Patent Publication Number: US-6664962-B1

Title: Shadow mapping in a low cost graphics system

Description:
This application claims the benefit of U.S. Provisional Application No. 60/227,006, filed Aug. 23, 2000, the entire content of which is hereby incorporated by reference in this application. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to computer graphics, and more particularly to interactive graphics systems such as home video game platforms. Still more particularly, this invention relates to techniques for generating shadows using full scene shadow mapping in a low cost graphics system, and to use of graphics pipeline texture coordinate generation and/or texture mapping arrangements to generate precision numerical values supporting shadow comparisons and other effects. 
     BACKGROUND AND SUMMARY OF THE INVENTION 
     Many of us have seen films containing remarkably realistic dinosaurs, aliens, animated toys and other fanciful creatures. Such animations are made possible by computer graphics. Using such techniques, a computer graphics artist can specify how each object should look and how it should change in appearance over time, and a computer then models the objects and displays them on a display such as your television or a computer screen. The computer takes care of performing the many tasks required to make sure that each part of the displayed image is colored and shaped just right based on the position and orientation of each object in a scene, the direction in which light seems to strike each object, the surface texture of each object, and other factors. 
     Because computer graphics generation is complex, computer-generated three-dimensional graphics just a few years ago were mostly limited to expensive specialized flight simulators, high-end graphics workstations and supercomputers. The public saw some of the images generated by these computer systems in movies and expensive television advertisements, but most of us couldn&#39;t actually interact with the computers doing the graphics generation. All this has changed with the availability of relatively inexpensive 3D graphics platforms such as, for example, the Nintendo 64® and various 3D graphics cards now available for personal computers. It is now possible to interact with exciting 3D animations and simulations on relatively inexpensive computer graphics systems in your home or office. 
     Shadows are important for creating realistic images and providing the viewer with visual cues about where objects appear relative to one another. Many different shadowing techniques are known. See, for example, Woo et al., “A Survey of Shadow Algorithms,”  IEEE Computer Graphics and Applications,  Volume 10, Number 6, pages 13-32 (November 1990). 
     A problem graphics system designers confronted in the past was how to draw shadows using low cost graphics systems. One known technique for accomplishing this is called shadow mapping. This technique allows a common z-buffer-based renderer to be used to generate shadows quickly on arbitrary objects. See Williams “Casting Curved Shadows on Curved Surfaces,”  Computer Graphics  ( SIGGRAPH &#39; 78  Proceedings ), Volume 12, Number 3, pages 270-274 (August 1978). Using this technique, the graphics system renders the scene using the z-buffer algorithm with respect to the position and direction of the light source. For each pixel in the z buffer, the resulting rendered z depth contains the distance to the object that is closest to the light source. This depth map is called a shadow map. The scene is then rendered a second time, but this time with respect to the viewer (camera). As each drawing primitive is being rendered, its location (depth from the light) is compared to the shadow map. If a rendered point is further away from the light source than the value in the shadow map, that point is in shadow and its brightness is attenuated. If the rendered point is closer to the light source than the shadow map value, the point is illuminated by the light and is not in shadow. 
     One efficient way to implement this shadow mapping technique is by exploiting texture mapping hardware to project the shadow map into the scene. See, e.g., Heidrich et al., “Applications of Pixel Textures in Visualization and Realistic Image Synthesis,”  Proceedings  1999  Symposium On Interactive  3D  Graphics,  pages 127-134 (April 1999); Segal et al., “Fast Shadows and Lighting Effects Using Texture Mapping,”  Computer Graphics  ( SIGGRAPH &#39; 92  Proceedings,  Volume 26, Number 2, pages 249-252 (July 1992). Using these techniques, the shadow map can be generated using z buffering (that is, lighting, texturing and the writing of color values into the color buffer can be turned off). Then, the scene is rendered from the viewer using only ambient lighting to resolve visibility. A shadow testing step is then performed to compare the z value in the z buffer with the z value (which is transformed from the coordinate system of the light source into the coordinate system of the viewer) in the shadow map. One technique is to set an additional value in the frame buffer for each pixel based on the result of the shadow comparison at that pixel. The whole scene is then rendered using the entire lighting equation—with the final color of each pixel being the color from the ambient lighting pass plus the color from the full rendering pass multiplied by the additional value in the frame buffer. 
     An extension of Williams&#39; shadow mapping technique proposed by Wang et al., “Second-Depth Shadow Mapping” (Department of Computer Science, University of North Carolina at Chapel Hill) solves certain self-shadowing problems (where a surface may cast a shadow onto itself due to lack of precision in the shadow comparison) by performing the shadow comparison based on the depth of a second surface defined by a primitive. Wang et al thus suggest using front-faced culling techniques to eliminate the first surface of primitives when generating the shadow map. This prevents limited precision depth comparisons from causing front surfaces to cast shadows upon themselves. 
     The above-described shadow mapping techniques allow general-purpose graphics hardware to render arbitrary shadows. However, using these techniques, the quality of the shadow produced depends on the resolution (in pixels) of the shadow map, and also on the numerical precision of the z buffer and the depth comparison. See Moller et al.,  Real-Time Rendering,  pages 179-183 (AK Peters Ltd., 1999). Achieving adequate numerical precision for the depth comparison can be a problem for low cost graphics systems such as video game platforms. In full scene shadowing, any object can cast a shadow on any object (including itself). The number of bits of information used to encode the distance value will determine where the near and far planes can be on the projection from the light source, and how much depth complexity can be provided in the rendered shadow map. To find out whether a surface is in shadow or outside of shadow, a depth comparison is performed between the actual distance from the light to the surface being rendered, and the nearest distance from the light (determined by rendering the scene from the light source into the shadow map). The number of bits in this distance value will determine the range that a particular light can cast shadows into the scene. The lower the precision, the less depth complexity that can be provided on the shadows and on the light. Hence, lower precision can limit the number of shadows the light can cast into the scene and how far ranging those shadows can be. 
     If the graphics pipeline does not provide sufficient numerical precision for shadow mapping effects, higher precision depth values can usually be obtained by having the graphics system host processor perform necessary calculations under software control. However, this places substantial additional loading on the host processor, and may make it difficult or impossible to render full-scene shadows in real time within the context of an interactive animated computer graphics system that allows the user to change the position(s) of one or more objects within the scene at will. 
     Another way to get around the limited precision problem is to use a form of shadow mapping which does not attempt the shadow depth comparison, but works instead by identifying what is seen by the light. See, e.g., Hourcade et al, “Algorithms for Antialiased Cast Shadows”,  Computers and Graphics,  vol. 9, no. 3, pp. 259-265 (1985). If an object is seen from the selected viewpoint and the shadow map indicates that the object is also seen by the light, then the object is illuminated. This technique has the advantage of avoiding the shadow depth comparison. However, areas where objects or polygons meet can be problematic. It is possible to resolve such problem areas by using different identifiers for different objects—although an object with a single identifier can never cast a shadow upon itself using this algorithm. 
     While much work has been done in the past, further improvements are possible. 
     The present invention solves the numerical precision problem while providing techniques and arrangements that perform full scene shadow mapping using low cost, limited precision hardware such as that found, for example, in home video game platforms and personal computer graphics accelerators. 
     One aspect of the invention uses a texture coordinate generator to assist in calculating distance between light position and a primitive surface at a precision that is based on the dynamic depth of the scene. A texture mapper uses the generated texture coordinates to look up a precision distance value from a ramp function stored as a texture. The resulting precision distance value can be compared with the corresponding depth value in the shadow map to determine whether or not the pixel is in shadow. 
     In one embodiment, the ramp function is stored as a 2-D texture in such a way that certain texels are redundant and not all texels are used. To eliminate lookup errors, redundant texel values are provided where the ramp function crosses texel row/column boundaries. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features and advantages provided by the invention will be better and more completely understood by referring to the following detailed description of presently preferred embodiments in conjunction with the drawings. The file of this patent contains at least one drawing executed in color. Copies of this patent with color drawing(s) will be provided by the Patent and Trademark Office upon request and payment of the necessary fee. The drawings are briefly described as follows: 
     FIG. 1 is an overall view of an example interactive computer graphics system; 
     FIG. 2 is a block diagram of the FIG. 1 example computer graphics system; 
     FIG. 3 is a block diagram of the example graphics and audio processor shown in FIG. 2; 
     FIG. 4 is a block diagram of the example 3D graphics processor shown in FIG. 3; 
     FIG. 5 is an example logical flow diagram of the FIG. 4 graphics and audio processor; 
     FIG. 6 shows an example shadow mapping procedure; 
     FIG. 7 shows an example more detailed shadow mapping procedure; 
     FIG. 8 shows example second-pass shadow mapping pipeline processing; 
     FIGS. 9A and 9B show example ramp lookup tables; 
     FIG. 10 shows an example technique for dynamically generating the FIG. 9A ramp lookup table; 
     FIG. 11 shows an example scaling operation; 
     FIGS. 12A and 12B show further embodiments of example ramp lookup tables that eliminate errors by incorporating redundant lookup values; 
     FIGS. 13A and 13B show an example shadow mapping procedure implementation; 
     FIG. 14 shows example recirculating shader stage configuration; 
     FIG. 15 shows example full scene shadow mapping image results; 
     FIG. 16 shows an example alternate shadow mapping technique based on IDs; and 
     FIGS. 17A and 17B show example alternative compatible implementations. 
    
