# Patent

## Title
Untransformed display lists in a tile based rendering system

## Abstract

A three-dimensional computer graphics rendering system allows a tile-based rendering system to operate with a reduced amount of storage required for tiled screen space geometry by using an untransformed display list to represent the screen's geometry.

## Background
This application is a continuation of U.S. patent application Ser. No. 12/383,119, filed on Mar. 19, 2009, entitled “UNTRANSFORMED DISPLAY LISTS IN A TILE BASED RENDERING SYSTEM”, now U.S. Pat. No. 8,368,691, issued on Feb. 5, 2013, which claims priority from GB 0805146.8, filed on Mar. 19, 2008.


This invention relates to a three-dimensional computer graphics rendering system and in particular to methods and apparatus associated with rendering three-dimensional graphic images utilising an untransformed display list within a tile based rendering system.
BACKGROUND TO THE INVENTION
Tile based rendering systems are well known, these subdivide an image into a plurality of rectangular blocks or tiles in order to increase efficiency of the rasterisation process.
FIG. 1 illustrates a traditional the based rendering system. Tile based rendering systems operate in two phases, a geometry processing phase and a rasterisation phase. During the geometry processing phase a primitive/command fetch unit 100 retrieves command and primitive data from memory and passes this to a geometry fetch unit 105 which fetches the geometry data 110 from memory and passes it to a transform unit 115. This transforms the primitive and command data into screen space and applies any lighting/attribute processing as required using well-known methods. The resulting data is passed to a culling unit 120 which culls any geometry that isn't visible using well known methods. The culling unit writes any remaining geometry data to the transformed parameter buffer 135 and also passes the position data of the remaining geometry to the tiling unit 125 which generates a set of screen space objects lists for each tile which are written to the tiled geometry lists 130. Each object list contains references to the transformed primitives that exist wholly or partially in that tile. The lists exist for every tile on the screen, although some object lists may have no data in them. This process continues until all the geometry within the scene has been processed.
During the rasterisation phase the object lists are fetched by a tiled parameter fetch unit 140 which first fetches the object references and then the object data referenced and supplies them to a hidden surface removal unit (HSR) 145 which removes surfaces which will not contribute to the final scene (usually because they are obscured by another surface). The HSR unit processes each primitive in the tile and passes only data for visible primitives/pixels to a texturing and shading unit (TSU) 150. The TSU takes the data from the HSR unit and uses it to fetch textures and apply shading to each pixel within a visible object using well-known techniques. The TSU then supplies the textured and shaded data to an alpha test/fogging/alpha blending unit 155. This is able to apply degrees of transparency/opacity to the surfaces again using well-known techniques. Alpha blending is performed using an on chip tile buffer 160 thereby eliminating the requirement to access external memory for this operation. It should be noted that the TSU and alpha test/fogging/alpha blend units may be fully programmable in nature.
Once each tile has been completed, a pixel processing unit 165 performs any necessary backend processing such as packing and anti-alias filtering before writing the resulting data to a rendered scene buffer 170, ready for display.
Typically modern computer graphics applications utilise a significant amount of geometry that remains static throughout a scene or across multiple scenes, this geometry data is stored in what is commonly known as static vertex buffers that typically reside in memory that is local to the graphics processing unit. Current tile based systems transform this data into screen space and store the resulting geometry within a parameter buffer/tiled screen spaced geometry list that can consume a considerable amount of additional storage and memory bandwidth.
SUMMARY OF THE INVENTION
Preferred embodiments of the present invention provide a method and apparatus that allow a the based rendering system to operate with a reduced amount of storage required for tiled screen space geometry. This is accomplished by the use of an untransformed display list to represent the scene's geometry. This removes the need for the transformed parameter buffer 135 in FIG. 1 by utilising the fact that the incoming scene geometry is static and so it can be referenced in both the geometry processing and rasterisation phases.



BRIEF DESCRIPTION OF THE DRAWINGS
Preferred embodiments of the invention will now be described in detail by way of example with reference to the accompanying drawings in which:
FIG. 1 illustrates a traditional tile based rendering system;
FIG. 2 illustrates a tile based rendering system using an untransformed display list;
FIG. 3 illustrates deferred lighting/attribute processing;
FIG. 4 illustrates the addition of a transformed data cache to the system; and
FIG. 5 illustrates a hybrid transformed/untransformed display list based tile based rendering system.



DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
FIG. 2 illustrates a tile based rendering system that has been modified to support an untransformed display list. During the geometry processing phase a primitive/command fetch unit 200 retrieves command and primitive data from memory and passes this to a position data fetch unit 205 which fetches a position part of static geometry data from memory 210 and passes it to transform 1 unit 215. This transforms the primitive into screen space only i.e. it does not apply any lighting/attribute processing as would occur in the system of FIG. 1. The resulting screen space position data is passed to a culling unit 220 which culls any geometry in the same manner as the system of FIG. 1. Unlike the system of FIG. 1 the culling unit does not write the remaining geometry data to a transformed parameter buffer, instead it only passes the position data of the remaining geometry to a tiling unit 225.
In the system of FIG. 1, the tiling unit generates references to transformed geometry that has been stored in the transformed parameter buffer, in the new system the tiling unit generates references to the untransformed static geometry data which are written to the tiled geometry lists 230 as before. These references are in the form of pointers to the geometry data in the memory 210. This process continues until all the geometry within the scene has been processed.
During the rasterisation phase object lists for each tile are fetched by a tiled parameter fetch unit 240 which supplies the static geometry references (pointers) from the total geometry lists to untransformed geometry fetch unit 245 which fetches the untransformed static geometry data from memory 210 and passes it to the transform 2 unit 250. The transform 2 unit retransforms the retrieved data to screen space and applies any required lighting/attribute processing etc to the geometry. The transformed geometry is then passed to hidden surface removal unit (HSR) 255 which removes surfaces which will not contribute to the final scene as in the system of FIG. 1. The remaining stages 260 through to 280 all operate in the same manner as stages 150 through 170 (in FIG. 1) as described above. [John: should FIG. 2 also include a source of dynamic geometry?]
In a further optimisation it is possible to defer any lighting or attribute processing that is required after hidden surface removal has been performed. This means that this processing is only applied to that geometry which is visible within the final scene giving significant improvements in both throughput and power consumption. FIG. 3 illustrates a modification to the system that implements deferred lighting/attribute processing. Units 300 and 305 operate as described for units 240 and 245 of FIG. 2, unlike unit 250 in FIG. 2 the transform 2 unit 310 only transforms the position data before passing it onto the hidden surface removal unit 315. The visible primitives emitted by the hidden surface removal unit are then passed to transform 3 unit 320 where any lighting/attribute processing is performed. The operation of units 325 to 350 is the same as units 145 to 170 in FIG. 1.
It should be noted that each of the three transformation units mentioned above could all be implemented in a single “universal” unit similar to that described in our British Patent Application GB-A-2430513. Although the above approaches eliminate the need for a transformed parameter buffer they have the disadvantage of requiring the position data to be transformed in both phases and for the transformation to be repeated for every tile that any piece of geometry overlaps. FIG. 4 illustrates a modification to the rasterisation phase of the untransformed display list system in which a cache is added in order to minimise the number of times the data is retransformed in the rasterisation phase. It should be noted that although FIG. 4 shows a modification with respect to a non deferred lighting/attribute processing system it is equally applicable to either. As in FIG. 2 the tiled parameter fetch unit 400 fetches the tiled object list references generated in the geometry processing phase from memory. The references are passed to a cache control unit 405 which checks to see if there is an entry in the transformed data cache memory 410 that corresponds to the object reference, if there is the cache control unit reads the data from the cache and passes it to the hidden surface removal unit 425. If there is no corresponding entry in the cache the cache control unit issues the reference to the untransformed geometry fetch unit 415 which fetch the data from memory and passes it to the transform 2 unit 420. The transform 2 unit transforms and applies any lighting/attribute process required to the geometry data and then passes it back to the cache control unit. The cache control unit then adds it to the transformed data cache memory for future reference before passing it to the hidden surface removal unit. The operation of units 425 to 450 is the same as units 145 to 170 in FIG. 1.
In order to eliminate the additional geometry processing pass used in the above approach the result of the position transform can be stored in a parameter buffer for use in the second pass. Although this results in the need for, transformed parameter storage it may be consider a useful trade off compared against transforming the position data multiple times. It should also be noted that there are cases were an application will update the vertex data during a scene, this type of vertex data is often referred to as dynamic, in these circumstances the data must be transformed and copied to a parameter buffer as per a conventional tile based rendering device.
FIG. 5 illustrates a hybrid system that allows the use of both untransformed and transformed display lists. During the geometry processing phase a primitive/command fetch unit 500 retrieves command and primitive data from memory and passes this to the geometry fetch unit 505 which fetches both the dynamic geometry data 507 and static geometry data 510 from memory and passes it to the transform 1 unit 515.
For dynamic geometry the transform 1 unit transforms the position and applies any required lighting/attribute processing as per a traditional tile based rendering system, for static geometry only the position is transformed as previously described. The resulting data is passed to a culling unit 520 which culls any geometry that isn't visible using well known methods. The culling unit writes any remaining dynamic geometry and static position data to the transformed parameter buffer 535 and also passes the position data of the remaining geometry to the tiling unit 525 which generates a set of screen objects lists for each tile which are written to the tiled geometry lists 530. It should be noted that the tiled geometry lists indicate which geometry is dynamic and which is static. As in FIG. 2 the tiled parameter fetch unit 540 fetches the tiled object list references generated in the geometry processing phase from memory. The references are passed to the cache control unit 545 which checks to see if there is an entry in the transformed data cache memory 550 that corresponds to the object reference, if there is the cache control unit reads the data from the cache and passes it to the hidden surface removal unit 565. If there is no corresponding entry in the cache the cache control unit issues the reference to either the transformed parameter fetch unit 547 or the untransformed geometry fetch unit 555 based on the type indicated in the tiled reference lists. Transformed geometry is fetched by the transformed parameter fetch unit and passed back to the cache control unit and untransformed geometry is fetched by the untransformed geometry fetch unit and processed by transform unit 2 560 before being passed back to the cache control unit. Both geometry types are then written to the cache by the control unit before being passed to the hidden surface removal unit. All subsequent units 565 through to 590 operate as previously described for units 145 through 170 in FIG. 1.

