Abstract:
A graphics system and method are disclosed that may optimize the rate of pixel generation to match the rate at which a memory may be designed to receive pixel data. If a memory is configured to store multiple pixels substantially simultaneously, it may be advantageous to render an equivalent number of pixels substantially simultaneously and at the same rate. An edge walker that utilizes multiple sets of accumulators to generate multiple scan lines substantially simultaneously and a span walker that utilizes multiple sets of accumulators to render multiple pixel values substantially simultaneously is described.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates generally to the field of computer graphics and, more particularly, to high performance computer graphics systems. 
   2. Description of the Related Art 
   Early graphics systems were limited to two-dimensional (2D) graphics, were configured to compute a gray scale value for each pixel displayed, and acted as simple translators or interfaces to a display device. Modem high performance graphics systems, however, may support three-dimensional (3D) graphics with one or more special effects such as anti-aliasing, texturing, shading, fogging, alpha-blending, and specular highlighting. 3D graphics data may be several orders of magnitude larger than comparable 2D graphics data. 3D graphics data may include a set of information components for each vertex of the geometric primitives used to model the objects to be imaged. 
   In recent years, demand for high performance graphics systems that can render complex three-dimensional (3D) objects and scenes have increased substantially. This increase is at least in part due to the demand for new applications such as computer-generated animation for motion pictures, virtual reality simulators/trainers, and interactive computer games. These new applications place tremendous computational loads upon graphics systems. Modem computer displays have also improved and have a significantly higher pixel resolution, greater color depth, and are able to display more complex images with higher refresh rates than earlier models. Consequently, modem high performance graphics systems incorporate graphics processors with a great deal of complexity and power, and the color value of one pixel may be the accumulated result of many calculations involving several models and mathematical approximations. 
   With each new generation of graphics system, there is more image data to process, the processing is more complex, and there is less time in which to process it. This need for more processing power is being met with the combination of more hardware resources and/or more efficient processes. 
   SUMMARY OF THE INVENTION 
   The problems set forth above may at least in part be solved in some embodiments by a graphics system and method that may optimize the rate of pixel generation to match the rate at which the memory architecture is designed to receive pixel data. If a memory is configured to store multiple pixels substantially simultaneously it may be advantageous to render an equivalent number of pixels substantially simultaneously and at the same rate. An edge walker and a span walker architecture described herein, that utilizes multiple sets of accumulators to generate multiple scan lines and render multiple pixel values substantially simultaneously, may achieve this goal. 
   The edge walker may have 2 sets of accumulators to generate parameter values for two sets of end points for two scan lines substantially simultaneously. The span walker may use four sets of accumulators to determine parameter values for a first and a second pixel on each of the 2 scan lines substantially simultaneously. This configuration may enable the edge walker and the span walker to generate parameter values for four pixels substantially simultaneously. In other embodiments, additional accumulators may be utilized to enable more than 2 scan lines to be processed substantially simultaneously. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing, as well as other objects, features, and advantages of this invention may be more completely understood by reference to the following detailed description when read together with the accompanying drawings in which: 
       FIG. 1  is a perspective view of one embodiment of a computer system; 
       FIG. 2  is a simplified block diagram of one embodiment of a computer system; 
       FIG. 3  is a functional block diagram of one embodiment of a graphics system; 
       FIG. 4  is a functional block diagram of one embodiment of the media processor of  FIG. 3 ; 
       FIG. 5  is a functional block diagram of one embodiment of the hardware accelerator of  FIG. 3 ; 
       FIG. 6  is a functional block diagram of one embodiment of the video output processor of  FIG. 3 ; 
       FIG. 7  is an illustration of a sample space partitioned into an array of bins; 
       FIG. 8  is a more detailed block diagram of a portion of the render pipeline of  FIG. 5 ; 
       FIG. 9  is a block diagram of the internal structure of the edge walker and span walker of  FIG. 8 ; and 
       FIG. 10  is a flowchart for a method for rendering multiple pixels substantially simultaneously. 