    
     DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS OF THE INVENTION 
     FIG. 1 shows an example interactive 3D computer graphics system  50 . System  50  can be used to play interactive 3D video games with interesting stereo sound. It can also be used for a variety of other applications. 
     In this example, system 50 is capable of processing, interactively in real time, a digital representation or model of a three-dimensional world. System  50  can display some or all of the world from any arbitrary viewpoint. For example, system  50  can interactively change the viewpoint in response to real time inputs from handheld controllers  52   a,    52   b  or other input devices. This allows the game player to see the world through the eyes of someone within or outside of the world. System  50  can be used for applications that do not require real time 3D interactive display (e.g., 2D display generation and/or non-interactive display), but the capability of displaying quality 3D images very quickly can be used to create very realistic and exciting game play or other graphical interactions. 
     To play a video game or other application using system  50 , the user first connects a main unit  54  to his or her color television set  56  or other display device by connecting a cable  58  between the two. Main unit  54  produces both video signals and audio signals for controlling color television set  56 . The video signals are what controls the images displayed on the television screen  59 , and the audio signals are played back as sound through television stereo loudspeakers  61 L,  61 R. 
     The user also needs to connect main unit  54  to a power source. This power source may be a conventional AC adapter (not shown) that plugs into a standard home electrical wall socket and converts the house current into a lower DC voltage signal suitable for powering the main unit  54 . Batteries could be used in other implementations. 
     The user may use hand controllers  52   a ,  52   b  to control main unit  54 . Controls  60  can be used, for example, to specify the direction (up or down, left or right, closer or further away) that a character displayed on television  56  should move within a 3D world. Controls  60  also provide input for other applications (e.g., menu selection, pointer/cursor control, etc.). Controllers  52  can take a variety of forms. In this example, controllers  52  shown each include controls  60  such as joysticks, push buttons and/or directional switches. Controllers  52  may be connected to main unit  54  by cables or wirelessly via electromagnetic (e.g., radio or infrared) waves. 
     To play an application such as a game, the user selects an appropriate storage medium  62  storing the video game or other application he or she wants to play, and inserts that storage medium into a slot  64  in main unit  54 . Storage medium  62  may, for example, be a specially encoded and/or encrypted optical and/or magnetic disk. The user may operate a power switch  66  to turn on main unit  54  and cause the main unit to begin running the video game or other application based on the software stored in the storage medium  62 . The user may operate controllers  52  to provide inputs to main unit  54 . For example, operating a control  60  may cause the game or other application to start. Moving other controls  60  can cause animated characters to move in different directions or change the user&#39;s point of view in a 3D world. Depending upon the particular software stored within the storage medium  62 , the various controls  60  on the controller  52  can perform different functions at different times. 
     Example Electronics of Overall System 
     FIG. 2 shows a block diagram of example components of system  50 . The primary components include: 
     a main processor (CPU)  110 , 
     a main memory  112 , and 
     a graphics and audio processor  114 . 
     In this example, main processor  110  (e.g., an enhanced IBM Power PC 750) receives inputs from handheld controllers  108  (and/or other input devices) via graphics and audio processor  114 . Main processor  110  interactively responds to user inputs, and executes a video game or other program supplied, for example, by external storage media  62  via a mass storage access device  106  such as an optical disk drive. As one example, in the context of video game play, main processor  110  can perform collision detection and animation processing in addition to a variety of interactive and control functions. 
     In this example, main processor  110  generates 3D graphics and audio commands and sends them to graphics and audio processor  114 . The graphics and audio processor  114  processes these commands to generate interesting visual images on display  59  and interesting stereo sound on stereo loudspeakers  61 R,  61 L or other suitable sound-generating devices. 
     Example system  50  includes a video encoder  120  that receives image signals from graphics and audio processor  114  and converts the image signals into analog and/or digital video signals suitable for display on a standard display device such as a computer monitor or home color television set  56 . System  50  also includes an audio codec (compressor/decompressor)  122  that compresses and decompresses digitized audio signals and may also convert between digital and analog audio signaling formats as needed. Audio codec  122  can receive audio inputs via a buffer  124  and provide them to graphics and audio processor  114  for processing (e.g., mixing with other audio signals the processor generates and/or receives via a streaming audio output of mass storage access device  106 ). Graphics and audio processor  114  in this example can store audio related information in an audio memory  126  that is available for audio tasks. Graphics and audio processor  114  provides the resulting audio output signals to audio codec  122  for decompression and conversion to analog signals (e.g., via buffer amplifiers  128 L,  128 R) so they can be reproduced by loudspeakers  61  L,  61  R. 
     Graphics and audio processor  114  has the ability to communicate with various additional devices that may be present within system  50 . For example, a parallel digital bus  130  may be used to communicate with mass storage access device  106  and/or other components. A serial peripheral bus  132  may communicate with a variety of peripheral or other devices including, for example: 
     a programmable read-only memory and/or real time clock  134 , 
     a modem  136  or other networking interface (which may in turn connect system  50  to a telecommunications network  138  such as the Internet or other digital network from/to which program instructions and/or data can be downloaded or uploaded), and 
     flash memory  140 . 
     A further external serial bus  142  may be used to communicate with additional expansion memory  144  (e.g., a memory card) or other devices. Connectors may be used to connect various devices to busses  130 ,  132 ,  142 . 
     Example Graphics And Audio Processor 
     FIG. 3 is a block diagram of an example graphics and audio processor  114 . Graphics and audio processor  114  in one example may be a single-chip ASIC (application specific integrated circuit). In this example, graphics and audio processor  114  includes: 
     a processor interface  150 , 
     a memory interface/controller  152 , 
     a 3D graphics processor  154 , 
     an audio digital signal processor (DSP)  156 , 
     an audio memory interface  158 , 
     an audio interface and mixer  160 , 
     a peripheral controller  162 , and 
     a display controller  164 . 
     3D graphics processor  154  performs graphics processing tasks. Audio digital signal processor  156  performs audio processing tasks. Display controller  164  accesses image information from main memory  112  and provides it to video encoder  120  for display on display device  56 . Audio interface and mixer  160  interfaces with audio codec  122 , and can also mix audio from different sources (e.g., streaming audio from mass storage access device  106 , the output of audio digital signal processor (DSP)  156 , and external audio input received via audio codec  122 ). Processor interface  150  provides a data and control interface between main processor  110  and graphics and audio processor  114 . 
     Memory interface  152  provides a data and control interface between graphics and audio processor  114  and memory  112 . In this example, main processor  110  accesses main memory  112  via processor interface  150  and memory interface  152  that are part of graphics and audio processor  114 . Peripheral controller  162  provides a data and control interface between graphics and audio processor  114  and the various peripherals mentioned above. Audio memory interface  158  provides an interface with audio memory  126 . 
     Example Graphics Pipeline 
     FIG. 4 shows a more detailed view of an example 3D graphics processor  154 . 3D graphics processor  154  includes, among other things, a command processor  200  and a 3D graphics pipeline  180 . Main processor  110  communicates streams of data (e.g., graphics command streams and display lists) to command processor  200 . Main processor  110  has a two-level cache  115  to minimize memory latency, and also has a write-gathering buffer  111  for uncached data streams targeted for the graphics and audio processor  114 . The write-gathering buffer  111  collects partial cache lines into full cache lines and sends the data out to the graphics and audio processor  114  one cache line at a time for maximum bus usage. 
     Command processor  200  receives display commands from main processor  110  and parses them—obtaining any additional data necessary to process them from shared memory  112 . The command processor  200  provides a stream of vertex commands to graphics pipeline  180  for 2D and/or 3D processing and rendering. Graphics pipeline  180  generates images based on these commands. The resulting image information may be transferred to main memory  112  for access by display controller/video interface unit  164 —which displays the frame buffer output of pipeline  180  on display  56 . 
     FIG. 5 is a logical flow diagram of graphics processor  154 . Main processor  110  may store graphics command streams  210 , display lists  212  and vertex arrays  214  in main memory  112 , and pass pointers to command processor  200  via bus interface  150 . The main processor  110  stores graphics commands in one or more graphics first-in-first-out (FIFO) buffers  210  it allocates in main memory  110 . The command processor  200  fetches: 
     command streams from main memory  112  via an on-chip FIFO memory buffer  216  that receives and buffers the graphics commands for synchronization/flow control and load balancing, 
     display lists  212  from main memory  112  via an on-chip call FIFO memory buffer  218 , and 
     vertex attributes from the command stream and/or from vertex arrays  214  in main memory  112  via a vertex cache  220 . 
     Command processor  200  performs command processing operations  200   a  that convert attribute types to floating point format, and pass the resulting complete vertex polygon data to graphics pipeline  180  for rendering/rasterization. A programmable memory arbitration circuitry  130  (see FIG. 4) arbitrates access to shared main memory  112  between graphics pipeline  180 , command processor  200  and display controller/video interface unit  164 . 
     FIG. 4 shows that graphics pipeline  180  may include: 
     a transform unit  300 , 
     a setup/rasterizer  400 , 
     a texture unit  500 , 
     a texture environment unit  600 , and 
     a pixel engine  700 . 
     Transform unit  300  performs a variety of 2D and 3D transform and other operations  300   a  (see FIG.  5 ). Transform unit  300  may include one or more matrix memories  300   b  for storing matrices used in transformation processing  300   a . Transform unit  300  transforms incoming geometry per vertex from object space to screen space; and transforms incoming texture coordinates and computes projective texture coordinates ( 300   c ). Transform unit  300  may also perform polygon clipping/culling  300   d.  Lighting processing  300 e also performed by transform unit  300   b  provides per vertex lighting computations for up to eight independent lights in one example embodiment. Transform unit  300  can also perform texture coordinate generation ( 300   c ) for embossed type bump mapping effects, as well as polygon clipping/culling operations ( 300   d ). 
     Setup/rasterizer  400  includes a setup unit which receives vertex data from transform unit  300  and sends triangle setup information to one or more rasterizer units ( 400   b ) performing edge rasterization, texture coordinate rasterization and color rasterization. 
     Texture unit  500  (which may include an on-chip texture memory (TMEM)  502 ) performs various tasks related to texturing including for example: 
     retrieving textures  504  from main memory  112 , 
     texture processing ( 500 a) including, for example, multi-texture handling, post-cache texture decompression, texture filtering, embossing, shadows and lighting through the use of projective textures, and BLIT with alpha transparency and depth, 
     bump map processing for computing texture coordinate displacements for bump mapping, pseudo texture and texture tiling effects ( 500   b ), and 
     indirect texture processing ( 500   c ). 
     Texture unit  500  outputs filtered texture values to the texture environment unit  600  for texture environment processing ( 600   a ). Texture environment unit  600  blends polygon and texture color/alpha/depth, and can also perform texture fog processing ( 600   b ) to achieve inverse range based fog effects. Texture environment unit  600  can provide multiple stages to perform a variety of other interesting environment-related functions based for example on color/alpha modulation, embossing, detail texturing, texture swapping, clamping, and depth blending. 
