The resultQueue can be returned to the SM  132  when a query completes or in mid-traversal if the need should arise for SM intervention (for example, if the number of alpha hits found within a single triangle range exceeds the storage capacity for alphaHits of the resultQueue). 
       FIG.  11    is a flowchart of an example non-limiting method for accelerated ray-primitives intersection test. The method may be performed by a TTU  700  disclosed in this application, but is not so limited. 
     The method includes receiving a request for a nearest intersection between a query data structure and a primitive range  1110 . The request may be received from an SM or may be the result of ray-complet test performed by the ray-complet test path of the TTU. In some embodiments the query data structure may be a ray given by its three-coordinate origin, three-coordinate direction, and minimum and maximum values for the t-parameter along the ray. The primitive range for the request may be identified in one or more stack entries of a stack management block in the TTU. The stack entries may be made based on results of finding intersected leaf nodes of a BVH in the ray-complet test path of the TTU. 
     The primitive range is retrieved from memory  1112 . The primitive range may be retrieved from the TTU memory (e.g., triangle cache  754  of L0 cache  750 ) or memory outside of the TTU. The primitive range may for example be provided in a contiguous group of cacheline-sized blocks. Each cacheline-sized block may include a header identifying the type of geometry expressed within the block and a primitive type of each primitive in the block. For example, the header may identify that the block includes triangles and indicate whether each triangle is an alpha primitive or an opaque primitive. The header may include an alpha bit for each primitive to designate whether the respective primitive is an alpha primitive or an opaque primitive. 
     The method includes testing primitives in the primitive range for intersection with the ray  1114 . Testing the primitives may include determining whether the ray intersects an area identified by the primitive (e.g., by vertices of a triangle in object-space coordinates). Each primitive in the primitive range may be tested for an intersection with the ray. To test the ray against the area identified by the vertices, the ray-triangle test path of the TTU  700  may transform the ray into the object-space coordinates of the primitive range using instance transforms. 
     The method includes omitting intersected primitives which can be determined to not have a functional impact on visualizing a resulting scene  1116 . In one embodiment, an Intersection Management Unit  722  (see  FIG.  9   ) may determine which spatially intersected primitives can be omitted from the results provided to the SM  132 . The TTU  700  may determine that one or more spatially intersected primitives in the primitive range are occluded along the ray by another intersecting opaque triangle that is closer to the ray origin. Because a closer opaque primitive in a scene will block the ray from passing through the primitive, the spatially intersected primitives occluded with respect to the ray origin by the opaque triangle do not need to be included in the result of intersected primitives and can be removed from the results to be reported to the SM  132  if they were previously added. 
     With reference to  FIG.  5 A , when the TTU determines that the ray intersects a front primitive which is opaque, the TTU can omit spatially intersected primitives behind the opaque primitive from the results. In  FIG.  5 B , the front spatially intersected primitive is an alpha primitive which may allow for the ray to pass through and hit other primitives. Primitives behind an alpha primitive should not be omitted from the reported results because the ray may, depending on location and or type of texture, pass though the alpha primitive and hit other primitives behind the alpha primitive. In some examples, the TTU may not able to determine by itself (e.g., without intervention of the SM) whether a spatial intersection alpha triangle will block a ray or allow the ray to pass to other primitives. 
     Accordingly, when the TTU determines that the ray spatially intersects an alpha primitive, the TTU adds the intersected alpha primitive to the results and continues the ray-primitive intersection test to determine any other spatially intersected primitives in the primitive range, which may be behind or in front of the alpha primitive with reference to the direction of the ray. With reference to  FIG.  5 C , because the first two spatially intersected primitives are alpha primitives, both of the alpha primitives will be added to the result together with the opaque primitive. 
     In one embodiment, the TTU may omit primitives behind a spatially intersected opaque primitive only if all of the intersected primitives in the range are opaque. In another embodiment, the TTU may ignore primitives behind a spatially intersected opaque primitive regardless of whether other spatially intersected primitives are opaque or alpha primitives. In this example, the spatially intersected primitives which are excluded from the results may include opaque and/or alpha primitives. 
     The spatially intersected primitives which could have a functional impact on a resulting scene may be stored in a result queue (e.g., in the Intersection Management Unit  722 ). The result queue provides a data structure within the TTU co-processor  700  for storing one or more opaque or alpha primitive intersections found during traversal. 
     In one embodiment, the operation of the TTU co-processor  700  and the result queue allows the SM to defer alpha tests and potentially avoid them altogether if a closer opaque intersection occludes an alpha intersection. Testing of some primitives can be eliminated by shortening the ray. However, when a primitive range is tested for intersections, all the primitives in the range may be tested by the TTU co-processor  700 . When a TTU result includes an alpha intersection with other remaining primitives, the remaining primitives are retested when the request is launched. This configuration of the SM and the TTU co-processor  700  allows for the alpha tests to be deferred from the SM perspective. 
       FIG.  12    illustrates a result queue  1200  according to an exemplary embodiment of this disclosure. The result queue  1200  may include an entry  1210  for one opaque primitive intersection or one alpha primitive intersection. The result queue  1200  may optionally include one or more additional entries  1220 ,  1230  for additional alpha intersections. 
     In one exemplary embodiment, each result queue entry specifies a hit type and a parametric length which specifies the point along the ray where the hit occurred and attributes of the hit such as the instance ID or material ID which can be used by the SM to select a specific material shader and a set of resource bindings, and primitive IDs and (u,v) coordinates which can be used by the SM during shading to retrieve and interpolate attributes or sample textures. 
     In other embodiments, the result queue may include separate queues for opaque primitives and alpha primitives. For example, the result queue may include a single entry for an opaque primitive intersection and one or more entries for alpha primitive intersections. 
     The method includes returning the results of intersected primitives to the SM  1118 . The results may be returned to the SM when a request completes or in mid-traversal if the need should arise for SM intervention (for example, if the number of alpha hits found within a single triangle range exceeds the storage capacity for alphaHits of the result queue). If all of the primitives in the request are opaque, then the TTU can return only the closest opaque primitive to the SM. However, if the primitives include a mixture of opaque and alpha primitives, a plurality of alpha primitives may be intersected by the ray. Intersection information for each of the intersected alpha primitives may be returned to the SM for further processing (e.g., to determine if there is a hit based on texture information associated with the alpha primitive). 
     The SM may use the results to build the scene, issue additional queries to the TTU, and/or modify previously issued queries to the TTU. In one embodiment, spatially intersected primitives with the alpha bit set are returned to SM even if traversal has not completed. The spatially intersected primitives may be returned to the SM with traversal stack information which contains the state required to continue traversal if software determines that the triangle was in fact transparent. In one embodiment, the TTU may return the results to the SM after all of the primitives in the primitive range have been tested. In this embodiment, the size of the result queue needs to be sufficient to store all of the results. 
     In practice, due to physical constraints, the result queue size may be limited to a single entry or few entries. For example, the result queue may include a single entry for the opaque intersection or the alpha intersection. In another example, the result queue may include a single entry for the opaque intersection and a plurality of entries (e.g., four entries) for the alpha intersections. When an intersection which cannot be discarded is identified and the result queue is full, the TTU may return the results in the result queue to the SM with information for the SM to be able to resubmit the ray test request after processing one or more of the primitives identified in the results. For example, SM may resubmit the ray test request which skips over one or more primitives that have already been returned to the SM. Accordingly, if multiple alpha primitives are intersected in a ray, intersection information for each intersected alpha primitive may be returned one by one to the SM. 
     In another example, when an intersection which cannot be omitted is identified and the result queue is full, the IMU may replace one of the entries in the queue with the new intersection. This may be done when deterministic results should be reported to the SM and the results of the intersections need to be returned in the order the intersections are found in memory address order of the primitive range. 
     Example Ray Continued Bounding Volume Hierarchy Traversal without Shader Intervention 
     In a more specific example, the TTU operates by tracing rays through a generated BVH which includes internal nodes, instance transforms, item ranges, and triangle ranges. Internal node intersections traverse further into the hierarchy. Instance node intersection can do a hardware accelerated instance transform and then continue traversal on the instanced child BVH. Item ranges are returned to the SM. Triangle ranges can use specific hardware acceleration inside the TTU in the Ray Triangle Test (RTT) block to determine an intersection between a ray and a specific triangle. In general a query is concerned about either the closest interaction to the ray origin or simply that there was an intersection. 
     The TTU has two types of triangles that it intersects: opaque and alpha. The type of triangle can be designated by a flag (e.g., an alpha triangle flag). An opaque triangle is one for which the intersection can be fully determined by the mathematical intersection of a ray and that triangle. This intersection can be determined in the ray-triangle test path of the TTU. An alpha triangle, on the other hand, is one for which the intersection cannot be fully determined by the mathematical intersection of a ray and that triangle. For alpha triangles, the ray-triangle test path of the TTU determines that there could be an intersection and the information about the possible intersection is sent to the SM to determine the presence of absence of an actual intersection. The SM can perform software based processing to determine specifics of the ray and alpha triangle intersection and modify the ray-triangle query based on this determination. An example use of an alpha triangle is in foliage where a leaf may be represented by a single triangle to which a texture is applied to define a tighter bound around the actual leaf shape. This concept is illustrated in  FIGS.  6 A and  6 B  where a texture  615  of a leaf is applied to a portion of the triangle  610 . The alpha triangle flag can also be applied to otherwise opaque triangles so that the TTU returns every triangle intersected by the ray. In one example, a flag can be set when the query is made to treat every triangle as if it is translucent to get back everything that is intersected and not just the closest. 
     In example non-limiting embodiments, alpha triangles need further processing by the SM to determine an actual intersection. Opaque triangles do not. The TTU is provided with the Intersection Management Unit (IMU) which allows the TTU to not have to return all intersected triangles to the SM. Reducing those returns is desirable since a return to the SM requires a warp-wide synchronization. 
     To handle the different primitive hit types, the IMU of the TTU maintains a resultQueue data structure which contains storage for each ray for one opaque primitive intersection, or one alpha primitive intersection, and zero or more additional entries for additional alpha intersections. The resultQueue can be returned to the SM when a query completes or in mid-traversal if the need should arise for SM intervention (for example, if the number of alpha hits found within a single triangle range exceeds the storage capacity for alphaHits of the resultQueue). 