   

   While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present invention as defined by the appended claims. Note, the headings are for organizational purposes only and are not meant to be used to limit or interpret the description or claims. Furthermore, note that the word “may” is used throughout this application in a permissive sense (i.e., having the potential to, being able to), not a mandatory sense (i.e., must).” The term “include”, and derivations thereof, mean “including, but not limited to”. The term “connected” means “directly or indirectly connected”, and the term “coupled” means “directly or indirectly connected”. 
   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Computer System— FIG. 1   
     FIG. 1  illustrates one embodiment of a computer system  80  that includes a graphics system. The graphics system may be included in any of various systems such as computer systems, network PCs, Internet appliances, televisions (e.g. HDTV systems and interactive television systems), personal digital assistants (PDAs), virtual reality systems, and other devices which display 2D and/or 3D graphics, among others. 
   As shown, the computer system  80  includes a system unit  82  and a video monitor or display device  84  coupled to the system unit  82 . The display device  84  may be any of various types of display monitors or devices (e.g., a CRT, LCD, or gas-plasma display). Various input devices may be connected to the computer system, including a keyboard  86  and/or a mouse  88 , or other input device (e.g., a trackball, digitizer, tablet, six-degree of freedom input device, head tracker, eye tracker, data glove, or body sensors). Application software may be executed by the computer system  80  to display graphical objects on display device  84 . 
   Computer System Block Diagram— FIG. 2   
     FIG. 2  is a simplified block diagram illustrating the computer system of FIG.  1 . As shown, the computer system  80  includes a central processing unit (CPU)  102  coupled to a high-speed memory bus or system bus  104  also referred to as the host bus  104 . A system memory  106  (also referred to herein as main memory) may also be coupled to high-speed bus  104 . 
   Host processor  102  may include one or more processors of varying types, e.g., microprocessors, multi-processors and CPUs. The system memory  106  may include any combination of different types of memory subsystems such as random access memories (e.g., static random access memories or “SRAMs,” synchronous dynamic random access memories or “SDRAMs,” and Rambus dynamic random access memories or “RDRAMs,” among others), read-only memories, and mass storage devices. The system bus or host bus  104  may include one or more communication or host computer buses (for communication between host processors, CPUs, and memory subsystems) as well as specialized subsystem buses. 
   In  FIG. 2 , a graphics system  112  is coupled to the high-speed memory bus  104 . The graphics system  112  may be coupled to the bus  104  by, for example, a crossbar switch or other bus connectivity logic. It is assumed that various other peripheral devices, or other buses, may be connected to the high-speed memory bus  104 . It is noted that the graphics system  112  may be coupled to one or more of the buses in computer system  80  and/or may be coupled to various types of buses. In addition, the graphics system  112  may be coupled to a communication port and thereby directly receive graphics data from an external source, e.g., the Internet or a network. As shown in the figure, one or more display devices  84  may be connected to the graphics system  112 . 
   Host CPU  102  may transfer information to and from the graphics system  112  according to a programmed input/output (I/O) protocol over host bus  104 . Alternately, graphics system  112  may access system memory  106  according to a direct memory access (DMA) protocol or through intelligent bus mastering. 
   A graphics application program conforming to an application programming interface (API) such as OpenGL® or Java 3D™ may execute on host CPU  102  and generate commands and graphics data that define geometric primitives such as polygons for output on display device  84 . Host processor  102  may transfer the graphics data to system memory  106 . Thereafter, the host processor  102  may operate to transfer the graphics data to the graphics system  112  over the host bus  104 . In another embodiment, the graphics system  112  may read in geometry data arrays over the host bus  104  using DMA access cycles. In yet another embodiment, the graphics system  112  may be coupled to the system memory  106  through a direct port, such as the Advanced Graphics Port (AGP) promulgated by Intel Corporation. 
   The graphics system may receive graphics data from any of various sources, including host CPU  102  and/or system memory  106 , other memory, or from an external source such as a network (e.g. the Internet), or from a broadcast medium, e.g., television, or from other sources. 