     Pixel engine  700  performs depth (z) compare ( 700   a ) and pixel blending ( 700   b ). In this example, pixel engine  700  stores data into an embedded (on-chip) frame buffer memory  702 . Graphics pipeline  180  may include one or more embedded DRAM memories  702  to store frame buffer and/or texture information locally. Z compares  700   a′ can also be performed at an earlier stage in the graphics pipeline  180  depending on the rendering mode currently in effect (e.g., z compares can be performed earlier if alpha blending is not required). The pixel engine  700  includes a copy operation  700   c  that periodically writes on-chip frame buffer  702  to main memory  112  for access by display/video interface unit  164 . This copy operation  700   c  can also be used to copy embedded frame buffer  702  contents to textures in the main memory  112  for dynamic texture synthesis effects. Anti-aliasing and other filtering can be performed during the copy-out operation. The frame buffer output of graphics pipeline  180  (which is ultimately stored in main memory  112 ) is read each frame by display/video interface unit  164 . Display controller/video interface  164  provides digital RGB pixel values for display on display  102 . 
     Example Shadow Mapping Technique 
     In the example embodiment, transform unit  300  includes lighting calculation hardware that may be programmed to calculate a lighting equation including a distance attenuation factor. See, for example, commonly-assigned U.S. provisional Application No. 60/227,007, filed Aug. 23, 2000 and its corresponding utility application Ser. No. 09/726,216, filed Nov. 28, 2000 (atty. dkt. no. 723-967), both entitled “Achromatic Lighting Functions In A Graphics System And Method” (Attorney Docket No. 723-748). Such lighting calculation can be used to determine the distance from a light to an arbitrary surface in the scene being rendered. Because the light intensity is attenuated (e.g., linearly) with distance, the resulting brightness value at a rendered surface can be used as an indication of the distance from the light to the surface. This computed depth value can be compared with a shadow map depth value to determine whether or not to shadow a pixel. 
     In the example embodiment, the lighting calculation output is eight bits wide. For example, this value might be contained in a single color component (e.g., the red component of an RGB triplet). While 8-bit precision is perfectly acceptable for lighting operations, it may be insufficient for performing the comparison with shadow map depth to provide a full scene shadowing technique for scenes having arbitrary shadow complexity. While it would be possible to modify transform unit  300  to provide more than eight bits per channel, this would increase the cost and complexity of the transform unit. 
     Texture unit  500  following the transform unit  300  in graphics pipeline  180  has substantial precision per color channel, and this precision is propagated through graphics pipeline  180 . Texture unit  500  is not a general purpose processor, but rather, in the preferred embodiment is designed to perform texture mapping functions. We discovered a way to use the texture coordinate generating hardware to develop a distance and then, once that distance is in the texture coordinate(s), to sample a texture and get more than eight bits of precision. For example, in one implementation we can obtain sixteen bits up to even nineteen bits of accuracy in this particular embodiment for shadow comparisons. 
     In more detail, we have discovered an alternative technique that allows us to use the higher precision texture coordinate generator and texture mapping unit  500  of system  50  to increase the precision of depth values calculated by graphics pipeline  180  for comparison with a shadow map. FIG. 6 shows an example routine  1000  that system  50  can provide full-scene shadow mapping with higher numerical precision depth comparisons. In this example, graphics pipeline  180  generates a shadow map depth texture in a first pass (block  1002 ). In a second pass, the shadow map depth texture is projected into the scene using conventional texture projection techniques (block  1002 ). Also in the second pass, transform unit  300  uses texture coordinate generation and texture mapping to determine a precision numerical value representing the distance from a light position to surfaces being rendered (block  1004 ). 
     In more detail, we control the transform unit  300  to generate texture coordinates representing the distance from the surface to the light (e.g., using a combination of a modelview transformation of vertex position from the light position, and an additional scaling transformation that takes the dynamic range of scene depth into account). We then apply the resulting texture coordinates to texture unit  500  for the purpose of looking up higher precision depth values stored in a ramp function table that a texture unit can process as an ordinary texture (block  1006 ). The high precision distance values so obtained can be compared to corresponding shadow map depth values to determine whether or not to shadow pixels (block  1008 ). System  50  can then display full-scene shadowed images produced by this technique (block  1010 ; see FIG. 15 for an example image). 
     More Detailed Example Implementation 
     FIG. 7 shows an example more detailed procedure  1020  for operating system  50  to provide the full-scene shadowed images, and FIG. 8 shows an example second pass graphics pipeline processing to provide full-scene shadowed images. FIG. 7 shows a shadow map generated in the first pass by rendering the scene depth from the light source viewpoint into the z buffer portion of embedded frame buffer  702  (block  1022 ). See commonly-assigned copending application provisional Application No. 60/226,900, filed Aug. 23, 2000 and its corresponding utility application Ser. No. 09/726,226, filed Nov. 28, 2000 (atty. dkt. no. 723-964), both entitled “Method And Apparatus For Anti-Aliasing In A Graphics System.” The example embodiment activates front-face culling during this operation to implement the Wang et al., “second depth” algorithm discussed above. The resulting depth map stored in the embedded frame buffer  702  is copied out into a texture by pixel engine  700  (block  1024 ). See commonly-assigned provisional Application No. 60/227,030, filed Aug. 23, 2000 and its corresponding utility application Ser. No. 09/722,663, filed Nov. 28, 2000 (atty. dkt. no. 723-963), both entitled “Graphics System With Copy Out Conversions Between Embedded Frame Buffer And Main Memory.” To increase efficiency, block  1022  can be performed with color frame buffer updates off since we are only concerned about generating depth values. 
     During a second pass, the scene is rendered from the viewpoint of the camera (i.e., the user) (block  1026 ). For each pixel, an appropriate texture projection matrix is applied to project the shadow map texture into the scene (block  1026 ). Additionally, the position of the vertex in object space is transformed using a modelview matrix defining a viewpoint at the light source—converting from object space to light space (the same space the shadow map was rendered in) (block  1028 ). The resulting z value represents the distance from the surface to the light. 
     In the example embodiment, the texture coordinate generation process performs an additional transformation on this depth value. This additional transformation is based on the dynamic depth range of the scene. The texture coordinate generation process is used, in the example embodiment, to transform the calculated distance value based on the dynamic distance range (i.e., the difference between the closest surface the camera can “see” and the furthest-away surface the camera can see). See FIG.  11 . 
     The result of the second transformation provides a texture coordinate(s) for indexing into a lookup table ramp “texture” that defines a (e.g., linearly) increasing depth ramp (block  1030 ). In the example embodiment, the texture unit  500  treats the ramp lookup table as any other texture and performs a normal texture mapping operation on it based on the texture coordinate(s) generated by transform unit  300 . See, for example, commonly-assigned U.S. provisional Application No. 60/226,891, filed Aug. 23, 2000 and its corresponding utility application Ser. No. 09/722,382, filed Nov. 28, 2000, (atty. dkt. no. 723-961), both entitled “Method And Apparatus For Direct and Indirect Texture Processing In A Graphics System.” However, resulting texel values are not used to immediately modify surface color or opacity. Rather, they provide precision depth values that are compared with corresponding projected shadow map depth values to make shadowing decisions (block  1032 ). 
     The ramp function might ideally be stored in a one-dimensional texture having sufficient resolution. However, many low cost graphics systems have a limitation on the maximum size of textures they can handle. For example, in one implementation, system  50  can handle textures with a maximum size of 1024×1024 texels. A one-dimensional 1024-texel ramp function texture provides 10 bits of numerical precision. This is better than 8 bits, but we want to do better. Therefore, we use texture coordinate generation to encode the depth value (distance from light to surface) in the combined values of two texture coordinates (s, t)—and we store the ramp function into a two-dimensional texture of a size that provides sufficient precision to suit our needs. For example, a 1024×1024 two-dimensional texture can provide (nearly) double the precision as compared to a 1024-texel one-dimensional texture. To maximize precision, we can also dynamically transform the depth value based on the depth range of the scene so that none of the 2D ramp texture values are wasted on distances that will never be accessed. 
     Referring again to FIG. 7, once we have used the texture coordinate generation and texture mapping process to obtain a surface depth value, we can compare this depth value with a corresponding depth value in the shadow map. If the shadow map depth is less than the pixel depth relative to the light, the pixel is in shadow and its brightness is attenuated (“yes” exit to decision block  1032 , block  1034 ). Operations  1026 - 1034  are performed for each pixel being rendered. When all pixels on all surfaces have been processed (“no” exit to decision block  1036 ), the full-scene shadowed image is complete and may be displayed (block  1038 ). 
     FIG. 8 shows operations performed by graphics pipeline  180  during the second pass. In this example, transform unit  300  performs a conventional modelview transform based on a camera at the viewer&#39;s viewpoint, and also lights incoming vertex definitions (block  1050 ) to provide the results to rasterizer  400  (block  1052 ) for production of rasterized Gouraud-shaded pixel color/alpha/z. In the example embodiment, these values are provided to texture environment unit  600  for blending (block  1054 ). In this example, a predetermined shadow color value is also provided to the blend operation ( 1054 ). 
     A conventional projective texture transformation (orthographic or perspective, depending on the preference of the artist who designed the scene) is performed by transform unit  300  (block  1056 ) to generate texture coordinates (block  1058 ). The texture unit  500  performs the corresponding texture mapping operation to project the shadow map texture onto the scene. The resulting depth values are supplied to a compare operation ( 1060 )—which in the example embodiment is performed by texture environment unit  600 . 
     The pixel depth value is provided by blocks  1062 ,  1064 ,  1066  via transform unit  300  and texture unit  500 . As explained above, a modelview transformation performed by transform unit  300  determines pixel depth from the light (block  1062 ) by transforming the location of the pixel to the location of the light that is casting the shadow. A further transform associated with texture coordinate generation can be used to transform the resulting z (depth) value based on the depth range of the scene (block  1064 ). These two transformations ( 1062 ,  1064 ) can be combined and performed by a single matrix multiplication that generates texture coordinates s, t. The resulting texture coordinates s, t are used by texture unit  500  to perform a texture mapping operation that looks up a precision distance value from a ramp texture lookup table (block  1066 ). This resulting value is used by compare block  1060  to compare with the shadow map depth. 
     Compare operation  1060  compares the shadow map depth value with a pixel depth value relative to the position of the light casting the shadow. The output of the comparison is provided to blend operation ( 1054 ) to control whether or not the predetermined shadow color is to be used instead of the rasterized shaded pixel color. In another embodiment, the blend operation  1054  could modulate (e.g., attenuate) the pixel color to darken pixels that the depth comparison reveals are in shadow. 
     Example Ramp Textures 
     As explained above, using one example implementation of system  50 , encoding the pixel distance value in a single texture coordinate to look up a one-dimensional texture providing an identity (i.e., pass-through) would provide ten bits of distance precision based on a maximum texture size of 1024 by 1024 texels. Using two texture coordinates (e.g., with one coordinate being a scaled version of the other) provides a way to propagate a ramp distance function through all (or most) of the texels in a two-dimensional texture. FIGS. 9A and 9A show illustrative example two-dimensional ramp textures for 8-bit (16×16) and 16-bit (256×256) texture formats respectively. Notice that in each case, a linearly increasing ramp function appears down each column of the texture. The ramp function is continued in successive columns to provide a continuous ramp function from a minimum value (e.g., 00 or 0000) to a maximum value (e.g., 0×FF or 0×FFFF). Such ramp textures can be dynamically generated by main processor  110  using a procedure (see FIG.  10 ), or they may be defined statically beforehand and simply loaded from mass storage when needed. In these examples, a particular texture column is selected by the S coordinate, and the T texture coordinate selects a particular distance-valued texel within the selected column. 
     The illustrative ramp textures allow two texture coordinates S, T to encode a precision distance value that can be generated through a texture mapping operation —providing essentially double the precision that would be available by using a single texture coordinate. For example, using the texture mapping operation based on the example textures shown in FIGS. 9A &amp; 9B produces the following correspondence between input texture coordinate values and output texel (depth) values: 
     