     The TTU may present a contract that is partially implemented in the IMU:
         1. The TTU is allowed to skip anything that is not intersected by the ray.   2. The TTU is allowed to skip anything that lies behind an opaque triangle when the triangle is hardware intersected.   3. Conversely, the TTU must report a hit for anything intersected by the ray that does not lie behind a hardware-intersected opaque triangle.   4. If the ray intersects only opaque triangles, the TTU reports the nearest hit and nothing else.   5. If the ray intersects primitives other than (or in addition to) opaque triangles, there may be multiple hits, which the TTU is allowed to report in any order.   6. For primitives that are not hardware-intersected (i.e., non-triangles), the TTU must report a hit for anything that is intersected by the ray and may report a hit for anything that might be intersected by the ray. In other words, false positives are allowed.   7. The application is allowed to shorten the ray when continuing traversal with a subsequent query, but the ray must otherwise remain unchanged.       

       FIG.  13    is a flowchart of an example non-limiting method for accelerated ray-triangle intersection test. The method may be performed by a TTU  700  disclosed in this application, but is not so limited. 
     The TTU  700  may be configured to handle a certain geometry type primitives in the ray-primitive test path of the TTU  700 . In some implementations, other primitive types may not be supported in the ray-primitive path of the TTU  700 . Primitive types not supported in the ray-primitive path of the TTU  700  may be returned to the SM for processing (e.g., after being identified in the ray-complet test path of the TTU). The flowchart in  FIG.  13    is explained with reference to a TTU that is configured to handle triangles in the ray-primitive test path, but is not so limited. The ray-primitive test path of the TTU may be configured to determine intersections for other primitives, and may be configured to determine intersections for a plurality of different primitives. In some embodiments, the TTU may include a plurality of ray-primitive test paths each configured to handle a different primitive. 
     The flowchart illustrated in  FIG.  13    includes the TTU  700  receiving a query requesting a nearest intersection between a ray and a triangle range (step  1310 ). The query may be received by the ray management block  730  from an SM  132  or may be initiated based on the result of ray-complet test performed by the ray-complet test path of the TTU. The query may request a closest hit intersection or any intersection. 
     The ray may be identified by its three-coordinate origin, three-coordinate direction, and minimum and maximum values for the t-parameter along the ray. The triangle range for the query may be identified in one or more stack entries of a stack management block  740  in the TTU. In one embodiment, the ray-complet test may identify one or more intersected leaf nodes of a tree which point to a range of triangles or items. The triangle range can be a set of consecutive triangles that start at the n-th triangle in a block of memory and run for zero or more blocks until the m-th triangle of the last block. A block may be a cache line (e.g.,  128 B). The triangles may be compressed in the cache lines, with a varying number of triangles in each cache line. A range of triangles could span over a varying number of cache lines. 
     The triangle range is retrieved from memory (step  1312 ). The triangle range may be retrieved from the TTU memory (e.g., triangle cache  754  of L0 cache  750 ) or memory  140  outside of the TTU  700 . The triangle range may be provided in a contiguous group of cacheline-sized block. Each cacheline-sized block may include a header identifying the type of geometry expressed within the block and a primitive type of each primitive in the block. In one example, the header may identify the type of geometry expressed within the block and the encoding scheme (for example, compressed triangles), the number of primitives in the block (minus one), and a set of ForceNoCull bits which indicate whether culling should be disabled for each primitive in the block, and a set of Alpha bits which indicate for each primitive in the block whether the primitive is an Alpha primitive or an Opaque primitive. 
     The method includes testing a triangle in the triangle range for intersection with the ray (step  1314 ). Because the block with the triangle range can encode multiple opaque or alpha primitives in any order, and the triangle range can span multiple blocks returned in arbitrary order by the memory subsystem of the TTU  700 , the first triangle tested may not be the closest triangle to the ray origin. The ray-triangle test and transform block  720  may test the triangle for intersection with the ray. If the ray-triangle test and transform block  720  determines an intersection, the ray-triangle test and transform block  720  may pass information about the intersection to the IMU  722 . Testing the triangle for intersection may include determining whether the ray intersects an area identified by vertices of the triangle. The ray-triangle test and transform block  720  may transform the ray into object-space coordinates of the triangle before performing the intersection test. 
     If there is no intersection between the triangle and the ray (No in step  1316 ), then the triangle can be omitted from the results and a determination is made as to whether there is another triangle in the range to be tested (step  1318 ). If there is another triangle to be tested in the triangle range (Yes in step  1318 ), either in the same block or another block associated with the triangle range, then the triangle-ray test may be performed on the next triangle (step  1314 ). If there are no other triangles in the triangle range to be tested (No in step  1318 ), then the ray-triangle test is complete and the results of the ray-triangle test can be sent to the SM or another block inside or outside of the TTU  700  initiating the query (step  1320 ). Returning the results, in step  1320 , may provide the parametric opaque intersection in the triangle range which is closest to the ray origin. The results may provide the opaque triangle ID, along with the t-value, and coordinates of the intersection (e.g., barycentric coordinates of the intersection). If no intersections are identified in the triangle range, the result to the SM will indicate that no intersections were found. 
     If there is an intersection between the triangle and the ray (Yes in step  1316 ), then a determination is made in step  1322  whether the triangle is an opaque triangle. This determination may be performed by the IMU  722  based on the alpha bit associated with the triangle. If the intersection is with an opaque triangle (Yes in step  1322 ), then the IMU  722  may determine whether the intersected opaque triangle is closer to the ray origin than an opaque triangle stored in the result queue (step  1324 ). If the intersected opaque triangle is closer to the ray origin than a previously stored opaque triangle in the result queue (Yes in step  1324 ), then the intersected opaque triangle intersection will replace the previously stored opaque triangle intersection in the result queue (step  1326 ). Similarly, if the there is no opaque triangle stored in the result queue, then the intersected opaque triangle is the closest intersection to the ray origin and will be stored in the queue (step  1326 ). 
     Determining whether the intersected opaque triangle is closer to the ray origin than an opaque triangle stored in the result queue may include the IMU  722  comparing whether a triangle&#39;s t-value representing a parametric length along the ray (point along the ray where the intersection occurred) is smaller than a t-value representing a parametric length of the opaque triangle stored in the queue. The triangle having the smaller t-value is stored in the result queue because it is closer to the ray origin and hides other triangles from the viewer along the ray. Triangles having a larger parametric length will be culled. 
     In determining which intersection is closer (step  1324 ), two triangles could have the same parametric intersection value. “T-fighting” may exist in two triangle blocks of the same triangle range. In one embodiment, whichever triangle would go first by memory return order would win and be stored in the result queue. 
     In some exemplary embodiments, when a new opaque triangle intersection is stored in the result queue (step  1326 ), the length of the ray may be shortened to the parametric length of the opaque triangle&#39;s intersection. In this manner, when the ray-triangle intersection is repeated for other triangles, the TTU will not record an incoming opaque or alpha intersection in the results queue if its t-value is beyond the t-value of the opaque intersection stored in the result queue. To provide deterministic results (e.g., when two triangles could have the same t-value), the ray may only be shortened after an entire triangle range has been processed. The closest intersection test in step  1324  may be sufficient to cull further triangles without needing to shorten the ray until all of the triangles in the triangle range are tested for intersections. 
     If the intersected opaque triangle is not closer to the ray origin than a previously stored opaque triangle in the result queue (No in step  1324 ), then the intersected opaque triangle is not added to the result queue and a determination is made whether there are any additional triangles in the range (step  1318 ). If there is another triangle to be tested in the triangle range (Yes in step  1318 ) either in same block or another block associated with the triangle range, then the triangle-ray test may be performed on the next triangle (step  1314 ). If there are no other triangles in the triangle range to be tested (No in step  1318 ), then the ray-triangle test is complete and the results of the ray-triangle test can be sent to the SM (step  1320 ). 
     Accordingly, after an intersection of an opaque triangle, rather than return that triangle intersection to be recorded by the SM, the IMU unit instead stores the triangle information and the distance at which the triangle was hit. The ray may be internally shortened in the TTU such that any item or triangles beyond that opaque triangle are culled. 
     When an alpha triangle intersection is determined with a ray, the TTU should return that alpha triangle to the SM. If there are no previous intersections, the TTU returns the alpha triangle to the SM for further processing, along with information on how to continue the traversal at the next step. In one implementation, that information is contained in a stack whose entries control traversal. It is not necessary that the traversal is controlled by a stack implantation. In the case that the SM determines an actual hit for the alpha triangle based on further processing, it may shorten the length of the ray such that any items or triangles beyond that alpha triangles are culled. 
     In an implementation with a single entry result queue (and single entry return to the SM), after an intersection of an alpha triangle, it is possible that a previous opaque triangle has already been intersected. To not lose that original intersection, the IMU first returns that opaque triangle to the SM along with information on how to continue traversal at the testing of that same intersected alpha triangle. When SM continues that traversal, the first thing to happen is the alpha triangle intersection which can then be returned since there would be no previous intersection in that same query. 
     As shown in  FIG.  13   , if the intersection is with an alpha triangle (No in step  1322 ), then a determination is made as to whether there is an opaque intersection stored in the results queue (step  1330 ). If there is already an opaque intersection stored in the results queue (Yes in step  1330 ), then the result queue is returned to the SM  132  along with information on how to continue traversal. After returning the opaque triangle to the SM  132 , the TTU may receive another query from the SM  132 . The other query may modify the previously issued query so that the triangle range to be tested does not include the intersected opaque triangle that was already returned to the SM  132 . The other query may also update the length of the ray to the parametric length of the intersection, so that any triangles spatially located behind the opaque triangle are not intersected by the ray. 
     In this example, the TTU  700  returns the stored intersection to the calling thread with a request to ask again for the ray-triangle query because there are more triangles that need to be tested. The SM  132  may use the information on how to continue traversal to issue a query to the TTU  700  to continue the ray intersection test for the triangle range starting at the triangle following in memory the returned opaque triangle. In this manner, the TTU  700  will once again identify the alpha triangle intersection, but this time there will not be an opaque triangle intersection stored in the result queue and the alpha triangle can be returned to the SM  132  for further processing (e.g., to determine if there is an actual hit). 
     If there is no opaque intersection stored in the results queue (No in step  1330 ), then the alpha triangle intersection may be returned (step  1334 ) to the SM  132  along with information on how to continue traversal. In this example, the intersected triangles with the alpha bit set are returned to SM even if traversal has not completed. After returning the opaque triangle to the SM  132 , the TTU  700  may receive another query from the SM  132 . The SM  132  may issue the other query for the TTU  700  based on the SM  132  resolving (e.g., based on texture information) whether or not the ray intersects a texture applied to the alpha triangle surface and/or whether the texture will block the ray. The other query may modify the previously issued query so that the triangle range does not include the intersected alpha triangle that was previously returned to the SM  132 . If the SM  132  determines that the alpha triangle is blocked by the ray, the SM  132  may shorten the length of the ray to the parametric length of the intersection and return the other query to the TTU  700  with a modified ray length so that any items or triangles beyond that alpha triangles are culled. If the SM  132  determines that the alpha triangle was not blocked by the ray, the SM  132  may maintain the same ray parameters in the other query but modify the range of triangles to be tested. 