   Note while graphics system  112  is depicted as part of computer system  80 , graphics system  112  may also be configured as a stand-alone device (e.g., with its own built-in display). Graphics system  112  may also be configured as a single chip device or as part of a system-on-a-chip or a multi-chip module. Additionally, in some embodiments, certain of the processing operations performed by elements of the illustrated graphics system  112  may be implemented in software. 
   Graphics System— FIG. 3   
     FIG. 3  is a functional block diagram illustrating one embodiment of graphics system  112 . Note that many other embodiments of graphics system  112  are possible and contemplated. Graphics system  112  may include one or more media processors  14 , one or more hardware accelerators  18 , one or more texture buffers  20 , one or more frame buffers  22 , and one or more video output processors  24 . Graphics system  112  may also include one or more output devices such as digital-to-analog converters (DACs)  26 , video encoders  28 , flat-panel-display drivers (not shown), and/or video projectors (not shown). Media processor  14  and/or hardware accelerator  18  may include any suitable type of high performance processor (e.g., specialized graphics processors or calculation units, multimedia processors, DSPs, or general purpose processors). 
   In some embodiments, one or more of these components may be removed. For example, the texture buffer may not be included in an embodiment that does not provide texture mapping. In other embodiments, all or part of the functionality incorporated in either or both of the media processor or the hardware accelerator may be implemented in software. 
   In one set of embodiments, media processor  14  is one integrated circuit and hardware accelerator is another integrated circuit. In other embodiments, media processor  14  and hardware accelerator  18  may be incorporated within the same integrated circuit. In some embodiments, portions of media processor  14  and/or hardware accelerator  18  may be included in separate integrated circuits. 
   As shown, graphics system  112  may include an interface to a host bus such as host bus  104  in  FIG. 2  to enable graphics system  112  to communicate with a host system such as computer system  80 . More particularly, host bus  104  may allow a host processor to send commands to the graphics system  112 . In one embodiment, host bus  104  may be a bi-directional bus. 
   Media Processor— FIG. 4   
     FIG. 4  shows one embodiment of media processor  14 . As shown, media processor  14  may operate as the interface between graphics system  112  and computer system  80  by controlling the transfer of data between computer system  80  and graphics system  112 . In some embodiments, media processor  14  may also be configured to perform transformations, lighting, and/or other general-purpose processing operations on graphics data. 
   Transformation refers to the spatial manipulation of objects (or portions of objects) and includes translation, scaling (e.g. stretching or shrinking), rotation, reflection, or combinations thereof. More generally, transformation may include linear mappings (e.g. matrix multiplications), nonlinear mappings, and combinations thereof. 
   Lighting refers to calculating the illumination of the objects within the displayed image to determine what color values and/or brightness values each individual object will have. Depending upon the shading algorithm being used (e.g., constant, Gourand, or Phong), lighting may be evaluated at a number of different spatial locations. 
   As illustrated, media processor  14  may be configured to receive graphics data via host interface  11 . A graphics queue  148  may be included in media processor  14  to buffer a stream of data received via the accelerated port of host interface  11 . The received graphics data may include one or more graphics primitives. As used herein, the term graphics primitive may include polygons, parametric surfaces, splines, NURBS (non-uniform rational B-splines), sub-divisions surfaces, fractals, volume primitives, voxels (i.e., three-dimensional pixels), and particle systems. In one embodiment, media processor  14  may also include a geometry data preprocessor  150  and one or more microprocessor units (MPUs)  152 . MPUs  152  may be configured to perform vertex transformation, lighting calculations and other programmable functions, and to send the results to hardware accelerator  18 . MPUs  152  may also have read/write access to texels (i.e. the smallest addressable unit of a texture map) and pixels in the hardware accelerator  18 . Geometry data preprocessor  150  may be configured to decompress geometry, to convert and format vertex data, to dispatch vertices and instructions to the MPUs  152 , and to send vertex and attribute tags or register data to hardware accelerator  18 . 