       
         
           
               
               
               
             
               
                   
                   
               
               
                   
                 S, T 
                 Texel Value 
               
               
                   
                   
               
             
            
               
                   
                 0, 0 
                 00 
               
               
                   
                 0, 1 
                 01 
               
               
                   
                 0, 2 
                 02 
               
               
                   
                 0, 3 
                 03 
               
               
                   
                 so on 
                 so on 
               
               
                   
                   
               
            
           
         
       
     
     In the example embodiment, each texel is interpreted as a numerical value. For example, 16-bit wide texels can be interpreted as 16-bit numbers. When texture mapping, the nearest sample value should be selected (e.g., bilinear interpolation should not be used). 
     FIG. 11 shows an example technique that uses the texture coordinate generating hardware within transform unit  300  to generate texture coordinates s, t representing distance for lookup into a ramp texture such as that shown in FIGS. 9A and 9B. In the example embodiment, a camera is configured to provide a certain angle of view, a certain position and a certain distance range. The camera distance range is parameterized by a nearz value (N) and a farz value (F)—with the depth range of the camera being specified by the distance between farz and nearz (i.e., F-N). These range parameters are used to transform a vertex position to provide an overall transformation of the vertex position from object space to light space that spreads out the dynamic range of the numerical precision the ramp provides so that it exactly “fits” the scene&#39;s depth range. 
     The particular function used in the example embodiment for transforming the distance into a pair of texture coordinates is different depending upon whether the projection is perspective or orthographic. The following shows example computations for these two cases: 
     &lt;Perspective&gt;         s   =       F     F   -   N       +       1   z          NF     F   -   N             ,                t   =     s   ×   scale                       
     example scale=16 (8bit mode)/256 (16bit mode)        M   =     (         0       0         F     F   -   N             NF     F   -   N               0       0         Sc   ·     F     F   -   N               Sc   ·     NF     F   -   N                 0       0       1       0         )                     
     The s becomes 0 when z=−N and 1 when z=−F. 
     &lt;Orthographic&gt;         s   =       -     z     F   -   N         -     N     F   -   N           ,                t   =     s   ×   scale                       
     example scale=16 (8 bit mode)/ 256 (16 bit mode)        M   =     (         0       0           -   1       F   -   N               -   N       F   -   N               0       0         Sc   ·       -   1       F   -   N               Sc   ·       -   N       F   -   N                 0       0       0       1         )                     
     The s becomes 0 when z=−N and 1 when z=−F. 
     Note that the z value is present in both of these computations. The resulting transformation matrices shown above convert the z value derived from a modelview transformation based on position of the light into s, t texture coordinates used to index into the ramp textures such as those shown in FIG. 9A,  9 B. Because the transform unit  300  provides relatively high-precision texture generation computations (especially when two texture coordinates are being generated in parallel) and the example ramp textures provide relatively high-precision lookup values (especially in the case where two texture coordinates are used to map a two-dimensional texture), relatively high-precision pixel depth values can be provided using this technique. 
     Although the precision provided is now adequate, ambiguities in the 2-D texture mapping process can occur which will occasionally cause an incorrect value to be selected from the ramp texture. In the ramp texture examples shown in FIGS. 9A and 9B, it is possible to cross a boundary and “jump” to an incorrect value, which may introduce artifacts into the shadow comparison. For example, errors can be introduced by providing the ramp function across the entire width of the texture such that every time the system traverses a texture it moves on to the next adjacent row or column of the texture. To eliminate these errors, it is possible to use a modified ramp texture containing a smaller number of unique depth values and stores redundant values to eliminate lookup errors. FIGS. 12A and 12B are examples of such ramp textures. 
     The ramp textures shown in FIGS. 12A and 12B provide a shallow ramp function which spans straight across the texture map in a one-dimensional format but which, every once in a while (i.e., where the ramp “crosses” between texture rows), provides an identical sample on the next line down. As shown in FIGS. 12A and 12B, a modified approach is to move down to the next line in the texture whenever the system traverses a full line minus one texel in the line. For example, referring to the simplified ramp texture of FIG. 12A, because the period of the ramp is not exactly the width of the texture, it is possible to replicate a certain number of texels on each adjacent one-dimensional texture line. Thus, in the example ramp texture shown in FIGS. 12A and 12B, not all of the texels represent unique values because some of them are replicated. Furthermore, some of the texels may contain null values because the ramp function will access them. In these examples, not every texel is used but very nearly every texel can be used. The equation used to calculate depth values shown above is used to come up with a shallow line that has exactly the right slope to traverse the texture. The texture generating hardware within transform unit  300  in the example embodiment and the texture sampling in generation/rasterizing of the texture coordinates are accurate enough to maintain this slope and not introduce any artifacts through the texture. The example ramp textures shown in FIGS. 12A,  12 B therefore introduce no errors under these conditions, and the shadow comparison is free from artifacts resulting from ramp texture lookup errors. 
     Example Detailed Implementation 
     FIGS. 13A and 13B show an example more complete detailed implementation of an application executing in main processor  110  to perform shadow mapping on system  50 . Procedure  1100  begins by initializing system  50  (block  1102 ), including vertex formats and scene parameters including lights (block  1104 ). Routine  1100  then creates or retrieves one or more ramp textures as described above (block  1106 ), and prepares for a first pass operation by turning off the display mode and reserving memory for a dynamic shadow map (block  1108 ). System  50  then disables the color frame buffer update (only z will be updated) (block  1110 ), and sets up the shader within texture environment unit  600  for a first pass operation to generate the shadow depth map (block  1112 ). System  50  enables front face culling to draw only “second” surfaces from the light (block  1114 ) and sets the viewport to the light position (block  1116 ). Graphics pipeline  180  then draws the scene, performing hidden surface removal to create a depth map within the z buffer (block  1118 ). Once the scene is drawn from the position of the light, the z buffer depth map (which now contains the depths of all closest “second” surfaces of all primitives within the scene relative to the light position), system  50  copies the z buffer into a texture and flushes the z buffer (block  1120 ). 
     Preparing for a second pass operation, system  50  enables color updates (block  1122 ) and turns off front-face culling (block  1124 ). System  50  then sets the viewport/camera/light for rendering the main image (block  1126 ), and sets up the shadow map texture (block  1128 ). System  50  sets the mode/stages for the recirculating shader within texture environment unit  600  for second pass parameters to draw the actual scene from the viewer with full-scene shadows (block  1130 ). As shown in FIG. 14, an example shader configuration will cause: 
     a stage zero shader operation to load the depth value from the ramp texture; 
     a stage one shader operation to compare the loaded ramp texture depth value with the shadow map depth value and output a 0 if the ramp texture depth value is greater than or equal to the shadow map depth value; and 
     in a stage two shader operation, outputting a preset shadow color if stage one produced zero and otherwise outputting the rasterized color. 
     For more information concerning the operation of the recirculating shader contained within texture environment unit  600 , see commonly assigned provisional Application No. 60/226,888, filed Aug. 23, 2000 and its corresponding utility application Ser. No. 09/722,367, filed Nov. 23,2000 (atty. dkt. no. 723-968), both entitled “Recirculating Shade Tree Blender For A Graphics System.” 
     Processor  110  then loads texture coordinate generation matrices for the shadow map texture projection and the ramp depth value texture lookup (block  1132 ). In one particular implementation, the following source code can be used to generate the shadow projection matrix and the depth lookup matrix (perspective or orthographic): 
     