     In this example, the result queue may include a single entry for an opaque triangle intersection or an alpha triangle intersection (see entry  1210  in  FIG.  12   ). However, other embodiments may include one or more additional entries for additional alpha triangle intersections (see entries  1220  and  1230  in  FIG.  12   ). 
     In one example, if the IMU includes a results queue with space for multiple alpha triangles (e.g., entries  1220  and  1230 ) and a new alpha triangle intersection is identified, the new alpha triangle intersection may be stored in the queue with other previously identified alpha triangle intersections. After testing all of the triangles in the triangle range, the alpha triangles in the result queue may be returned to the SM  132  for further processing. 
     In some cases, the size of the result queue may not be sufficient to store all of the alpha triangle intersections in the triangle range being tested. In one embodiment, the triangles in the queue may be returned to the SM  132  when the alpha triangle queue is full and another intersected alpha triangle is determined. Together with the results the SM  132  is notified that other alpha triangles exist in the triangle range and that the query needs to continue testing of the triangle range. The SM  132  may issue another query to test intersections in the remaining triangles of the triangle range. 
     In other embodiments, when a new alpha triangle intersection is determined and the result queue is full, the IMU  722  may replace one of the stored alpha triangle entries in the queue with the new alpha triangle entry, if the new alpha triangle appears earlier in the memory address order than one of the alpha triangles stored in the result queue. Accordingly, the TTU  700  will test all of the triangles in the triangle range and return a set of alpha triangle intersections in memory address order regardless of whether the alpha triangles intersections are parametrically closer or further away from the ray origin. When the set of alpha triangle intersections are returned to the SM  132 , the results may be returned to the SM  132  with traversal stack information which contains the state required to continue traversal to determine any remaining alpha triangle intersections in the triangle range. 
     In one embodiment with the result queue including a single entry for alpha intersection, the IMU  722  may apply a protocol to provide deterministic results to the SM  132  independent of the memory order in which the triangles of the triangle range are provided to the TTU  700  for ray-triangle intersection testing. In this protocol, the nearest alpha bit in memory order is maintained in the result queue instead of the parametrically nearest alpha hit. This protocol may be applied when a plurality of block include triangles of the triangle range and the blocks may be provided to the TTU  700  for processing in any order. In this example, an alpha hit is stored in the alpha queue and the traversal continues to identify if any other alpha hits exist in the triangle range. The alpha queue is updated with any alpha hit occurring earlier in memory order. All of the alpha hits are identified within the TTU  700  but only the alpha hit occurring first in memory order is stored in the queue. The first alpha hit in memory order is returned to the SM  132  and the triangle range in the stack entry is modified so that the triangle that was returned to the SM  132  is not returned again. Upon continuing the traversal, the triangle range in the stack entry points to other triangles for which a next intersection in memory order is identified and provided to the SM  132 . The process is repeated until all of the alpha triangles intersecting the ray are returned to the SM  132 . The process may be repeated as long as a bit is set indicating that there are remaining alphas triangles in the triangle range to be tested. 
     The same mechanism is used in the case of intersections of bounding boxes containing item ranges whose individual intersections are not hardware-accelerated and thus must be returned to the SM. Item ranges provide a method of specifying within an acceleration data structure a group of primitives of a class of geometry which may or may not be supported by the co-processor which are encoded within a sequence of blocks of memory. 
     In the case of multiple alpha triangles intersected in the same triangle range (i.e., leaf node of the BVH), the IMU returns a pointer to the first alpha triangle intersected in that range in memory order. Within a triangle range, all triangles within a triangle range are guaranteed to be tested if they come after the starting triangle in memory order. The alpha triangles are then returned one by one with each subsequent call of the query. 
     Some implementations may use large buffers in IMU to accumulate a single opaque triangle and/or multiple alpha triangles before returning to the SM. Given the nature of opaque triangles, it is never necessary to accumulate more than one. This further reduces the number of returns. 
     Some implementations may use large buffers also to record item range intersections and continue traversal beyond the first. 
     The primary difference, and the use of a hardware solution enables this, is that we can hide those items or triangles which are provably capable of being hidden without a functional impact on the resulting scene. Previous implementations done in software need to inspect all triangles or item ranges that are intersected. 
     This invention is a key component of the Tree Traversal Unit (TTU) used for ray tracing in DirectX Raytracing (DXR) and OptiX™. The reduction in results returned to SM and synchronization steps between threads greatly improves the overall performance of traversal. 
     Example Image Generation Pipeline Including Ray Tracing 
     The ray tracing and other capabilities described above can be used in a variety of ways. For example, in addition to being used to render a scene using ray tracing, they may be implemented in combination with scan conversion techniques such as in the context of scan converting geometric building blocks (i.e., polygon primitives such as triangles) of a 3D model for generating image for display (e.g., on display  150  illustrated in  FIG.  1   ).  FIG.  14    illustrates an example flowchart for processing primitives to provide image pixel values of an image, in accordance with an embodiment. 
     As  FIG.  14    shows, an image of a 3D model may be generated in response to receiving a user input (Step  1652 ). The user input may be a request to display an image or image sequence, such as an input operation performed during interaction with an application (e.g., a game application). In response to the user input, the system performs scan conversion and rasterization of 3D model geometric primitives of a scene using conventional GPU 3D graphics pipeline (Step  1654 ). The scan conversion and rasterization of geometric primitives may include for example processing primitives of the 3D model to determine image pixel values using conventional techniques such as lighting, transforms, texture mapping, rasterization and the like as is well known to those skilled in the art and discussed below in connection with  FIG.  18   . The generated pixel data may be written to a frame buffer. 
     In step  1656 , one or more rays may be traced from one or more points on the rasterized primitives using TTU hardware acceleration. The rays may be traced in accordance with the one or more ray-tracing capabilities disclosed in this application. Based on the results of the ray tracing, the pixel values stored in the buffer may be modified (Step  1658 ). Modifying the pixel values may in some applications for example improve the image quality by, for example, applying more realistic reflections and/or shadows. An image is displayed (Step  1660 ) using the modified pixel values stored in the buffer. 
     In one example, scan conversion and rasterization of geometric primitives may be implemented using the processing system described in relation to  FIGS.  15 - 17 ,  19 ,  20 ,  21  and/or  22   , and ray tracing may be implemented by the SM&#39;s  132  using the TTU architecture described in relation to  FIG.  9   , to add further visualization features (e.g., specular reflection, shadows, etc.).  FIG.  14    is just a non-limiting example—the SM&#39;s  132  could employ the described TTU by itself without texture processing or other conventional 3D graphics processing to produce images, or the SM&#39;s could employ texture processing and other conventional 3D graphics processing without the described TTU to produce images. The SM&#39;s can also implement any desired image generation or other functionality in software depending on the application to provide any desired programmable functionality that is not bound to the hardware acceleration features provided by texture mapping hardware, tree traversal hardware or other graphics pipeline hardware. 
     Example Parallel Processing Architecture Including Ray Tracing 
     The TTU structure described above can be implemented in, or in association with, an example non-limiting parallel processing system architecture such as that described below in relation to  FIGS.  15 - 22   . Such a parallel processing architecture can be used for example to implement the GPU  130  of  FIG.  1   . 
     Example Parallel Processing Architecture 
       FIG.  15    illustrates an example non-limiting parallel processing unit (PPU)  1700 . In an embodiment, the PPU  1700  is a multi-threaded processor that is implemented on one or more integrated circuit devices. The PPU  1700  is a latency hiding architecture designed to process many threads in parallel. A thread (i.e., a thread of execution) is an instantiation of a set of instructions configured to be executed by the PPU  1700 . In an embodiment, the PPU  1700  is configured to implement a graphics rendering pipeline for processing three-dimensional (3D) graphics data in order to generate two-dimensional (2D) image data for display on a display device such as a liquid crystal display (LCD) device, an organic light emitting diode (OLED) device, a transparent light emitting diode (TOLED) device, a field emission display (FEDs), a field sequential display, a projection display, a head mounted display or any other desired display. In other embodiments, the PPU  1700  may be utilized for performing general-purpose computations. While one exemplary parallel processor is provided herein for illustrative purposes, it should be noted that such processor is set forth for illustrative purposes only, and that any processor may be employed to supplement and/or substitute for the same. 
     For example, one or more PPUs  1700  may be configured to accelerate thousands of High Performance Computing (HPC), data center, and machine learning applications. The PPU  1700  may be configured to accelerate numerous deep learning systems and applications including autonomous vehicle platforms, deep learning, high-accuracy speech, image, and text recognition systems, intelligent video analytics, molecular simulations, drug discovery, disease diagnosis, weather forecasting, big data analytics, astronomy, molecular dynamics simulation, financial modeling, robotics, factory automation, real-time language translation, online search optimizations, and personalized user recommendations, and the like. 
     The PPU  1700  may be included in a desktop computer, a laptop computer, a tablet computer, servers, supercomputers, a smart-phone (e.g., a wireless, hand-held device), personal digital assistant (PDA), a digital camera, a vehicle, a head mounted display, a hand-held electronic device, and the like. In an embodiment, the PPU  1700  is embodied on a single semiconductor substrate. In another embodiment, the PPU  1700  is included in a system-on-a-chip (SoC) along with one or more other devices such as additional PPUs  1700 , the memory  1704 , a reduced instruction set computer (RISC) CPU, a memory management unit (MMU), a digital-to-analog converter (DAC), and the like. 
     In an embodiment, the PPU  1700  may be included on a graphics card that includes one or more memory devices  1704 . The graphics card may be configured to interface with a PCIe slot on a motherboard of a desktop computer. In yet another embodiment, the PPU  1700  may be an integrated graphics processing unit (iGPU) or parallel processor included in the chipset of the motherboard. 
     As shown in  FIG.  15   , the PPU  1700  includes an Input/Output (I/O) unit  1705 , a front end unit  1715 , a scheduler unit  1720 , a work distribution unit  1725 , a hub  1730 , a crossbar (Xbar)  1770 , one or more general processing clusters (GPCs)  1750 , and one or more partition units  1780 . The PPU  1700  may be connected to a host processor or other PPUs  1700  via one or more high-speed NVLink  1710  interconnect. The PPU  1700  may be connected to a host processor or other peripheral devices via an interconnect  1702 . The PPU  1700  may also be connected to a local memory comprising a number of memory devices  1704 . In an embodiment, the local memory may comprise a number of dynamic random access memory (DRAM) devices. The DRAM devices may be configured as a high-bandwidth memory (HBM) subsystem, with multiple DRAM dies stacked within each device. 