   As shown, media processor  14  may have other possible interfaces, including an interface to one or more memories. For example, as shown, media processor  14  may include direct Rambus interface  156  to a direct Rambus DRAM (DRDRAM)  16 . A memory such as DRDRAM  16  may be used for program and/or data storage for MPUs  152 . DRDRAM  16  may also be used to store display lists and/or vertex texture maps. 
   Media processor  14  may also include interfaces to other functional components of graphics system  112 . For example, media processor  14  may have an interface to another specialized processor such as hardware accelerator  18 . In the illustrated embodiment, controller  160  includes an accelerated port path that allows media processor  14  to control hardware accelerator  18 . Media processor  14  may also include a direct interface such as bus interface unit (BIU)  154 . Bus interface unit  154  provides a path to memory  16  and a path to hardware accelerator  18  and video output processor  24  via controller  160 . 
   Hardware Accelerator— FIG. 5   
   One or more hardware accelerators  18  may be configured to receive graphics instructions and data from media processor  14  and to perform a number of functions on the received data according to the received instructions. For example, hardware accelerator  18  may be configured to perform rasterization, 2D and/or 3D texturing, pixel transfers, imaging, fragment processing, clipping, depth cueing, transparency processing, set-up, and/or screen space rendering of various graphics primitives occurring within the graphics data. 
   Clipping refers to the elimination of graphics primitives or portions of graphics primitives that lie outside of a 3D view volume in world space. The 3D view volume may represent that portion of world space that is visible to a virtual observer (or virtual camera) situated in world space. For example, the view volume may be a solid truncated pyramid generated by a 2D view window, a viewpoint located in world space, a front clipping plane and a back clipping plane. The viewpoint may represent the world space location of the virtual observer. In most cases, primitives or portions of primitives that lie outside the 3D view volume are not currently visible and may be eliminated from further processing. Primitives or portions of primitives that lie inside the 3D view volume are candidates for projection onto the 2D view window. 
   Set-up refers to mapping primitives to a three-dimensional viewport. This involves translating and transforming the objects from their original “world-coordinate” system to the established viewport&#39;s coordinates. This creates the correct perspective for three-dimensional objects displayed on the screen. 
   Screen-space rendering refers to the calculations performed to generate the data used to form each pixel that will be displayed. For example, hardware accelerator  18  may calculate “samples.” Samples are points that have color information but no real area. Samples allow hardware accelerator  18  to “super-sample,” or calculate more than one sample per pixel. Super-sampling may result in a higher quality image. 
   Hardware accelerator  18  may also include several interfaces. For example, in the illustrated embodiment, hardware accelerator  18  has four interfaces. Hardware accelerator  18  has an interface  161  (referred to as the “North Interface”) to communicate with media processor  14 . Hardware accelerator  18  may receive commands and/or data from media processor  14  through interface  161 . Additionally, hardware accelerator  18  may include an interface  176  to bus  32 . Bus  32  may connect hardware accelerator  18  to boot PROM  30  and/or video output processor  24 . Boot PROM  30  may be configured to store system initialization data and/or control code for frame buffer  22 . Hardware accelerator  18  may also include an interface to a texture buffer  20 . For example, hardware accelerator  18  may interface to texture buffer  20  using an eight-way interleaved texel bus that allows hardware accelerator  18  to read from and write to texture buffer  20 . Hardware accelerator  18  may also interface to a frame buffer  22 . For example, hardware accelerator  18  may be configured to read from and/or write to frame buffer  22  using a four-way interleaved pixel bus. 
   The vertex processor  162  may be configured to use the vertex tags received from the media processor  14  to perform ordered assembly of the vertex data from the MPUs  152 . Vertices may be saved in and/or retrieved from a mesh buffer  164 . 