       
         
           
               
             
               
                   
               
             
            
               
                 { 
               
            
           
           
               
               
            
               
                   
                 // Shadow projection matrix, Perspective projection 
               
               
                   
                 MTXLightFrustum( 
               
               
                   
                  proj, 
               
               
                   
                  - (cam-&gt;cfg.top), // t = −y in projected texture 
               
               
                   
                  cam-&gt;cfg.top, 
               
               
                   
                  cam-&gt;cfg.left, 
               
               
                   
                  - (cam-&gt;cfg.left), 
               
               
                   
                  cam-&gt;cfg.znear, 
               
               
                   
                  0.50F, 
               
               
                   
                  0.50F, 
               
               
                   
                  0.50F, 
               
               
                   
                  0.50F ); 
               
               
                   
                 // Depth lookup matrix, Perspective Projection 
               
               
                   
                 // in order to generate: 
               
               
                   
                 //  s = (1 + N/z) * F / (F − N) 
               
               
                   
                 //  t = s * tscale due to the texture size 
               
               
                   
                 MTXRowCol(dp, 0, 2) = f / range; 
               
               
                   
                 MTXRowCol(dp, 0, 3) = f * n / range; 
               
               
                   
                 MTXRowCol(dp, 1, 2) = MTXRowCol (dp, 0, 2) * tscale; 
               
               
                   
                 MTXRowCol(dp, 1, 3) = MTXRowCol (dp, 0, 3) * tscale; 
               
               
                   
                 MTXRowCol(dp, 2, 2) = 1.0F; 
               
            
           
           
               
            
               
                 } 
               
               
                 { 
               
            
           
           
               
               
            
               
                   
                 // Shadow projection matrix, Orthographic Projection 
               
               
                   
                 MTXLightOrtho ( 
               
               
                   
                  proj, 
               
               
                   
                  - (cam-&gt;cfg.top), // t = −y in projected texture 
               
               
                   
                  cam-&gt;cfg.top, 
               
               
                   
                  cam-&gt;cfg.left, 
               
               
                   
                  - (cam-&gt;cfg.left), 
               
               
                   
                  0.50F, 
               
               
                   
                  0.50F, 
               
               
                   
                  0.50F, 
               
               
                   
                  0.50F ); 
               
               
                   
                 // Depth lookup matrix, Orthographic projection 
               
               
                   
                 // in order to generate: 
               
               
                   
                 //  s = − (z + N) / (F − N) 
               
               
                   
                 //  t = s * tscale due to the texture size 
               
               
                   
                 MTXRowCol(dp, 0, 2) = − 1.0F / range; 
               
               
                   
                 MTXRowCol(dp, 0, 3) = − n / range; 
               
               
                   
                 MTXRowCol(dp, 1, 2) = MTXRowCol(dp, 0, 2) * tscale; 
               
               
                   
                 MTXRowCol(dp, 1, 3) = MTXRowCol(dp, 0, 3) * tscale; 
               
               
                   
                 MTXRowCol(dp, 2, 3) = 1.0F; 
               
            
           
           
               
            
               
                 } 
               
               
                 MTXConcat(proj, cam-&gt;view, tmo-&gt;texProj); 
               
               
                 MTXConcat(dp, cam-&gt;view, tmo-&gt;depth); 
               
               
                   
               
            
           
         
       
     
     The following source code example fragment sets up parameters for a light including an appropriate transformation matrix that is multiplied by the modelview matrix: 
     
       
         
           
               
             
               
                   
               
             
            
               
                 /*--------------------------------------------------------------------------- 
               
               
                 * 
               
            
           
           
               
               
               
            
               
                   
                 Name: 
                 SetLight 
               
               
                   
                 Description: 
                 Set up light parameters 
               
            
           
           
               
               
               
               
            
               
                   
                 Arguments: 
                 light 
                 : pointer to a MyLightObj structure 
               
               
                   
                   
                 v 
                 : view matrix 
               
               
                   
                 Returns: 
                 none 
               
            
           
           
               
            
               
                 /*--------------------------------------------------------------------------- 
               
               
                 / 
               
               
                 void SetLight( MyLightObj* light, Mtx v ) 
               
               
                 { 
               
            
           
           
               
               
            
               
                   
                 Vec 1pos = light-&gt;cam.cfg.location; 
               
               
                   
                 // Multipled by view matrix 
               
               
                   
                 MTXMultVec(v, &amp;1pos, &amp;1pos); 
               
               
                   
                 GxInitLightPos(&amp;light-&gt;1obj, 1pos.x, 1pos.y, 1pos.z); 
               
               
                   
                 GxInitLightColor(&amp;light-&gt;1obj, COL_LIGHT); 
               
               
                   
                 GXLoadLightObjImm(&amp;light-&gt;1obj, GX_LIGHT0); 
               
            
           
           
               
            
               
                 } 
               
               
                   
               
            
           
         
       
     
     Once the various transformation matrices are loaded, processor  110  then loads the shadow map and ramp textures into texture unit  500  (block  1134 ). Graphics pipeline  180  may now draw the scene (block  1136 ) which may be displayed (block  1138 ) upon completion. 
     As discussed above, the recirculating shader within texture environment unit  600  is used in the example embodiment to both perform the shadow depth comparison and blend in a shadow color based on the comparison results. Different recirculating shader stages may be used depending upon the particular precision of the shadow mapping operation being performed. 
     In one example implementation, an 8/16/24-bit comparator is available within the recirculating shader to compare the two depth values in a single stage. In embodiments where such a comparator is not available (e.g., only lower precision, 8-bit comparisons can be performed), the recirculating shader can still perform a higher precision comparison but may require multiple stages to do so. For example, one stage might be used to compare a most significant portion of each depth value (e.g., contained in one texel color component), and (an)other stage(s) might be used to compare a least significant position(s) of each value (e.g., contained in another texel color component). The following show some example recirculating shader configurations for different precision shader operations that require only an 8-bit comparator. In these examples, R 0  holds shadow color, RASC is lit vertex color, and RP=TEVPREV register. 
     &lt;Example 8bit comparison mode&gt; 
     Stage 0 : RP RGB =Ramp tex(I 8 ) 
     Stage 1 : RP RGB =RP RGB &gt;=Shadow ma PRRR ?0:255 
     Stage 2 : RP RGB =RP RGB ==0 ? RO RGB :RASC 
     &lt;Example 16bit comparison mode using a sequence of 8-bit compares&gt; 
     R 2 =constant {1, 1, 1, 1} 
     Stage 0 : R 1   RGB/A =Ramp tex(IA8) 
     Stage 1 : RP RGB =R1 RGB &gt;=Shadow ma PGGG ? 0:255 
     Stage 2 : R 1   RGB =R1 A −Shadow ma PRRR    
     Stage 3 : RP RGB =R 1   RGB −RP RGB ×R2 RGB &gt;=0 ?0:255 
     Stage 4 : RP RGB =RP RGB ==0 ?R0 RGB :RASC 
     &lt;Example 4 stage 16bit mode (may introduce some artifacts)&gt; 
     R 2 =constant {1, 1, 1, 1} 
     Stage 0 : R 1   RGB/A =(255−R2 RGB/A )×Ramp tex(IA8) 
     Stage 1 : RP RGB =R1 RGB− (255−R2 RGB )×Shadow map GGG &gt;=0 ?0:255 
     Stage 2 : RP RGB =R1 RGB −(255−R2 RGB )×Shadow ma PRRR −R2 RGB ×RP RGB &gt;=0 ? 0:255 
     Stage 3 : RP RGB =RP RGB ==0?R0 RGB :RASC 
     The following example source code provides additional details of how to control an example recirculating shader to perform desired comparisons and shadow drawing: 
     