     The NVLink  1710  interconnect enables systems to scale and include one or more PPUs  1700  combined with one or more CPUs, supports cache coherence between the PPUs  1700  and CPUs, and CPU mastering. Data and/or commands may be transmitted by the NVLink  1710  through the hub  1730  to/from other units of the PPU  1700  such as one or more copy engines, a video encoder, a video decoder, a power management unit, etc. (not explicitly shown). The NVLink  1710  is described in more detail in conjunction with  FIG.  21   . 
     The I/O unit  1705  is configured to transmit and receive communications (i.e., commands, data, etc.) from a host processor (not shown) over the interconnect  1702 . The I/O unit  1705  may communicate with the host processor directly via the interconnect  1702  or through one or more intermediate devices such as a memory bridge. In an embodiment, the I/O unit  1705  may communicate with one or more other processors, such as one or more of the PPUs  1700  via the interconnect  1702 . In an embodiment, the I/O unit  1705  implements a Peripheral Component Interconnect Express (PCIe) interface for communications over a PCIe bus and the interconnect  1702  is a PCIe bus. In alternative embodiments, the I/O unit  1705  may implement other types of well-known interfaces for communicating with external devices. 
     The I/O unit  1705  decodes packets received via the interconnect  1702 . In an embodiment, the packets represent commands configured to cause the PPU  1700  to perform various operations. The I/O unit  1705  transmits the decoded commands to various other units of the PPU  1700  as the commands may specify. For example, some commands may be transmitted to the front end unit  1715 . Other commands may be transmitted to the hub  1730  or other units of the PPU  1700  such as one or more copy engines, a video encoder, a video decoder, a power management unit, etc. (not explicitly shown). In other words, the I/O unit  1705  is configured to route communications between and among the various logical units of the PPU  1700 . 
     In an embodiment, a program executed by the host processor encodes a command stream in a buffer that provides workloads to the PPU  1700  for processing. A workload may comprise several instructions and data to be processed by those instructions. The buffer is a region in a memory that is accessible (i.e., read/write) by both the host processor and the PPU  1700 . For example, the I/O unit  1705  may be configured to access the buffer in a system memory connected to the interconnect  1702  via memory requests transmitted over the interconnect  1702 . In an embodiment, the host processor writes the command stream to the buffer and then transmits a pointer to the start of the command stream to the PPU  1700 . The front end unit  1715  receives pointers to one or more command streams. The front end unit  1715  manages the one or more streams, reading commands from the streams and forwarding commands to the various units of the PPU  1700 . 
     The front end unit  1715  is coupled to a scheduler unit  1720  that configures the various GPCs  1750  to process tasks defined by the one or more streams. The scheduler unit  1720  is configured to track state information related to the various tasks managed by the scheduler unit  1720 . The state may indicate which GPC  1750  a task is assigned to, whether the task is active or inactive, a priority level associated with the task, and so forth. The scheduler unit  1720  manages the execution of a plurality of tasks on the one or more GPCs  1750 . 
     The scheduler unit  1720  is coupled to a work distribution unit  1725  that is configured to dispatch tasks for execution on the GPCs  1750 . The work distribution unit  1725  may track a number of scheduled tasks received from the scheduler unit  1720 . In an embodiment, the work distribution unit  1725  manages a pending task pool and an active task pool for each of the GPCs  1750 . The pending task pool may comprise a number of slots (e.g., 32 slots) that contain tasks assigned to be processed by a particular GPC  1750 . The active task pool may comprise a number of slots (e.g., 4 slots) for tasks that are actively being processed by the GPCs  1750 . As a GPC  1750  finishes the execution of a task, that task is evicted from the active task pool for the GPC  1750  and one of the other tasks from the pending task pool is selected and scheduled for execution on the GPC  1750 . If an active task has been idle on the GPC  1750 , such as while waiting for a data dependency to be resolved, then the active task may be evicted from the GPC  1750  and returned to the pending task pool while another task in the pending task pool is selected and scheduled for execution on the GPC  1750 . 
     The work distribution unit  1725  communicates with the one or more GPCs  1750  via XBar  1770 . The XBar  1770  is an interconnect network that couples many of the units of the PPU  1700  to other units of the PPU  1700 . For example, the XBar  1770  may be configured to couple the work distribution unit  1725  to a particular GPC  1750 . Although not shown explicitly, one or more other units of the PPU  1700  may also be connected to the XBar  1770  via the hub  1730 . 
     The tasks are managed by the scheduler unit  1720  and dispatched to a GPC  1750  by the work distribution unit  1725 . The GPC  1750  is configured to process the task and generate results. The results may be consumed by other tasks within the GPC  1750 , routed to a different GPC  1750  via the XBar  1770 , or stored in the memory  1704 . The results can be written to the memory  1704  via the partition units  1780 , which implement a memory interface for reading and writing data to/from the memory  1704 . The results can be transmitted to another PPU  1704  or CPU via the NVLink  1710 . In an embodiment, the PPU  1700  includes a number U of partition units  1780  that is equal to the number of separate and distinct memory devices  1704  coupled to the PPU  1700 . A partition unit  1780  will be described in more detail below in conjunction with  FIG.  16   . 
     In an embodiment, a host processor (e.g., processor  120  of  FIG.  1   ) executes a driver kernel that implements an application programming interface (API) that enables one or more applications executing on the host processor to schedule operations for execution on the PPU  1700 . In an embodiment, multiple compute applications are simultaneously executed by the PPU  1700  and the PPU  1700  provides isolation, quality of service (QoS), and independent address spaces for the multiple compute applications. An application may generate instructions (i.e., API calls) that cause the driver kernel to generate one or more tasks for execution by the PPU  1700 . The driver kernel outputs tasks to one or more streams being processed by the PPU  1700 . Each task may comprise one or more groups of related threads, referred to herein as a warp. In an embodiment, a warp comprises 32 related threads that may be executed in parallel. Cooperating threads may refer to a plurality of threads including instructions to perform the task and that may exchange data through shared memory. Threads and cooperating threads are described in more detail in conjunction with  FIG.  19   . 
     Example Memory Partition Unit 
     The MMU  1890  provides an interface between the GPC  1750  and the partition unit  1780 . The MMU  1890  may provide translation of virtual addresses into physical addresses, memory protection, and arbitration of memory requests. In an embodiment, the MMU  1890  provides one or more translation lookaside buffers (TLBs) for performing translation of virtual addresses into physical addresses in the memory  1704 . 
       FIG.  16    illustrates a memory partition unit  1780  of the PPU  1700  of  FIG.  15   , in accordance with an embodiment. As shown in  FIG.  16   , the memory partition unit  1780  includes a Raster Operations (ROP) unit  1850 , a level two (L2) cache  1860 , and a memory interface  1870 . The memory interface  1870  is coupled to the memory  1704 . Memory interface  1870  may implement 32, 64, 128, 1024-bit data buses, or the like, for high-speed data transfer. In an embodiment, the PPU  1700  incorporates U memory interfaces  1870 , one memory interface  1870  per pair of partition units  1780 , where each pair of partition units  1780  is connected to a corresponding memory device  1704 . For example, PPU  1700  may be connected to up to Y memory devices  1704 , such as high bandwidth memory stacks or graphics double-data-rate, version 5, synchronous dynamic random access memory, or other types of persistent storage. 
     In an embodiment, the memory interface  1870  implements an HBM2 memory interface and Y equals half U. In an embodiment, the HBM2 memory stacks are located on the same physical package as the PPU  1700 , providing substantial power and area savings compared with conventional GDDR5 SDRAM systems. In an embodiment, each HBM2 stack includes four memory dies and Y equals 4, with HBM2 stack including two 128-bit channels per die for a total of 8 channels and a data bus width of 1024 bits. 
     In an embodiment, the memory  1704  supports Single-Error Correcting Double-Error Detecting (SECDED) Error Correction Code (ECC) to protect data. ECC provides higher reliability for compute applications that are sensitive to data corruption. Reliability is especially important in large-scale cluster computing environments where PPUs  1700  process very large datasets and/or run applications for extended periods. 
     In an embodiment, the PPU  1700  implements a multi-level memory hierarchy. In an embodiment, the memory partition unit  1780  supports a unified memory to provide a single unified virtual address space for CPU and PPU  1700  memory, enabling data sharing between virtual memory systems. In an embodiment the frequency of accesses by a PPU  1700  to memory located on other processors is traced to ensure that memory pages are moved to the physical memory of the PPU  1700  that is accessing the pages more frequently. In an embodiment, the NVLink  1710  supports address translation services allowing the PPU  1700  to directly access a CPU&#39;s page tables and providing full access to CPU memory by the PPU  1700 . 
     In an embodiment, copy engines transfer data between multiple PPUs  1700  or between PPUs  1700  and CPUs. The copy engines can generate page faults for addresses that are not mapped into the page tables. The memory partition unit  1780  can then service the page faults, mapping the addresses into the page table, after which the copy engine can perform the transfer. In a conventional system, memory is pinned (i.e., non-pageable) for multiple copy engine operations between multiple processors, substantially reducing the available memory. With hardware page faulting, addresses can be passed to the copy engines without worrying if the memory pages are resident, and the copy process is transparent. 
     Data from the memory  1704  or other system memory may be fetched by the memory partition unit  1780  and stored in the L2 cache  1860 , which is located on-chip and is shared between the various GPCs  1750 . As shown, each memory partition unit  1780  includes a portion of the L2 cache  1860  associated with a corresponding memory device  1704 . Lower level caches may then be implemented in various units within the GPCs  1750 . For example, each of the SMs  1840  may implement a level one (L1) cache. The L1 cache is private memory that is dedicated to a particular SM  1840 . Data from the L2 cache  1860  may be fetched and stored in each of the L1 caches for processing in the functional units of the SMs  1840 . The L2 cache  1860  is coupled to the memory interface  1870  and the XBar  1770 . 
     The ROP unit  1850  performs graphics raster operations related to pixel color, such as color compression, pixel blending, and the like. The ROP unit  1850  also implements depth testing in conjunction with the raster engine  1825 , receiving a depth for a sample location associated with a pixel fragment from the culling engine of the raster engine  1825 . The depth is tested against a corresponding depth in a depth buffer for a sample location associated with the fragment. If the fragment passes the depth test for the sample location, then the ROP unit  1850  updates the depth buffer and transmits a result of the depth test to the raster engine  1825 . It will be appreciated that the number of partition units  1780  may be different than the number of GPCs  1750  and, therefore, each ROP unit  1850  may be coupled to each of the GPCs  1750 . The ROP unit  1850  tracks packets received from the different GPCs  1750  and determines which GPC  1750  that a result generated by the ROP unit  1850  is routed to through the Xbar  1770 . Although the ROP unit  1850  is included within the memory partition unit  1780  in  FIG.  16   , in other embodiment, the ROP unit  1850  may be outside of the memory partition unit  1780 . For example, the ROP unit  1850  may reside in the GPC  1750  or another unit. 