   The render pipeline  166  may be configured to rasterize 2D window system primitives and 3D primitives into fragments. A fragment may contain one or more samples. Each sample may contain a vector of color data and perhaps other data such as alpha and control tags. 2D primitives include objects such as dots, fonts, Bresenham lines and 2D polygons. 3D primitives include objects such as smooth and large dots, smooth and wide DDA (Digital Differential Analyzer) lines and 3D polygons (e.g. 3D triangles). 
   For example, the render pipeline  166  may be configured to receive vertices defining a triangle, to identify fragments that intersect the triangle. 
   The render pipeline  166  may be configured to handle full-screen size primitives, to calculate plane and edge slopes, and to interpolate data (such as color) down to tile resolution (or fragment resolution) using interpolants or components such as: 
   r, g, b (i.e., red, green, and blue vertex color); 
   r2, g2, b2 (i.e., red, green, and blue specular color from lit textures); 
   alpha (i.e. transparency); 
   z (i.e. depth); and 
   s, t, r, and w (i.e. texture components). 
   In embodiments using super-sampling, the sample generator  174  may be configured to generate samples from the fragments output by the render pipeline  166  and to determine which samples are inside the rasterization edge. Sample positions may be defined by user-loadable tables to enable stochastic sample-positioning patterns. 
   Hardware accelerator  18  may be configured to write textured fragments from 3D primitives to frame buffer  22 . The render pipeline  166  may send pixel tiles defining r, s, t and w to the texture address unit  168 . The texture address unit  168  may use the r, s, t and w texture coordinates to compute texel addresses (e.g. addresses for a set of neighboring texels) and to determine interpolation coefficients for the texture filter  170 . The texel addresses are used to access texture data (i.e. texels) from texture buffer  20 . The texture buffer  20  may be interleaved to obtain as many neighboring texels as possible in each clock. The texture filter  170  may perform bilinear, trilinear or quadlinear interpolation. The pixel transfer unit  182  may also scale and bias and/or lookup texels. The texture environment  180  may apply texels to samples produced by the sample generator  174 . The texture environment  180  may also be used to perform geometric transformations on images (e.g., bilinear scale, rotate, flip) as well as to perform other image filtering operations on texture buffer image data (e.g., bicubic scale and convolutions). 
   In the illustrated embodiment, the pixel transfer MUX  178  controls the input to the pixel transfer unit  182 . The pixel transfer unit  182  may selectively unpack pixel data received via north interface  161 , select channels from either the frame buffer  22  or the texture buffer  20 , or select data received from the texture filter  170  or sample filter  172 . 
   The pixel transfer unit  182  may be used to perform scale, bias, and/or color matrix operations, color lookup operations, histogram operations, accumulation operations, normalization operations, and/or min/max functions. Depending on the source of (and operations performed on) the processed data, the pixel transfer unit  182  may output the processed data to the texture buffer  20  (via the texture buffer MUX  186 ), the frame buffer  22  (via the texture environment unit  180  and the fragment processor  184 ), or to the host (via north interface  161 ). For example, in one embodiment, when the pixel transfer unit  182  receives pixel data from the host via the pixel transfer MUX  178 , the pixel transfer unit  182  may be used to perform a scale and bias or color matrix operation, followed by a color lookup or histogram operation, followed by a min/max function. The pixel transfer unit  182  may then output data to either the texture buffer  20  or the frame buffer  22 . 
   Fragment processor  184  may be used to perform standard fragment processing operations such as the OpenGL® fragment processing operations. For example, the fragment processor  184  may be configured to perform the following operations: fog, area pattern, scissor, alpha/color test, ownership test (WID), stencil test, depth test, alpha blends or logic ops (ROP), plane masking, buffer selection, pick hit/occlusion detection, and/or auxiliary clipping in order to accelerate overlapping windows. 
   Texture Buffer  20   
   Texture buffer  20  may include several SDRAMs. Texture buffer  20  may be configured to store texture maps, image processing buffers, and accumulation buffers for hardware accelerator  18 . Texture buffer  20  may have many different capacities (e.g., depending on the type of SDRAM included in texture buffer  20 ). In some embodiments, each pair of SDRAMs may be independently row and column addressable. 