       
         
           
               
             
               
                   
               
             
            
               
                 // ------------------------------------------- 
               
            
           
           
               
               
            
               
                   
                 // 8bit comparison version 
               
               
                   
                 // ------------------------------------------- 
               
               
                   
                 GXSetNumTevStages (3); 
               
               
                   
                 // TEV Stage 0 ( Loads a depth value from ramp texture ) 
               
               
                   
                 GXSetTevOrder(GX_TEVSTAGE0, GX_TEXCOORD1, 
               
            
           
           
               
               
            
               
                   
                 GX_TEXMAP1, GX_COLOR_NULL); 
               
            
           
           
               
               
            
               
                   
                 GXSetTevColorIn(GX_TEVSTAGE0, GX_CC_ZERO, GX_CC_ZERO, 
               
            
           
           
               
               
            
               
                   
                 GX_CC_ZERO, GX_CC_TEXC); 
               
            
           
           
               
               
            
               
                   
                 GXSetTevColorOp(GX_TEVSTAGE0, GX_TEV_ADD, GX_TB_ZERO, 
               
            
           
           
               
               
            
               
                   
                 GX_CS_SCALE 1, GX_TRUE, GX_TEVPREV); 
               
            
           
           
               
               
            
               
                   
                 GXSetTevClampMode(GX_TEVSTAGE0, GX_TC_LINEAR); 
               
               
                   
                 // TEV Stage 1 ( REGPREV &gt;= shadow map texture ? 0 : 255 ) 
               
               
                   
                 GXSetTevOrder(GX_TEVSTAGE1, GX_TEXCOORD0, 
               
            
           
           
               
               
            
               
                   
                 GX_TEXMAP0, GX_COLOR_NULL); 
               
            
           
           
               
               
            
               
                   
                 GXSetTevColorIn(GX_TEVSTAGE1, GX_CC_ZERO, GX_CC_TEXRRR, 
               
            
           
           
               
               
            
               
                   
                 GX_CC_ONE, GX_CC_CPREV); 
               
            
           
           
               
               
            
               
                   
                 GXSetTevColorOp(GX_TEVSTAGE1, GX_TEV_SUB, GX_TB_ZERO, 
               
            
           
           
               
               
            
               
                   
                 GX_CS_DIVIDE_2, GX_FALSE, GX_TEVPREV); 
               
            
           
           
               
               
            
               
                   
                 GXSetTevClampMode(GX_TEVSTAGE1, GX_TC_LE); 
               
               
                   
                 // TEV Stage 2 ( REGPREV == 0 ? shadow color : rasterized color ) 
               
               
                   
                 // Register 0 is supporsed to hold shadow color 
               
               
                   
                 GXSetTevOrder(GX_TEVSTAGE2, GX_TEXCOORD_NULL, 
               
            
           
           
               
               
            
               
                   
                 GX_TEXMAP_NULL, GX_COLOR0A0); 
               
            
           
           
               
               
            
               
                   
                 GXSetTevColorIn(GX_TEVSTAGE2, GX_CC_C0, GX_CC_RASC, 
               
            
           
           
               
               
            
               
                   
                 GX_CC_CPREV, GX_CC_ZERO); 
               
            
           
           
               
               
            
               
                   
                 GXSetTevColorOp(GX_TEVSTAGE2, GX_TEV_ADD, GX_TB_ZERO, 
               
            
           
           
               
               
            
               
                   
                 GX_CS_SCALE_1, GX_TRUE, GX_TEVPREV); 
               
            
           
           
               
               
            
               
                   
                 GXSetTevClampMode(GX_TEVSTAGE2, GX_TC_LINEAR); 
               
            
           
           
               
            
               
                 } 
               
               
                 else 
               
               
                 { 
               
            
           
           
               
               
            
               
                   
                 // ------------------------------------------- 
               
               
                   
                 // 16bit comparison version 
               
               
                   
                 // ------------------------------------------- 
               
               
                   
                 GxSetNumTevStages(5); 
               
               
                   
                 // REG2 = constant {1, 1, 1, 1} 
               
               
                   
                 GXSetTevColor(GX_TEVREG2, col_one); 
               
               
                   
                 // TEV Stage 0 ( Loads a depth value from ramp texture ) 
               
               
                   
                 // TEXA -&gt; REG1(A) / TEXC -&gt; REG1(C) 
               
               
                   
                 GxSetTevOrder(GX_TEVSTAGE0, GX_TEXCOORD1, 
               
            
           
           
               
               
            
               
                   
                 GX_TEXMAP1, GX_COLOR_NULL); 
               
            
           
           
               
               
            
               
                   
                 GxSetTevColorIn(GX_TEVSTAGE0, GX_CC_ZERO, GX_CC_ZERO, 
               
            
           
           
               
               
            
               
                   
                 GX_CC_ZERO, GX_CC_TEXC); 
               
            
           
           
               
               
            
               
                   
                 GXSetTevColorOp(GX_TEVSTAGE0, GX_TEV_ADD, GX_TB_ZERO, 
               
            
           
           
               
               
            
               
                   
                 GX_CS_SCALE_1, GX_TRUE, GX_TEVREG1); 
               
            
           
           
               
               
            
               
                   
                 GXSetTevAlphaIn(GX_TEVSTAGE0, GX_CA_ZERO, GX_CA_ZERO, 
               
            
           
           
               
               
            
               
                   
                 GX_CA_ZERO, GX_CA_TEXA); 
               
            
           
           
               
               
            
               
                   
                 GXSetTevAlphaOp(GX_TEVSTAGE0, GX_TEV_ADD, GX_TB_ZERO, 
               
            
           
           
               
               
            
               
                   
                 GX_CS_SCALE_1, GX_TRUE, GX_TEVREG1); 
               
            
           
           
               
               
            
               
                   
                 GXSetTevClampMode(GX_TEVSTAGE0, GX_TC_LINEAR); 
               
               
                   
                 // TEV Stage 1 ( Compare Lower 8bit ) 
               
               
                   
                 // REGPREV(C) = REG1(C) &gt;= shadow map(G) ? 0 : 255 
               
               
                   
                 GXSetTevOrder(GX_TEVSTAGE1, GX_TEXCOORD0, 
               
            
           
           
               
               
            
               
                   
                 GX_TEXMAP0, GX_COLOR_NULL); 
               
            
           
           
               
               
            
               
                   
                 GXSetTevColorIn(GX_TEVSTAGE1, GX_CC_ZERO, GX_CC_TEXGGG, 
               
            
           
           
               
               
            
               
                   
                 GX_CC_ONE, GX_CC_C1); 
               
            
           
           
               
               
            
               
                   
                 GXSetTevColorOp(GX_TEVSTAGE1, GX_TEV_SUB, GX_TB_ZERO, 
               
            
           
           
               
               
            
               
                   
                 GX_CS_DIVIDE_2, GX_TRUE, GX_TEVPREV); 
               
            
           
           
               
               
            
               
                   
                 GXSetTevAlphaIn(GX_TEVSTAGE1, GX_CA_ZERO, GX_CA_ZERO, 
               
            
           
           
               
               
            
               
                   
                 GX_CA_ZERO, GX_CA_ZERO); // Dummy 
               
            
           
           
               
               
            
               
                   
                 GXSetTevAlphaOp(GX_TEVSTAGE1, GX_TEV_ADD, GX_TB_ZERO, 
               
            
           
           
               
               
            
               
                   
                 GX_CS_SCALE_1, GX_TRUE, GX_TEVPREV); // Dummy out 
               
            
           
           
               
               
            
               
                   
                 GXSetTevClampMode(GX_TEVSTAGE1, GX_TC_GE); 
               
               
                   
                 // TEV Stage 2 ( Compare Higher 8 bit ) 
               
               
                   
                 // REG1(C) = ( REG1(A) − shadow map(R) ) without clamp 
               
               
                   
                 GXSetTevOrder(GX_TEVSTAGE2, GX_TEXCOORD0, 
               
            
           
           
               
               
            
               
                   
                 GX_TEXMAP0, GX_COLOR_NULL); 
               
            
           
           
               
               
            
               
                   
                 GXSetTevColorIn(GX_TEVSTAGE2, GX_CC_ZERO, GX_CC_TEXRRR, 
               
            
           