     Example General Processing Clusters 
       FIG.  17    illustrates a GPC  1750  of the PPU  1700  of  FIG.  15   , in accordance with an embodiment. As shown in  FIG.  17   , each GPC  1750  includes a number of hardware units for processing tasks. In an embodiment, each GPC  1750  includes a pipeline manager  1810 , a pre-raster operations unit (PROP)  1815 , a raster engine  1825 , a work distribution crossbar (WDX)  1880 , a memory management unit (MMU)  1890 , and one or more Data Processing Clusters (DPCs)  1820 . It will be appreciated that the GPC  1750  of  FIG.  17    may include other hardware units in lieu of or in addition to the units shown in  FIG.  17   . 
     In an embodiment, the operation of the GPC  1750  is controlled by the pipeline manager  1810 . The pipeline manager  1810  manages the configuration of the one or more DPCs  1820  for processing tasks allocated to the GPC  1750 . In an embodiment, the pipeline manager  1810  may configure at least one of the one or more DPCs  1820  to implement at least a portion of a graphics rendering pipeline. 
     Each DPC  1820  included in the GPC  1750  includes an M-Pipe Controller (MPC)  1830 , a primitive engine  1835 , one or more SMs  1840 , one or more Texture Units  1842 , and one or more TTUs  700 . The SM  1840  may be structured similarly to SM  132  described above. The MPC  1830  controls the operation of the DPC  1820 , routing packets received from the pipeline manager  1810  to the appropriate units in the DPC  1820 . For example, packets associated with a vertex may be routed to the primitive engine  1835 , which is configured to fetch vertex attributes associated with the vertex from the memory  1704 . In contrast, packets associated with a shader program may be transmitted to the SM  1840 . 
     When configured for general purpose parallel computation, a simpler configuration can be used compared with graphics processing. Specifically, the fixed function graphics processing units shown in  FIG.  15   , are bypassed, creating a much simpler programming model. In the general purpose parallel computation configuration, the work distribution unit  1725  assigns and distributes blocks of threads directly to the DPCs  1820 . The threads in a block execute the same program, using a unique thread ID in the calculation to ensure each thread generates unique results, using the SM  1840  to execute the program and perform calculations, shared memory/L1 cache  1970  to communicate between threads, and the LSU  1954  to read and write global memory through the shared memory/L1 cache  1970  and the memory partition unit  1780 . When configured for general purpose parallel computation, the SM  1840  can also write commands that the scheduler unit  1720  can use to launch new work on the DPCs  1820 . The TTU  700  can be used to accelerate spatial queries in the context of general purpose computation. 
     Graphics Rendering Pipeline 
     A DPC  1820  may be configured to execute a vertex shader program on the programmable streaming multiprocessor (SM)  1840  which may accelerate certain shading operations with TTU  700 . The pipeline manager  1810  may also be configured to route packets received from the work distribution unit  1725  to the appropriate logical units within the GPC  1750 . For example, some packets may be routed to fixed function hardware units in the PROP  1815  and/or raster engine  1825  while other packets may be routed to the DPCs  1820  for processing by the primitive engine  1835  or the SM  1840 . In an embodiment, the pipeline manager  1810  may configure at least one of the one or more DPCs  1820  to implement a neural network model and/or a computing pipeline. 
     The PROP unit  1815  is configured to route data generated by the raster engine  1825  and the DPCs  1820  to a Raster Operations (ROP) unit, described in more detail in conjunction with  FIG.  16   . The PROP unit  1815  may also be configured to perform optimizations for color blending, organize pixel data, perform address translations, and the like. 
     The raster engine  1825  includes a number of fixed function hardware units configured to perform various raster operations. In an embodiment, the raster engine  1825  includes a setup engine, a coarse raster engine, a culling engine, a clipping engine, a fine raster engine, and a tile coalescing engine. The setup engine receives transformed vertices and generates plane equations associated with the geometric primitive defined by the vertices. The plane equations are transmitted to the coarse raster engine to generate coverage information (e.g., an x,y coverage mask for a tile) for the primitive. The output of the coarse raster engine is transmitted to the culling engine where fragments associated with the primitive that fail a z-test are culled, and non-culled fragments are transmitted to a clipping engine where fragments lying outside a viewing frustum are clipped. Those fragments that survive clipping and culling may be passed to the fine raster engine to generate attributes for the pixel fragments based on the plane equations generated by the setup engine. The output of the raster engine  1825  comprises fragments to be processed, for example, by a fragment shader implemented within a DPC  1820   
     In more detail, the PPU  1700  is configured to receive commands that specify shader programs for processing graphics data. Graphics data may be defined as a set of primitives such as points, lines, triangles, quads, triangle strips, and the like. Typically, a primitive includes data that specifies a number of vertices for the primitive (e.g., in a model-space coordinate system) as well as attributes associated with each vertex of the primitive. The PPU  1700  can be configured to process the graphics primitives to generate a frame buffer (i.e., pixel data for each of the pixels of the display) using for example TTU  700  as a hardware acceleration resource. 
     An application writes model data for a scene (i.e., a collection of vertices and attributes) to a memory such as a system memory or memory  1704 . The model data defines each of the objects that may be visible on a display. The model data may also define one or more BVH&#39;s as described above. The application then makes an API call to the driver kernel that requests the model data to be rendered and displayed. The driver kernel reads the model data and writes commands to the one or more streams to perform operations to process the model data. The commands may reference different shader programs to be implemented on the SMs  1840  of the PPU  1700  including one or more of a vertex shader, hull shader, domain shader, geometry shader, a pixel shader, a ray generation shader, a ray intersection shader, a ray hit shader, and a ray miss shader (these correspond to the shaders defined by the DirectX Raytracing (DXR) API, ignoring any distinction between “closest-hit” and “any-hit” shaders; see https://devblogs.nvidia.com/introduction-nvidia-rtx-directx-ray-tracing/). For example, one or more of the SMs  1840  may be configured to execute a vertex shader program that processes a number of vertices defined by the model data. In an embodiment, the different SMs  1840  may be configured to execute different shader programs concurrently. For example, a first subset of SMs  1840  may be configured to execute a vertex shader program while a second subset of SMs  1840  may be configured to execute a pixel shader program. The first subset of SMs  1840  processes vertex data to produce processed vertex data and writes the processed vertex data to the L2 cache  1860  and/or the memory  1704  (see  FIG.  16   ). After the processed vertex data is rasterized (i.e., transformed from three-dimensional data into two-dimensional data in screen space) to produce fragment data, the second subset of SMs  1840  executes a pixel shader to produce processed fragment data, which is then blended with other processed fragment data and written to the frame buffer in memory  1704 . The vertex shader program and pixel shader program may execute concurrently, processing different data from the same scene in a pipelined fashion until all of the model data for the scene has been rendered to the frame buffer. Then, the contents of the frame buffer are transmitted to a display controller for display on a display device. 
       FIG.  18    is a conceptual diagram of a graphics processing pipeline  2000  implemented by the PPU  1700  of  FIG.  15   . The graphics processing pipeline  2000  is an abstract flow diagram of the processing steps implemented to generate 2D computer-generated images from 3D geometry data. As is well-known, pipeline architectures may perform long latency operations more efficiently by splitting up the operation into a plurality of stages, where the output of each stage is coupled to the input of the next successive stage. Thus, the graphics processing pipeline  2000  receives input data  2001  that is transmitted from one stage to the next stage of the graphics processing pipeline  2000  to generate output data  2002 . In an embodiment, the graphics processing pipeline  2000  may represent a graphics processing pipeline defined by the OpenGL@ API. As an option, the graphics processing pipeline  2000  may be implemented in the context of the functionality and architecture of the previous Figures and/or any subsequent Figure(s). As discussed above with reference to  FIG.  14   , the ray tracing may be used to improve the image quality generated by rasterization of geometric primitives. In some embodiments, ray tracing operations and TTU structure disclosed in this application may be applied to one or more states of the graphics processing pipeline  2000  to improve the subjective image quality. 
     As shown in  FIG.  18   , the graphics processing pipeline  2000  comprises a pipeline architecture that includes a number of stages. The stages include, but are not limited to, a data assembly stage  2010 , a vertex shading stage  2020 , a primitive assembly stage  2030 , a geometry shading stage  2040 , a viewport scale, cull, and clip (VSCC) stage  2050 , a rasterization stage  2060 , a fragment shading stage  2070 , and a raster operations stage  2080 . In an embodiment, the input data  2001  comprises commands that configure the processing units to implement the stages of the graphics processing pipeline  2000  and geometric primitives (e.g., points, lines, triangles, quads, triangle strips or fans, etc.) to be processed by the stages. The output data  2002  may comprise pixel data (i.e., color data) that is copied into a frame buffer or other type of surface data structure in a memory. 
     The data assembly stage  2010  receives the input data  2001  that specifies vertex data for high-order surfaces, primitives, or the like. The data assembly stage  2010  collects the vertex data in a temporary storage or queue, such as by receiving a command from the host processor that includes a pointer to a buffer in memory and reading the vertex data from the buffer. The vertex data is then transmitted to the vertex shading stage  2020  for processing. 
     The vertex shading stage  2020  processes vertex data by performing a set of operations (i.e., a vertex shader or a program) once for each of the vertices. Vertices may be, e.g., specified as a 4-coordinate vector (i.e., &lt;x, y, z, w&gt;) associated with one or more vertex attributes (e.g., color, texture coordinates, surface normal, etc.). The vertex shading stage  2020  may manipulate individual vertex attributes such as position, color, texture coordinates, and the like. In other words, the vertex shading stage  2020  performs operations on the vertex coordinates or other vertex attributes associated with a vertex. Such operations commonly including lighting operations (i.e., modifying color attributes for a vertex) and transformation operations (i.e., modifying the coordinate space for a vertex). For example, vertices may be specified using coordinates in an object-coordinate space, which are transformed by multiplying the coordinates by a matrix that translates the coordinates from the object-coordinate space into a world space or a normalized-device-coordinate (NCD) space. The vertex shading stage  2020  generates transformed vertex data that is transmitted to the primitive assembly stage  2030 . 