   Frame Buffer  22   
   Graphics system  112  may also include a frame buffer  22 . In one embodiment, frame buffer  22  may include multiple memory devices such as 3D-RAM memory devices manufactured by Mitsubishi Electric Corporation. Frame buffer  22  may be configured as a display pixel buffer, an offscreen pixel buffer, and/or a super-sample buffer. Furthermore, in one embodiment, certain portions of frame buffer  22  may be used as a display pixel buffer, while other portions may be used as an offscreen pixel buffer and sample buffer. 
   Video Output Processor— FIG. 6   
   A video output processor  24  may also be included within graphics system  112 . Video output processor  24  may buffer and process pixels output from frame buffer  22 . For example, video output processor  24  may be configured to read bursts of pixels from frame buffer  22 . Video output processor  24  may also be configured to perform double buffer selection (dbsel) if the frame buffer  22  is double-buffered, overlay transparency (using transparency/overlay unit  190 ), plane group extraction, gamma correction, psuedocolor or color lookup or bypass, and/or cursor generation. For example, in the illustrated embodiment, the output processor  24  includes WID (Window ID) lookup tables (WLUTs)  192  and gamma and color map lookup tables (GLUTs, CLUTs)  194 . In one embodiment, frame buffer  22  may include multiple 3DRAM64s  201  that include the transparency overlay  190  and all or some of the WLUTs  192 . Video output processor  24  may also be configured to support two video output streams to two displays using the two independent video raster timing generators  196 . For example, one raster (e.g.,  196 A) may drive a 1280×1024 CRT while the other (e.g.,  196 B) may drive a NTSC or PAL device with encoded television video. 
   DAC  26  may operate as the final output stage of graphics system  112 . The DAC  26  translates the digital pixel data received from GLUT/CLUTs/Cursor unit  194  into analog video signals that are then sent to a display device. In one embodiment, DAC  26  may be bypassed or omitted completely in order to output digital pixel data in lieu of analog video signals. This may be useful when a display device is based on a digital technology (e.g., an LCD-type display or a digital micro-mirror display). 
   DAC  26  may be a red-green-blue digital-to-analog converter configured to provide an analog video output to a display device such as a cathode ray tube (CRT) monitor. In one embodiment, DAC  26  may be configured to provide a high resolution RGB analog video output at dot rates of 240 MHz. Similarly, encoder  28  may be configured to supply an encoded video signal to a display. For example, encoder  28  may provide encoded NTSC or PAL video to an S-Video or composite video television monitor or recording device. 
   In other embodiments, the video output processor  24  may output pixel data to other combinations of displays. For example, by outputting pixel data to two DACs  26  (instead of one DAC  26  and one encoder  28 ), video output processor  24  may drive two CRTs. Alternately, by using two encoders  28 , video output processor  24  may supply appropriate video input to two television monitors. Generally, many different combinations of display devices may be supported by supplying the proper output device and/or converter for that display device. 
   Sample-to-Pixel Processing Flow 
   In one set of embodiments, hardware accelerator  18  may receive geometric parameters defining primitives such as triangles from media processor  14 , and render the primitives in terms of samples. The samples may be stored in a sample storage area (also referred to as the sample buffer) of frame buffer  22 . The samples are then read from the sample storage area of frame buffer  22  and filtered by sample filter  22  to generate pixels. The pixels are stored in a pixel storage area of frame buffer  22 . The pixel storage area may be double-buffered. Video output processor  24  reads the pixels from the pixel storage area of frame buffer  22  and generates a video stream from the pixels. The video stream may be provided to one or more display devices (e.g. monitors, projectors, head-mounted displays, and so forth) through DAC  26  and/or video encoder  28 . 