           
               
               
            
               
                   
                 GX_CC_ONE, GX_CC_A1); 
               
            
           
           
               
               
            
               
                   
                 GXSetTevColorOp(GX_TEVSTAGE2, GX_TEV_SUB, GX_TB_ZERO, 
               
            
           
           
               
               
            
               
                   
                 GX_CS_SCALE_1, GX_FALSE, GX_TEVREG1); 
               
            
           
           
               
               
            
               
                   
                 GXSetTevClampMode(GX_TEVSTAGE2, GX_TC_LINEAR); 
               
               
                   
                 // TEV Stage 3 ( Compare Higher 8 bit (cont.) ) 
               
               
                   
                 // REGPREV(C) = REG1(C) − REGPREV(C) * 1/255 &gt;= 0 ? 0 : 255 
               
               
                   
                 GXSetTevOrder (GX_TEVSTAGE3, GX_TEXCOORD_NULL, 
               
            
           
           
               
               
            
               
                   
                 GX_TEXMAP_NULL, GX_COLOR_NULL); 
               
            
           
           
               
               
            
               
                   
                 GXSetTevColorIn(GX_TEVSTAGE3, GX_CC_ZERO, GX_CC_C2, 
               
            
           
           
               
               
            
               
                   
                 GX_CC_CPREV, GX_CC_C1); 
               
            
           
           
               
               
            
               
                   
                 GXSetTevColorOp(GX_TEVSTAGE3, GX_TEV_SUB, GX_TB_ZERO, 
               
            
           
           
               
               
            
               
                   
                 GX_CS_DIVIDE_2, GX_TRUE, GX_TEVPREV); 
               
            
           
           
               
               
            
               
                   
                 GXSetTevClampMode(GX_TEVSTAGE3, GX_TC_GE); 
               
               
                   
                 // TEV Stage 4 ( Select shadow/lit color ) 
               
               
                   
                 // output = REGPREV == 0 ? shadow color : rasterized color 
               
               
                   
                 // Register 0 is supporsed to hold shadow color 
               
               
                   
                 GXSetTevOrder(GX_TEVSTAGE4, GX_TEXCOORD_NULL, 
               
            
           
           
               
               
            
               
                   
                 GX_TEXMAP_NULL, GX_COLOR0A0); 
               
            
           
           
               
               
            
               
                   
                 GXSetTevColorIn(GX_TEVSTAGE4, GX_CC_C0, GX_CC_RASC, 
               
            
           
           
               
               
            
               
                   
                 GX_CC_CPREV, GX_CC_ZERO); 
               
            
           
           
               
               
            
               
                   
                 GXSetTevColorOp(GX_TEVSTAGE4, GX_TEV_ADD, GX_TB_ZERO 
               
            
           
           
               
               
            
               
                   
                 GX_CS_SCALE_1, GX_TRUE, GX_TEVPREV); 
               
            
           
           
               
               
            
               
                   
                 GXSetTevClampMode(GX_TEVSTAGE4, GX_TC_LINEAR); 
               
               
                   
                 // ------------------------------------------- 
               
               
                   
                 // Following is the TEV setting for 16bit 
               
               
                   
                 // comparison by 4 stages. But it may 
               
               
                   
                 // generate some error artifacts on surfaces. 
               
               
                   
                 // ------------------------------------------- 
               
               
                   
                 /* 
               
               
                   
                 GXSetNumTevstages(4); 
               
               
                   
                 // REG2 = constant {1, 1, 1, 1} 
               
               
                   
                 GxSetTevColor(GX_TEVREG2, col_one); 
               
               
                   
                 // TEV Stage 0 ( Loads a depth value from ramp texture ) 
               
               
                   
                 // TEXA * 254/255 -&gt; REG1(A) 
               
               
                   
                 // TEXC * 254/255 -&gt; REG1(C) 
               
               
                   
                 GXSetTevOrder(GX_TEVSTAGE0, GX_TEXCOORD1, 
               
            
           
           
               
               
            
               
                   
                 GX_TEXMAP1, GX_COLOR_NULL); 
               
            
           
           
               
               
            
               
                   
                 GXSetTevColorIn(GX_TEVSTAGE0, GX_CC_TEXC, GX_CC_ZERO, 
               
            
           
           
               
               
            
               
                   
                 GX_CC_C2, GX_CC_ZERO); 
               
            
           
           
               
               
            
               
                   
                 GXSetTevColorOp(GX_TEVSTAGE0, GX_TEV_ADD, GX_TB_ZERO, 
               
            
           
           
               
               
            
               
                   
                 GX_CS_SCALE_1, GX_TRUE, GX_TEVREG1); 
               
            
           
           
               
               
            
               
                   
                 GXSetTevAlphaIn(GX_TEVSTAGE0, GX_CA_TEXA, GX_CA_ZERO, 
               
            
           
           
               
               
            
               
                   
                 GX_CA_A2, GX_CA_ZERO); 
               
            
           
           
               
               
            
               
                   
                 GXSetTevAlphaOp(GX_TEVSTAGE0, GX_TEV_ADD, GX_TB_ZERO, 
               
            
           
           
               
               
            
               
                   
                 GX_CS_SCALE_1, GX_TRUE, GX_TEVREG1); 
               
            
           
           
               
               
            
               
                   
                 GXSetTevClampMode(GX_TEVSTAGE0, GX_TC_LINEAR); 
               
               
                   
                 // TEV Stage 1 ( Compare Lower 8bit ) 
               
               
                   
                 // REGPREV(C) = REG1(C) &gt;= shadow map(G) * 254/255 ? 0 : 255 
               
               
                   
                 GXSetTevOrder(GX_TEVSTAGE1, GX_TEXCOORD0, 
               
            
           
           
               
               
            
               
                   
                 GX_TEXMAP0, GX_COLOR_NULL); 
               
            
           
           
               
               
            
               
                   
                 GXSetTevColorIn(GX_TEVSTAGE1, GX_CC_TEXGGG, GX_CC_ZERO, 
               
            
           
           
               
               
            
               
                   
                 GX_CC_C2, GX_CC_C1); 
               
            
           
           
               
               
            
               
                   
                 GXSetTevColorOp(GX_TEVSTAGE1, GX_TEV_SUB, GX_TB_ZERO, 
               
            
           
           
               
               
            
               
                   
                 GX_CS_DIVIDE_2, GX_TRUE, GX_TEVPREV); 
               
            
           
           
               
               
            
               
                   
                 GXSetTevAlphaIn(GX_TEVSTAGE1, GX_CA_ZERO, GX_CA_ZERO, 
               
            
           
           
               
               
            
               
                   
                 GX_CA_ZERO, GX_CA_ZERO); // Dummy 
               
            
           
           
               
               
            
               
                   
                 GXSetTevAlphaOp(GX_TEVSTAGE1, GX_TEV_ADD, GX_TB_ZERO, 
               
            
           
           
               
               
            
               
                   
                 GX_CS_SCALE_1, GX_TRUE, GX_TEVPREV); // Dummy out 
               
            
           
           
               
               
            
               
                   
                 GXSetTevClampMode(GX_TEVSTAGE1, GX_TC_GE); 
               
               
                   
                 // TEV Stage 2 ( Compare Higher 8 bit ) 
               
               
                   
                 // REGPREV(C) = REG1(A) &gt;= shadow map(R) * 254/255 + PREV(C) 
               
            
           
           
               
               
               
            
               
                   
                 // 
                 ? 0 : 255 
               
            
           
           
               
               
            
               
                   
                 GXSetTevOrder(GX_TEVSTAGE2, GX_TEXCOORD0, 
               
            
           
           
               
               
            
               
                   
                 GX_TEXMAP0, GX_COLOR_NULL); 
               
            
           
           
               
               
            
               
                   
                 GXSetTevColorIn(GX_TEVSTAGE2, GX_CC_TEXRRR, GX_CC_CPREV, 
               
            
           
           
               
               
            
               
                   
                 GX_CC_C2, GX_CC_A1); 
               
            
           
           
               
               
            
               
                   
                 GXSetTevColorOp(GX_TEVSTAGE2, GX_TEV_SUB, GX_TB_ZERO, 
               
            
           
           
               
               
            
               
                   
                 GX_CS_SCALE_1, GX_TRUE, GX_TEVPREV); 
               
            
           
           
               
               
            
               
                   
                 GXSetTevClampMode(GX_TEVSTAGE2, GX_TC_GE); 
               
               
                   
                 // TEV Stage 3 ( Select shadow/lit color ) 
               
               
                   
                 // output = REGPREV == 0 ? shadow color : rasterized color 
               
               
                   
                 // Register 0 is supposed to hold shadow color 
               
               
                   
                 GXSetTevOrder(GX_TEVSTAGE3, GX_TEXCOORD_NULL, 
               
            
           
           
               
               
            
               
                   
                 GX_TEXMAP_NULL, GX_COLOR0A0); 
               
            
           
           
               
               
            
               
                   
                 GXSetTevColorIn(GX_TEVSTAGE3, GX_CC_C0, GX_CC_RASC, 
               
            
           
           
               
               
            
               
                   
                 GX_CC_CPREV, GX_CC_ZERO); 
               
            
           
           
               
               
            
               
                   