## Claims

1. A rendering method for a tile-based rendering system, comprising:
retrieving, from at least one memory, untransformed 3-D position data for elements of dynamic geometry and elements of static geometry, wherein dynamic geometry comprises geometry that may be updated during a rendering;
transforming the positions of dynamic geometry;
transforming the positions of the static geometry;
culling hidden surfaces based on the transformed positions of the dynamic geometry and of the static geometry;
performing one or more of lighting and attribute processing only for the dynamic geometry remaining after the culling, if any;
storing remaining transformed dynamic geometry data and outputs resulting from the performing of the one or more of lighting and attributing processing of the remaining dynamic geometry if any, in a buffer; and
generating respective tile object lists for a plurality of tiles of pixels using the transformed positions of the dynamic and static geometry, wherein the tile object lists include references to the untransformed 3-D position data for elements of static geometry found to overlap any pixel of that tile, and references, for elements of dynamic geometry found to overlap any pixel of that tile, to stored transformed dynamic geometry data and the outputs resulting from the performing of the one or more of lighting and attribute processing of such elements of dynamic geometry.

2. The rendering method for a tile-based rendering system of claim 1, further comprising, in a rasterizer, traversing each of the tile object lists, and for each element of static geometry, determining whether transformed 3-D position data for the element of static geometry is stored in a cache managed by the rasterizer, if so, then retrieving the transformed 3-D position data for the element of static geometry, if not, then retrieving the untransformed 3-D position data for the element of static geometry and transforming the untransformed 3-D position data for the element of static geometry, and for each element of dynamic geometry, retrieving transformed 3-D position data from the buffer, and performing hidden surface removal for each pixel on both the static geometry and the dynamic geometry for each pixel of each tile of pixels.

3. The rendering method for a tile-based rendering system of claim 1, further comprising, during rasterization, accessing a tile object list for a tile, supplying references for static geometry in the tile object list to a geometry fetch unit, transforming fetched untransformed 3-D position data for the static geometry into screen space, retrieving transformed position data for dynamic geometry, and performing hidden surface removal for pixels of the tile using the transformed 3-D position data for the static geometry and the retrieved transformed position data for the dynamic geometry.

4. A rendering method in a tile-based rendering system, comprising:
accessing, in a rasterization portion of the tile-based rendering system, data from a tile object list, which lists elements of geometry that overlap, in a 2-D screen space, with any pixel of a tile of pixels to be rendered, wherein the elements of geometry may be static or dynamic, and the tile object list distinguishes static elements of geometry from dynamic elements of geometry, wherein dynamic elements of geometry may be updated during a rendering;
for each element of geometry listed in the tile object list,
if the element of geometry is dynamic geometry, then retrieving 2-D screen space position information for the element of geometry, from a buffer, and
if the element of geometry is static geometry, then retrieving untransformed 3-D position data of the element of geometry, and transforming the 3-D position data into the 2-D screen space; and

performing hidden surface removal for the static geometry based on the transformed 3-D position data and for the dynamic geometry using the 3-D position data retrieved from the buffer.