   The samples are computed at positions in a two-dimensional sample space (also referred to as rendering space). The sample space may be partitioned into an array of bins (also referred to herein as fragments). The storage of samples in the sample storage area of frame buffer  22  may be organized according to bins as illustrated in FIG.  7 . Each bin may contain one or more samples. The number of samples per bin may be a programmable parameter. 
   Render Pipeline— FIG. 8   
     FIG. 8  illustrates a more detailed block diagram of one embodiment of the render system or render pipeline  166 . As shown, the render pipeline  166  may comprise a Vertex Processor (VP)  162 , a Pre-Setup Unit (PSU)  302 , a Setup Unit (SU)  304 , an Edge Walker (EW)  306 , and Span Walkers (SW)  308 . 
   The Vertex Processor  162  operates to assemble triangles (or polygons) from the vertex information received from the media processor  14 . The Pre-Setup Unit  302  operates to pre-process the triangle data. The Setup Unit  304  defines the edges of the triangle and the variation of parameter values (x,y,z,r,g,b . . . ) across the triangle. The Edge Walker  306  operates to define start and end points of scan lines that span the triangle from a controlling edge (longest or major edge) to a subordinate edge of the triangle. The scan lines are stepped in the major direction by steps of 1. The Span Walker  308  operates to interpolate pixel values along the scan lines. For each span line issued by the EW  306 , the SW  308  interpolates parameter values for each pixel location that lies on the scan line between the controlling edge point and the subordinate edge point on the scan line. 
   Internal Structure of Edge Walker and Span Walker— FIG. 9   
     FIG. 9  illustrates one embodiment of the EW  306  and the SW  308 . The EW  306  may have 2 sets of accumulators  410 - 420  to generate parameter values for two sets of end points for two scan lines substantially simultaneously. The SW  308  may use four sets of accumulators  430 - 460  to determine parameter values for a first and a second pixel on each of the 2 scan lines substantially simultaneously. This configuration may enable the EW  306  and the SW  308  to generate parameter values for four pixels substantially simultaneously. In other embodiments, additional accumulators may be utilized to enable more than 2 scan lines to be processed substantially simultaneously. 
   Method for Rendering Multiple Pixels Substantially Simultaneously— FIG. 10   
     FIG. 10  provides a flowchart for one embodiment of a method for rendering a triangle (or polygon) in a graphics system. The method includes receiving information regarding the triangle that includes data for the controlling edge (major edge or longest edge) and the two subordinate edges of the triangle and the variation of parameter values (x,y,z,r,g,b . . . ) across the triangle (step  500 ). The Edge Walker  306  determines the x coordinate for a first scan line (also referred to herein as a span or slice) (step  510 ) and then substantially simultaneously calculates the y coordinates for the start and end points for the two scan lines at x and x+1 (step  520 ). The Span Walker  308  substantially simultaneously interpolates parameter values for 4 pixel locations at a time, 2 on the first scan line at x and 2 on the second scan line at x+1 (step  530 ) and outputs each 2×2 tile of pixels (step  540 ). The Edge Walker  306  replaces x with x+2 and tests to determine if the new x coordinate is greater than x max, the x coordinate for the end of the triangle (step  560 ). If x is greater than x max, the triangle is fully rendered (step  570 ). If x is not greater than x max, the Edge Walker  306  calculates y coordinates for the start and end points for two new scan lines at the new x and x+1 coordinates (step  520 ) and steps  530 - 560  are repeated. In some embodiments, the 4 pixels of the 2×2 tile of pixels may be written substantially simultaneously to a memory  22 . 
   In other embodiments, Edge Walker  306  may be configured to process substantially simultaneously 3 or more scan lines, Span Walker  308  may be configured to process substantially simultaneously 2×2 or larger pixel arrays, and a memory  22  may be configured to substantially simultaneously receive 4 or more pixels. 
   Although the embodiments above have been described in considerable detail, other versions are possible. Numerous variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated. It is intended that the following claims be interpreted to embrace all such variations and modifications. Note the section headings used herein are for organizational purposes only and are not meant to limit the description provided herein or the claims attached hereto.