     The primitive assembly stage  2030  collects vertices output by the vertex shading stage  2020  and groups the vertices into geometric primitives for processing by the geometry shading stage  2040 . For example, the primitive assembly stage  2030  may be configured to group every three consecutive vertices as a geometric primitive (i.e., a triangle) for transmission to the geometry shading stage  2040 . In some embodiments, specific vertices may be reused for consecutive geometric primitives (e.g., two consecutive triangles in a triangle strip may share two vertices). The primitive assembly stage  2030  transmits geometric primitives (i.e., a collection of associated vertices) to the geometry shading stage  2040 . 
     The geometry shading stage  2040  processes geometric primitives by performing a set of operations (i.e., a geometry shader or program) on the geometric primitives. Tessellation operations may generate one or more geometric primitives from each geometric primitive. In other words, the geometry shading stage  2040  may subdivide each geometric primitive into a finer mesh of two or more geometric primitives for processing by the rest of the graphics processing pipeline  2000 . The geometry shading stage  2040  transmits geometric primitives to the viewport SCC stage  2050 . 
     In an embodiment, the graphics processing pipeline  2000  may operate within a streaming multiprocessor and the vertex shading stage  2020 , the primitive assembly stage  2030 , the geometry shading stage  2040 , the fragment shading stage  2070 , a ray tracing shader, and/or hardware/software associated therewith, may sequentially perform processing operations. Once the sequential processing operations are complete, in an embodiment, the viewport SCC stage  2050  may utilize the data. In an embodiment, primitive data processed by one or more of the stages in the graphics processing pipeline  2000  may be written to a cache (e.g. L1 cache, a vertex cache, etc.). In this case, in an embodiment, the viewport SCC stage  2050  may access the data in the cache. In an embodiment, the viewport SCC stage  2050  and the rasterization stage  2060  are implemented as fixed function circuitry. 
     The viewport SCC stage  2050  performs viewport scaling, culling, and clipping of the geometric primitives. Each surface being rendered to is associated with an abstract camera position. The camera position represents a location of a viewer looking at the scene and defines a viewing frustum that encloses the objects of the scene. The viewing frustum may include a viewing plane, a rear plane, and four clipping planes. Any geometric primitive entirely outside of the viewing frustum may be culled (i.e., discarded) because the geometric primitive will not contribute to the final rendered scene. Any geometric primitive that is partially inside the viewing frustum and partially outside the viewing frustum may be clipped (i.e., transformed into a new geometric primitive that is enclosed within the viewing frustum. Furthermore, geometric primitives may each be scaled based on a depth of the viewing frustum. All potentially visible geometric primitives are then transmitted to the rasterization stage  2060 . 
     The rasterization stage  2060  converts the 3D geometric primitives into 2D fragments (e.g. capable of being utilized for display, etc.). The rasterization stage  2060  may be configured to utilize the vertices of the geometric primitives to setup a set of plane equations from which various attributes can be interpolated. The rasterization stage  2060  may also compute a coverage mask for a plurality of pixels that indicates whether one or more sample locations for the pixel intercept the geometric primitive. In an embodiment, z-testing may also be performed to determine if the geometric primitive is occluded by other geometric primitives that have already been rasterized. The rasterization stage  2060  generates fragment data (i.e., interpolated vertex attributes associated with a particular sample location for each covered pixel) that are transmitted to the fragment shading stage  2070 . 
     The fragment shading stage  2070  processes fragment data by performing a set of operations (i.e., a fragment shader or a program) on each of the fragments. The fragment shading stage  2070  may generate pixel data (i.e., color values) for the fragment such as by performing lighting operations or sampling texture maps using interpolated texture coordinates for the fragment. The fragment shading stage  2070  generates pixel data that is transmitted to the raster operations stage  2080 . 
     The raster operations stage  2080  may perform various operations on the pixel data such as performing alpha tests, stencil tests, and blending the pixel data with other pixel data corresponding to other fragments associated with the pixel. When the raster operations stage  2080  has finished processing the pixel data (i.e., the output data  2002 ), the pixel data may be written to a render target such as a frame buffer, a color buffer, or the like. 
     It will be appreciated that one or more additional stages may be included in the graphics processing pipeline  2000  in addition to or in lieu of one or more of the stages described above. Various implementations of the abstract graphics processing pipeline may implement different stages. Furthermore, one or more of the stages described above may be excluded from the graphics processing pipeline in some embodiments (such as the geometry shading stage  2040 ). Other types of graphics processing pipelines are contemplated as being within the scope of the present disclosure. Furthermore, any of the stages of the graphics processing pipeline  2000  may be implemented by one or more dedicated hardware units within a graphics processor such as PPU  200 . Other stages of the graphics processing pipeline  2000  may be implemented by programmable hardware units such as the SM  1840  of the PPU  1700 . 
     The graphics processing pipeline  2000  may be implemented via an application executed by a host processor, such as a CPU  120 . In an embodiment, a device driver may implement an application programming interface (API) that defines various functions that can be utilized by an application in order to generate graphical data for display. The device driver is a software program that includes a plurality of instructions that control the operation of the PPU  1700 . The API provides an abstraction for a programmer that lets a programmer utilize specialized graphics hardware, such as the PPU  1700 , to generate the graphical data without requiring the programmer to utilize the specific instruction set for the PPU  1700 . The application may include an API call that is routed to the device driver for the PPU  1700 . The device driver interprets the API call and performs various operations to respond to the API call. In some instances, the device driver may perform operations by executing instructions on the CPU. In other instances, the device driver may perform operations, at least in part, by launching operations on the PPU  1700  utilizing an input/output interface between the CPU and the PPU  1700 . In an embodiment, the device driver is configured to implement the graphics processing pipeline  2000  utilizing the hardware of the PPU  1700 . 
     Various programs may be executed within the PPU  1700  in order to implement the various stages of the graphics processing pipeline  2000 . For example, the device driver may launch a kernel on the PPU  1700  to perform the vertex shading stage  2020  on one SM  1840  (or multiple SMs  1840 ). The device driver (or the initial kernel executed by the PPU  1800 ) may also launch other kernels on the PPU  1800  to perform other stages of the graphics processing pipeline  2000 , such as the geometry shading stage  2040  and the fragment shading stage  2070 . In addition, some of the stages of the graphics processing pipeline  2000  may be implemented on fixed unit hardware such as a rasterizer or a data assembler implemented within the PPU  1800 . It will be appreciated that results from one kernel may be processed by one or more intervening fixed function hardware units before being processed by a subsequent kernel on an SM  1840 . 
     Example Streaming Multiprocessor 
     The SM  1840  comprises a programmable streaming processor that is configured to process tasks represented by a number of threads. Each SM  1840  is multi-threaded and configured to execute a plurality of threads (e.g., 32 threads comprising a warp) from a particular group of threads concurrently. In an embodiment, the SM  1840  implements a SIMD (Single-Instruction, Multiple-Data) architecture where each thread in a group of threads (i.e., a warp) is configured to process a different set of data based on the same set of instructions. All threads in the group of threads execute the same instructions. In another embodiment, the SM  1840  implements a SIMT (Single-Instruction, Multiple Thread) architecture where each thread in a group of threads is configured to process a different set of data based on the same set of instructions, but where individual threads in the group of threads are allowed to diverge during execution. In an embodiment, a program counter, call stack, and execution state is maintained for each warp, enabling concurrency between warps and serial execution within warps when threads within the warp diverge. In another embodiment, a program counter, call stack, and execution state is maintained for each individual thread, enabling equal concurrency between all threads, within and between warps. When execution state is maintained for each individual thread, threads executing the same instructions may be converged and executed in parallel for maximum efficiency. 
       FIG.  19    illustrates the streaming multi-processor  1840  of  FIG.  17   , in accordance with an embodiment. As shown in  FIG.  19   , the SM  1840  includes an instruction cache  1905 , one or more scheduler units  1910 , a register file  1920 , one or more processing cores  1950 , one or more special function units (SFUs)  1952 , one or more load/store units (LSUs)  1954 , an interconnect network  1980 , a shared memory/L1 cache  1970 . 
     As described above, the work distribution unit  1725  dispatches tasks for execution on the GPCs  1750  of the PPU  1700 . The tasks are allocated to a particular DPC  1820  within a GPC  1750  and, if the task is associated with a shader program, the task may be allocated to an SM  1840 . The scheduler unit  1910  receives the tasks from the work distribution unit  1725  and manages instruction scheduling for one or more thread blocks assigned to the SM  1840 . The scheduler unit  1910  schedules thread blocks for execution as warps of parallel threads, where each thread block is allocated at least one warp. In an embodiment, each warp executes 32 threads. The scheduler unit  1910  may manage a plurality of different thread blocks, allocating the warps to the different thread blocks and then dispatching instructions from the plurality of different cooperative groups to the various functional units (i.e., cores  1950 , SFUs  1952 , and LSUs  1954 ) during each clock cycle. 
     Cooperative Groups is a programming model for organizing groups of communicating threads that allows developers to express the granularity at which threads are communicating, enabling the expression of richer, more efficient parallel decompositions. Cooperative launch APIs support synchronization amongst thread blocks for the execution of parallel algorithms. Conventional programming models provide a single, simple construct for synchronizing cooperating threads: a barrier across all threads of a thread block (i.e., the syncthreads( ) function). However, programmers would often like to define groups of threads at smaller than thread block granularities and synchronize within the defined groups to enable greater performance, design flexibility, and software reuse in the form of collective group-wide function interfaces. 
     Cooperative Groups enables programmers to define groups of threads explicitly at sub-block (i.e., as small as a single thread) and multi-block granularities, and to perform collective operations such as synchronization on the threads in a cooperative group. The programming model supports clean composition across software boundaries, so that libraries and utility functions can synchronize safely within their local context without having to make assumptions about convergence. Cooperative Groups primitives enable new patterns of cooperative parallelism, including producer-consumer parallelism, opportunistic parallelism, and global synchronization across an entire grid of thread blocks. 
     A dispatch unit  1915  is configured to transmit instructions to one or more of the functional units. In the embodiment, the scheduler unit  1910  includes two dispatch units  1915  that enable two different instructions from the same warp to be dispatched during each clock cycle. In alternative embodiments, each scheduler unit  1910  may include a single dispatch unit  1915  or additional dispatch units  1915 . 
     Each SM  1840  includes a register file  1920  that provides a set of registers for the functional units of the SM  1840 . In an embodiment, the register file  1920  is divided between each of the functional units such that each functional unit is allocated a dedicated portion of the register file  1920 . In another embodiment, the register file  1920  is divided between the different warps being executed by the SM  1840 . The register file  1920  provides temporary storage for operands connected to the data paths of the functional units.  FIG.  20    illustrates an example configuration of the registers files in the SM  1840 . 