5. The rendering method in a tile-based rendering system of claim 4, further comprising, after identifying a visible surface for a pixel in the tile of pixels, and if the visible surface is static geometry, responsively performing one or more of lighting and attribute processing for the static geometry constituting the visible surface in that pixel.

6. The rendering method in a tile-based rendering system of claim 5, further comprising, storing outputs of the performing one or more of lighting and attribute processing for the element of static geometry in a cache memory maintained by the rasterization portion.

7. The rendering method in a tile-based rendering system of claim 4, wherein the tile object lists comprise, for each element of static geometry, a reference to untransformed 3-D position data for that element of static geometry, and the method comprises determining whether the cache memory stores transformed position data for that element of static geometry, prior to retrieving the untransformed 3-D position data.

8. The rendering method in a tile-based rendering system of claim 4, wherein the untransformed 3-D position data is retrieved responsive to determining that a transformed data cache, maintained by the rasterization portion, does not have transformed position data for the element of geometry.

9. The rendering method in a tile-based rendering system of claim 4, further comprising attempting to retrieve transformed position data, from a transformed data cache, if the element of geometry is static geometry, before retrieving untransformed 3-D position data and transforming that 3-D position data to produce transformed position data for use in performing the hidden surface removal, and storing the transformed position data in the transformed data cache.

10. A tile-based 3-D rendering system comprising a rasterization portion, the rasterization portion comprising:
a fetch unit, operable to operable to obtain, from a tile object list for a tile of pixels being processed, a reference, for each element of static geometry appearing in the tile object list, to untransformed 3-D position data for that element of static geometry, and to retrieve the untransformed 3-D position data for that element of static geometry;
a transform unit, operable to receive the untransformed 3-D position data and transform the 3-D position data into 2-D screen space coordinates for the element of static geometry;
a hidden surface removal unit coupled to receive the 2-D screen space coordinates for the element of static geometry from the transform unit and perform hidden surface removal for each pixel in the tile of pixels, wherein dynamic geometry may be updated during a rendering; and
a shading unit operable to shade a visible surface for each pixel in the tile, wherein the shading unit is operable to use parameters produced during geometry processing for shading dynamic geometry and parameters produced within the rasterization portion for shading static geometry.

11. The tile-based 3-D rendering system of claim 10, further comprising a non-transitory memory storing 3-D position data and texturing and lighting parameter data for elements of static and dynamic geometry, and the rasterization portion is operable to retrieve the untransformed 3-D position information from the non-transitory memory.

12. The tile-based 3-D rendering system of claim 10, further comprising
one or more non-transitory memories storing 3-D position data and parameter data for elements of static and dynamic geometry;
a geometry transformation unit operable to transform the 3-D position data for elements of both the dynamic geometry and the static geometry, to produce respective 2-D position data, in a 2-D screen space, for the transformed elements of geometry, and to perform attribute processing only for elements of the dynamic geometry;
a tiling unit operable to assemble the object tile lists based on the transformed elements of static and dynamic geometry, each object tile list comprising references to memory locations storing untransformed 3-D position and parameter data for the elements of static geometry;
a parameter buffer for receiving and storing transformed parameters for elements of dynamic geometry, the parameter buffer operable to be read by the rasterization portion.

13. The tile-based 3-D rendering system of claim 12, wherein the geometry transformation unit and the transform unit of the rastizeration portion are implemented by a unitary geometry transform unit.

14. The 3-D rendering system of claim 12, further comprising a cull unit operable to receive transformed position information for dynamic geometry from the geometry transformation unit and to cull elements of geometry determined not to be visible, wherein transformed parameters for culled elements of geometry are not stored in the parameter buffer.

15. The 3-D rendering system of claim 10, wherein the fetch unit is operable to attempt to fetch transformed parameters of the static geometry, corresponding to the untransformed parameters, from a cache, before retrieving the untransformed parameters again.

16. The 3-D rendering system for producing renderings from static and dynamic 3-D geometry of claim 15, wherein the memory fetch unit is operable to update the cache with transformed parameters for static geometry.

17. The rendering method in a tile-based rendering system of claim 9, wherein the data in the transformed data cache is maintained during hidden surface removal for a plurality of tiles.

18. The tile-based 3-D rendering system of claim 10, wherein the transform unit of the rasterization portion is implemented by a programmable unit, and the programmable unit further implements a transform unit within a geometry processing portion of the tile-based 3-D rendering system.