     Each SM  1840  comprises L processing cores  1950 . In an embodiment, the SM  1840  includes a large number (e.g., 128, etc.) of distinct processing cores  1950 . Each core  1950  may include a fully-pipelined, single-precision, double-precision, and/or mixed precision processing unit that includes a floating point arithmetic logic unit and an integer arithmetic logic unit. In an embodiment, the floating point arithmetic logic units implement the IEEE 754-2008 standard for floating point arithmetic. In an embodiment, the cores  1950  include 64 single-precision (32-bit) floating point cores, 64 integer cores, 32 double-precision (64-bit) floating point cores, and 8 tensor cores. 
     Tensor cores are configured to perform matrix operations, and, in an embodiment, one or more tensor cores are included in the cores  1950 . In particular, the tensor cores are configured to perform deep learning matrix arithmetic, such as convolution operations for neural network training and inferencing. In an embodiment, each tensor core operates on a 4×4 matrix and performs a matrix multiply and accumulate operation D=A×B+C, where A, B, C, and D are 4×4 matrices. 
     In an embodiment, the matrix multiply inputs A and B are 16-bit floating point matrices, while the accumulation matrices C and D may be 16-bit floating point or 32-bit floating point matrices. Tensor Cores operate on 16-bit floating point input data with 32-bit floating point accumulation. The 16-bit floating point multiply requires 64 operations and results in a full precision product that is then accumulated using 32-bit floating point addition with the other intermediate products for a 4×4×4 matrix multiply. In practice, Tensor Cores are used to perform much larger two-dimensional or higher dimensional matrix operations, built up from these smaller elements. An API, such as CUDA 9 C++ API, exposes specialized matrix load, matrix multiply and accumulate, and matrix store operations to efficiently use Tensor Cores from a CUDA-C++ program. At the CUDA level, the warp-level interface assumes 16×16 size matrices spanning all 32 threads of the warp. 
     Each SM  1840  also comprises M SFUs  1952  that perform special functions (e.g., attribute evaluation, reciprocal square root, and the like). In an embodiment, the SFUs  1952  may include a tree traversal unit configured to traverse a hierarchical tree data structure. In an embodiment, the SFUs  1952  may include texture unit configured to perform texture map filtering operations. In an embodiment, the texture units are configured to load texture maps (e.g., a 2D array of texels) from the memory  1704  and sample the texture maps to produce sampled texture values for use in shader programs executed by the SM  1840 . In an embodiment, the texture maps are stored in the shared memory/L1 cache  1970 . The texture units implement texture operations such as filtering operations using mip-maps (i.e., texture maps of varying levels of detail). In an embodiment, each SM  1740  includes two texture units. 
     Each SM  1840  also comprises N LSUs  1954  that implement load and store operations between the shared memory/L1 cache  1970  and the register file  1920 . Each SM  1840  includes an interconnect network  1980  that connects each of the functional units to the register file  1920  and the LSU  1954  to the register file  1920 , shared memory/L1 cache  1970 . In an embodiment, the interconnect network  1980  is a crossbar that can be configured to connect any of the functional units to any of the registers in the register file  1920  and connect the LSUs  1954  to the register file and memory locations in shared memory/L1 cache  1970 . 
     The shared memory/L1 cache  1970  is an array of on-chip memory that allows for data storage and communication between the SM  1840  and the primitive engine  1835  and between threads in the SM  1840 . In an embodiment, the shared memory/L1 cache  1970  comprises 128 KB of storage capacity and is in the path from the SM  1840  to the partition unit  1780 . The shared memory/L1 cache  1970  can be used to cache reads and writes. One or more of the shared memory/L1 cache  1970 , L2 cache  1860 , and memory  1704  are backing stores. 
     Combining data cache and shared memory functionality into a single memory block provides the best overall performance for both types of memory accesses. The capacity is usable as a cache by programs that do not use shared memory. For example, if shared memory is configured to use half of the capacity, texture and load/store operations can use the remaining capacity. Integration within the shared memory/L1 cache  1970  enables the shared memory/L1 cache  1970  to function as a high-throughput conduit for streaming data while simultaneously providing high-bandwidth and low-latency access to frequently reused data. 
       FIG.  20    illustrates one example architecture for the SM  1840 . As illustrated in  FIG.  17   , the SM  1840  may be coupled to one or more Texture Unit  1842  and/or one or more TTUs  700 . As a compromise between performance and area, one example non-limiting embodiment may include a single Texture Unit  1842  and/or a single TTU  700  per groups of SMs  1840  (e.g., See  FIG.  17   ). The TTU  700  may communicate with the SMs  1840  via a TTU input/output block in memory input-output and with a L1 cache via a dedicated read interface. In one example embodiment, the TTU  700  only reads from the main memory and does not write to the main memory. 
     Example More Detailed TTU Architecture 
     As discussed above, the TTU  700  may be a coprocessor to the SM  1840 . Like a texture processor, it is exposed via a set of SM instructions, accesses memory as a read-only client of the L1 cache, and returns results into the SM register file. Unlike some texture processors, the amount of data that may need to be passed into and out of the TTU  700  for a typical query makes it difficult in some embodiments to specify all the source and destination registers in a single instruction (and because most of this data is unique per-thread, there is no TTU analogue of texture headers and samplers). As a consequence, the TTU  700  in some embodiments is programmed via a multi-instruction sequence. This sequence can be conceptualized as a single “macro-instruction” in some implementations. 
     Also like a Texture Units  1842 , the TTU  700  in some implementations may rely on certain read-only data structures in memory that are prepopulated by software. These include:
         One or more BVHs, where each BVH is for example a tree of axis-aligned bounding boxes, stored in a compressed format that greatly reduces memory traffic compared to an uncompressed representation. Each node in the BVH is stored as a complet structure, with size and alignment in some implementations matched to that of an L1 cache line. Child complets of a given parent are preferably stored contiguously in memory and child pointers are stored in compressed form.   Zero or more instance nodes, which provide a way to connect a leaf of one BVH to the root of another. An instance node may be a data structure that is also aligned. This structure may contain a pointer to the sub-BVH, flags that affect back-face culling behavior in the sub-BVH, and a matrix that corresponds to the first three rows of an arbitrary transformation matrix (in homogeneous coordinates) from the coordinate system of the top-level BVH (commonly “world space”) to that of the sub-BVH (commonly “object space”). The final row of the matrix in some embodiments is in some implementations implicitly (0, 0, 0, 1).   Zero or more triangle or other primitive buffers, containing for example triangles stored either as a triplet of coordinates per vertex or in a lossless compressed format understood by the TTU  700 . In addition, an alpha bit may be provided per triangle or other primitive, indicating triangles that require special handling by software to determine whether the triangle is actually intersected by a given ray. Triangle buffers can be organized into blocks. There may also be a per-triangle force-no-cull function bit. When set, that bit indicates that both sides of the triangle should be treated as front-facing or back-facing with respect to culling, i.e., the triangle should not be culled because the ray intersects the “back” instead of the “front”. The simplest use case for this is a single triangle used to represent a leaf, where we can still see the leaf if the ray hits it on the back surface.       

     The TTU  700  in some embodiments is stateless, meaning that no architectural state is maintained in the TTU between queries. At the same time, it is often useful for software running on the SM  1840  to request continuation of a previous query, which implies that relevant state should be written to registers by the TTU  700  and then passed back to the TTU in registers (often in-place) to continue. This state may take the form of a traversal stack that tracks progress in the traversal of the BVH. 
     A small number of stack initializers may also be provided for beginning a new query of a given type, for example:
         Traversal starting from a complet   Intersection of a ray with a range of triangles   Intersection of a ray with a range of triangles, followed by traversal starting from a complet   Vertex fetch from a triangle buffer for a given triangle   Optional support for instance transforms in front of the “traversal starting from a complet” and “intersection of a ray with a range of triangles”.       

     Vertex fetch is a simple query that may be specified with request data that consists of a stack initializer and nothing else. Other query types may require the specification of a ray or beam, along with the stack or stack initializer and various ray flags describing details of the query. A ray is given by its three-coordinate origin, three-coordinate direction, and minimum and maximum values for the t-parameter along the ray. A beam is additionally given by a second origin and direction. 
     Various ray flags can be used to control various aspects of traversal behavior, back-face culling, and handling of the various child node types, subject to a pass/fail status of an optional rayOp test. RayOps add considerable flexibility to the capabilities of the TTU. In some example embodiments, the RayOps portion introduces two Ray Flag versions can be dynamically selected based on a specified operation on data conveyed with the ray and data stored in the complet. To explore such flags, it&#39;s first helpful to understand the different types of child nodes allowed within a BVH, as well as the various hit types that the TTU  700  can return to the SM. Example node types are:
         A child complet (i.e., an internal node)
 
By default, the TTU  700  continues traversal by descending into child complets.
   A triangle range, corresponding to a contiguous set of triangles within a triangle buffer   (1) By default, triangle ranges encountered by a ray are handled natively by the TTU  700  by testing the triangles for intersection and shortening the ray accordingly. If traversal completes and a triangle was hit, default behavior is for the triangle ID to be returned to SM  1840 , along with the t-value and barycentric coordinates of the intersection. This is the “Triangle” hit type.   (2) By default, intersected triangles with the alpha bit set are returned to SM  1840  even if traversal has not completed. The returned traversal stack contains the state required to continue traversal if software determines that the triangle was in fact transparent.   (3) Triangle intersection in some embodiments is not supported for beams, so encountered triangle ranges are by default returned to SM  1840  as a “TriRange” hit type, which includes a pointer to the first triangle block overlapping the range, parameters specifying the range, and the t-value of the intersection with the leaf bounding box.   An item range, consisting of an index (derived from a user-provided “item range base” stored in the complet) and a count of items.       

     By default, item ranges are returned to SM  1840  as an “ItemRange” hit type, consisting of for example an index, a count, and the t-value of the intersection with the leaf bounding box.
         An instance node.       

     The TTU  700  in some embodiments can handle one level of instancing natively by transforming the ray into the coordinate system of the instance BVH. Additional levels of instancing (or every other level of instancing, depending on strategy) may be handled in software. The “InstanceNode” hit type is provided for this purpose, consisting of a pointer to the instance node and the tvalue of the intersection with the leaf bounding box. In other implementations, the hardware can handle two, three or more levels of instancing. 
     In addition to the node-specific hit types, a generic “NodeRef” hit type is provided that consists of a pointer to the parent complet itself, as well as an ID indicating which child was intersected and the t-value of the intersection with the bounding box of that child. 
     An “Error” hit type may be provided for cases where the query or BVH was improperly formed or if traversal encountered issues during traversal. 
     A “None” hit type may be provided for the case where the ray or beam misses all geometry in the scene. 
     How the TTU handles each of the four possible node types is determined by a set of node-specific mode flags set as part of the query for a given ray. The “default” behavior mentioned above corresponds to the case where the mode flags are set to all zeroes. 
     Alternative values for the flags allow for culling all nodes of a given type, returning nodes of a given type to SM as a NodeRef hit type, or returning triangle ranges or instance nodes to SM using their corresponding hit types, rather than processing them natively within the TTU  700 . 
     Additional mode flags may be provided for control handling of alpha triangles. 
     Exemplary Computing System 
     Systems with multiple GPUs and CPUs are used in a variety of industries as developers expose and leverage more parallelism in applications such as artificial intelligence computing. High-performance GPU-accelerated systems with tens to many thousands of compute nodes are deployed in data centers, research facilities, and supercomputers to solve ever larger problems. As the number of processing devices within the high-performance systems increases, the communication and data transfer mechanisms need to scale to support the increased data transmission between the processing devices. 
       FIG.  21    is a conceptual diagram of a processing system  1900  implemented using the PPU  1700  of  FIG.  15   , in accordance with an embodiment. The exemplary system  1900  may be configured to implement one or more methods disclosed in this application. The processing system  1900  includes a CPU  1930 , switch  1912 , and multiple PPUs  1700  each and respective memories  1704 . The NVLink  1710  provides high-speed communication links between each of the PPUs  1700 . Although a particular number of NVLink  1710  and interconnect  1702  connections are illustrated in  FIG.  21   , the number of connections to each PPU  1700  and the CPU  1930  may vary. The switch  1912  interfaces between the interconnect  1702  and the CPU  1930 . The PPUs  1700 , memories  1704 , and NVLinks  1710  may be situated on a single semiconductor platform to form a parallel processing module  1925 . In an embodiment, the switch  1912  supports two or more protocols to interface between various different connections and/or links. 
     In another embodiment (not shown), the NVLink  1710  provides one or more high-speed communication links between each of the PPUs  1700  and the CPU  1930  and the switch  1912  interfaces between the interconnect  1702  and each of the PPUs  1700 . The PPUs  1700 , memories  1704 , and interconnect  1702  may be situated on a single semiconductor platform to form a parallel processing module  1925 . In yet another embodiment (not shown), the interconnect  1702  provides one or more communication links between each of the PPUs  1700  and the CPU  1930  and the switch  1912  interfaces between each of the PPUs  1700  using the NVLink  1710  to provide one or more high-speed communication links between the PPUs  1700 . In another embodiment (not shown), the NVLink  1710  provides one or more high-speed communication links between the PPUs  1700  and the CPU  1930  through the switch  1912 . In yet another embodiment (not shown), the interconnect  1702  provides one or more communication links between each of the PPUs  1700  directly. One or more of the NVLink  1710  high-speed communication links may be implemented as a physical NVLink interconnect or either an on-chip or on-die interconnect using the same protocol as the NVLink  1710 . 
     In the context of the present description, a single semiconductor platform may refer to a sole unitary semiconductor-based integrated circuit fabricated on a die or chip. It should be noted that the term single semiconductor platform may also refer to multi-chip modules with increased connectivity which simulate on-chip operation and make substantial improvements over utilizing a conventional bus implementation. Of course, the various circuits or devices may also be situated separately or in various combinations of semiconductor platforms per the desires of the user. Alternately, the parallel processing module  1925  may be implemented as a circuit board substrate and each of the PPUs  1700  and/or memories  1704  may be packaged devices. In an embodiment, the CPU  1930 , switch  1912 , and the parallel processing module  1925  are situated on a single semiconductor platform. 
     In an embodiment, the signaling rate of each NVLink  1710  is 20 to 25 Gigabits/second and each PPU  1700  includes six NVLink  1710  interfaces (as shown in  FIG.  21   , five NVLink  1710  interfaces are included for each PPU  1700 ). Each NVLink  1710  provides a data transfer rate of 25 Gigabytes/second in each direction, with six links providing 1700 Gigabytes/second. The NVLinks  1710  can be used exclusively for PPU-to-PPU communication as shown in  FIG.  21   , or some combination of PPU-to-PPU and PPU-to-CPU, when the CPU  1930  also includes one or more NVLink  1710  interfaces. 
     In an embodiment, the NVLink  1710  allows direct load/store/atomic access from the CPU  1930  to each PPU&#39;s  1700  memory  1704 . In an embodiment, the NVLink  1710  supports coherency operations, allowing data read from the memories  1704  to be stored in the cache hierarchy of the CPU  1930 , reducing cache access latency for the CPU  1930 . In an embodiment, the NVLink  1710  includes support for Address Translation Services (ATS), allowing the PPU  1700  to directly access page tables within the CPU  1930 . One or more of the NVLinks  1710  may also be configured to operate in a low-power mode. 
       FIG.  22    illustrates an exemplary system  1965  in which the various architecture and/or functionality of the various previous embodiments may be implemented. The exemplary system  1965  may be configured to implement one or more methods disclosed in this application. 
     As shown, a system  1965  is provided including at least one central processing unit  1930  that is connected to a communication bus  1975 . The communication bus  1975  may be implemented using any suitable protocol, such as PCI (Peripheral Component Interconnect), PCI-Express, AGP (Accelerated Graphics Port), HyperTransport, or any other bus or point-to-point communication protocol(s). The system  1965  also includes a main memory  1940 . Control logic (software) and data are stored in the main memory  1940  which may take the form of random access memory (RAM). 
     The system  1965  also includes input devices  1960 , the parallel processing system  1925 , and display devices  1945 , i.e. a conventional CRT (cathode ray tube), LCD (liquid crystal display), LED (light emitting diode), plasma display or the like. User input may be received from the input devices  1960 , e.g., keyboard, mouse, touchpad, microphone, and the like. Each of the foregoing modules and/or devices may even be situated on a single semiconductor platform to form the system  1965 . Alternately, the various modules may also be situated separately or in various combinations of semiconductor platforms per the desires of the user. 
     Further, the system  1965  may be coupled to a network (e.g., a telecommunications network, local area network (LAN), wireless network, wide area network (WAN) such as the Internet, peer-to-peer network, cable network, or the like) through a network interface  1935  for communication purposes. 
     The system  1965  may also include a secondary storage (not shown). The secondary storage includes, for example, a hard disk drive and/or a removable storage drive, representing a floppy disk drive, a magnetic tape drive, a compact disk drive, digital versatile disk (DVD) drive, recording device, universal serial bus (USB) flash memory. The removable storage drive reads from and/or writes to a removable storage unit in a well-known manner. 
     Computer programs, or computer control logic algorithms, may be stored in the main memory  1940  and/or the secondary storage. Such computer programs, when executed, enable the system  1965  to perform various functions. The memory  1940 , the storage, and/or any other storage are possible examples of computer-readable media. 
     The architecture and/or functionality of the various previous figures may be implemented in the context of a general computer system, a circuit board system, a game console system dedicated for entertainment purposes, an application-specific system, and/or any other desired system. For example, the system  1965  may take the form of a desktop computer, a laptop computer, a tablet computer, servers, supercomputers, a smart-phone (e.g., a wireless, hand-held device), personal digital assistant (PDA), a digital camera, a vehicle, a head mounted display, a hand-held electronic device, a mobile phone device, a television, workstation, game consoles, embedded system, and/or any other type of logic. 
     Machine Learning 
     Deep neural networks (DNNs) developed on processors, such as the PPU  1700  have been used for diverse use cases, from self-driving cars to faster drug development, from automatic image captioning in online image databases to smart real-time language translation in video chat applications. Deep learning is a technique that models the neural learning process of the human brain, continually learning, continually getting smarter, and delivering more accurate results more quickly over time. A child is initially taught by an adult to correctly identify and classify various shapes, eventually being able to identify shapes without any coaching. Similarly, a deep learning or neural learning system needs to be trained in object recognition and classification for it get smarter and more efficient at identifying basic objects, occluded objects, etc., while also assigning context to objects. 
     At the simplest level, neurons in the human brain look at various inputs that are received, importance levels are assigned to each of these inputs, and output is passed on to other neurons to act upon. An artificial neuron or perceptron is the most basic model of a neural network. In one example, a perceptron may receive one or more inputs that represent various features of an object that the perceptron is being trained to recognize and classify, and each of these features is assigned a certain weight based on the importance of that feature in defining the shape of an object. 
     A deep neural network (DNN) model includes multiple layers of many connected perceptrons (e.g., nodes) that can be trained with enormous amounts of input data to quickly solve complex problems with high accuracy. In one example, a first layer of the DLL model breaks down an input image of an automobile into various sections and looks for basic patterns such as lines and angles. The second layer assembles the lines to look for higher level patterns such as wheels, windshields, and mirrors. The next layer identifies the type of vehicle, and the final few layers generate a label for the input image, identifying the model of a specific automobile brand. 
     Once the DNN is trained, the DNN can be deployed and used to identify and classify objects or patterns in a process known as inference. Examples of inference (the process through which a DNN extracts useful information from a given input) include identifying handwritten numbers on checks deposited into ATM machines, identifying images of friends in photos, delivering movie recommendations to over fifty million users, identifying and classifying different types of automobiles, pedestrians, and road hazards in driverless cars, or translating human speech in real-time. 
     During training, data flows through the DNN in a forward propagation phase until a prediction is produced that indicates a label corresponding to the input. If the neural network does not correctly label the input, then errors between the correct label and the predicted label are analyzed, and the weights are adjusted for each feature during a backward propagation phase until the DNN correctly labels the input and other inputs in a training dataset. Training complex neural networks requires massive amounts of parallel computing performance, including floating-point multiplications and additions that are supported by the PPU  1700 . Inferencing is less compute-intensive than training, being a latency-sensitive process where a trained neural network is applied to new inputs it has not seen before to classify images, translate speech, and generally infer new information. 
     Neural networks rely heavily on matrix math operations, and complex multi-layered networks require tremendous amounts of floating-point performance and bandwidth for both efficiency and speed. With thousands of processing cores, optimized for matrix math operations, and delivering tens to hundreds of TFLOPS of performance, the PPU  1700  is a computing platform capable of delivering performance required for deep neural network-based artificial intelligence and machine learning applications. 
     All patents &amp; publications cited above are incorporated by reference as if expressly set forth. 
     While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not to be limited to the disclosed embodiments, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.