                 GXSetTevColorOp(GX_TEVSTAGE3, GX_TEV_ADD, GX_TB_ZERO, 
               
            
           
           
               
               
            
               
                   
                 GX_CS_SCALE_1, GX_TRUE, GX_TEVPREV); 
               
            
           
           
               
               
            
               
                   
                 GXSetTevClampMode(GX_TEVSTAGE3, GX_TC_LINEAR); 
               
               
                   
                 */ 
               
            
           
           
               
            
               
                 } 
               
               
                   
               
            
           
         
       
     
     The following is a list of a repertoire of example illustrative application programming interface commands used by system  50  to perform shadow mapping as discussed above: 
     
       
         
           
               
               
               
             
               
                   
               
               
                 Function 
                 Parameters 
                 Description 
               
               
                   
               
             
            
               
                 GXLoadTexMtxImm 
                 matrix 
                 appropriate 
               
               
                   
                   
                 projection mtx 
               
               
                   
                 matrix destination 
                 GX_TEXMTX1 
               
               
                   
                 type 
                 GX_MTX3x4 
               
               
                 GXSetTexCoordGen 
                 destination coord 
                 GX_TEXCOORD1 
               
               
                   
                 texgen type 
                 GX_TG —   
               
               
                   
                   
                 MTX3x4 
               
               
                   
                 texgen src 
                 GX_TG_POS 
               
               
                   
                 mtx src 
                 GX_TEXMTX1 
               
               
                 GxInitTexObj 
                 image pointer 
                 appropriate 
               
               
                   
                 width/height 
                 appropriate 
               
               
                   
                 format 
                 GX_TF_IA4, 
               
               
                   
                   
                 GX_TF_RGB5A3 
               
               
                   
                 wrap mode(s, t) 
                 GX_CLAMP, 
               
               
                   
                   
                 GX_REPEAT 
               
               
                   
                 mipmap 
                 always GX_FALSE 
               
               
                 GXLoadTexObj 
                 destination texture name 
                 GX_TEXMAP0, 
               
               
                   
                   
                 GX_TEXMAP1 
               
               
                 GXInvalidateTexAll 
                 (no parameter) 
               
               
                 GXSetTexCopySrc 
                 top/left/width/height 
                 only fixed size 
               
               
                 GXSetTexCopyDst 
                 width/height 
                 only fixed size 
               
               
                   
                 format 
                 always GX_TF_IA4 
               
               
                   
                 mipmap filter 
                 GX_TRUE 
               
               
                 GXCopyTex 
                 image pointer 
                 appropriate 
               
               
                   
                 clear operation 
                 GX_TRUE 
               
               
                 GXSetCopyClear 
                 color 
                 only one color 
               
               
                   
                 Z value 
                 always 0xFFFFFF 
               
               
                 GXSetTevOp 
                 tev stage ID 
                 GX_TEVSTAGE0, 
               
               
                   
                   
                 GX_TEVSTAGE1 
               
               
                   
                 operation mode 
                 GX_MODULATE 
               
               
                   
                   
                 for stage0, 
               
               
                   
                   
                 GX_DECAL 
               
               
                   
                   
                 for stage1 
               
               
                 GXSetPixelFmt 
                 pixel format 
                 GX_PF —   
               
               
                   
                   
                 RGBA6_Z24 
               
               
                   
                 Z compression format 
                 always GX —   
               
               
                   
                   
                 ZC_LINEAR 
               
               
                 GXSetDrawSync 
                 token number 
                 various (count up 
               
               
                   
                   
                 in every frame) 
               
               
                 GXReadDrawSync 
                 (no parameter) 
               
               
                   
               
            
           
         
       
     
     Example Image Results 
     FIG. 15 shows an example image produced by full-scene shadow mapping. In this example, the torus object projects a shadow onto tessellated floor panels. The light direction appears to be coming from a point away from the viewer&#39;s viewpoint. Example system  50  can render a scene such as that shown in FIG. 15 in real time. 
     Further Embodiment Using Identification Technique 
     FIGS. 17A and 17B show a further shadow mapping embodiment that can be performed by system  50  based on the object identification technique. This embodiment renders the entire scene from the point of view of the light as discussed above—but instead of using a resulting z buffer as a shadow map for depth comparison, this alternative embodiment paints each object by an individual identification number. For example, this identification number may be encoded as a gray scale value in one or more of the color components of the embedded color frame buffer  702 . Unique object IDs are assigned to each object In one example, the IDs are eight bits wide, and are written by main processor  110  to transform unit  300  as material colors. The object IDs are thereby carried down graphics pipeline  180  and are written into one of the color channels of embedded color frame buffer  702 . The resulting color frame buffer at the end of the first pass rendering process contains the IDs of all objects the light can “see.” See FIG. 16, blocks  2100 - 2118 . 
     At the end of the first pass, the appropriate contents (IDs) of the color frame buffer are copied into a texture (block  2120 ) for use in a second-pass texture mapping process. The entire scene is then rendered again as viewed from the camera (user) viewpoint. The shadow map object ID texture is projected onto the scene using conventional texture projection, and texture environment unit  600  compares—at each pixel—the ID number of the object to the drawn and the ID number from the projected shadow map texture. If the two ID numbers are not similar, it means that a ray from the light is obstructed by another object containing another ID. Texture environment unit  600  in this instance blends a predetermined shadow color into the pixel (or otherwise attenuates the pixel brightness). The comparison is performed by texture environment unit  600  in the example embodiment in two stages: the first shader stage is used to receive the object ID from the shadow map, and the second shader stage is used to blend a predetermined shadow color based on the results of a comparison between the shadow map ID and the rendered object ID. See FIG. 16B, blocks  2126 - 2136 . 
     Other Example Compatible Implementations 
     Certain of the above-described system components  50  could be implemented as other than the home video game console configuration described above. For example, one could run graphics application or other software written for system  50  on a platform with a different configuration that emulates system  50  or is otherwise compatible with it. If the other platform can successfully emulate, simulate and/or provide some or all of the hardware and software resources of system  50 , then the other platform will be able to successfully execute the software. 
     As one example, an emulator may provide a hardware and/or software configuration (platform) that is different from the hardware and/or software configuration (platform) of system  50 . The emulator system might include software and/or hardware components that emulate or simulate some or all of hardware and/or software components of the system for which the application software was written. For example, the emulator system could comprise a general purpose digital computer such as a personal computer, which executes a software emulator program that simulates the hardware and/or firmware of system  50 . 
     Some general purpose digital computers (e.g., IBM or MacIntosh personal computers and compatibles) are now equipped with 3D graphics cards that provide 3D graphics pipelines compliant with DirectX or other standard 3D graphics command application programming interfaces (APIs). They may also be equipped with stereophonic sound cards that provide high quality stereophonic sound based on a standard set of sound commands. Such multimedia-hardware-equipped personal computers running emulator software may have sufficient performance to approximate the graphics and sound performance of system  50 . Emulator software controls the hardware resources on the personal computer platform to simulate the processing, 3D graphics, sound, peripheral and other capabilities of the home video game console platform for which the game programmer wrote the game software. 
     FIG. 16A illustrates an example overall emulation process using a host platform  1201 , an emulator component  1303 , and a game software executable binary image provided on a storage medium  62 . Host  1201  may be a general or special purpose digital computing device such as, for example, a personal computer, a video game console, or any other platform with sufficient computing power. Emulator  1303  may be software and/or hardware that runs on host platform  1201 , and provides a real-time conversion of commands, data and other information from storage medium  62  into a form that can be processed by host  1201 . For example, emulator  1303  fetches “source” binary-image program instructions intended for execution by system  50  from storage medium  62  and converts these program instructions to a target format that can be executed or otherwise processed by host  1201 . 
     As one example, in the case where the software is written for execution on a platform using an IBM PowerPC or other specific processor and the host  1201  is a personal computer using a different (e.g., Intel) processor, emulator  1303  fetches one or a sequence of binary-image program instructions from storage medium  1305  and converts these program instructions to one or more equivalent Intel binary-image program instructions. The emulator  1303  also fetches and/or generates graphics commands and audio commands intended for processing by the graphics and audio processor  114 , and converts these commands into a format or formats that can be processed by hardware and/or software graphics and audio processing resources available on host  1201 . As one example, emulator  1303  may convert these commands into commands that can be processed by specific graphics and/or or sound hardware of the host  1201  (e.g., using standard DirectX, OpenGL and/or sound APIs). 
     An emulator  1303  used to provide some or all of the features of the video game system described above may also be provided with a graphic user interface (GUI) that simplifies or automates the selection of various options and screen modes for games run using the emulator. In one example, such an emulator  1303  may further include enhanced functionality as compared with the host platform for which the software was originally intended. 
     FIG. 16B illustrates an emulation host system  1201  suitable for use with emulator  1303 . System  1201  includes a processing unit  1203  and a system memory  1205 . A system bus  1207  couples various system components including system memory  1205  to processing unit  1203 . System bus  1207  may be any of several types of bus structures including a memory bus or memory controller, a peripheral bus, and a local bus using any of a variety of bus architectures. System memory  1207  includes read only memory (ROM)  1252  and random access memory (RAM)  1254 . A basic input/output system (BIOS)  1256 , containing the basic routines that help to transfer information between elements within personal computer system  1201 , such as during start-up, is stored in the ROM  1252 . System  1201  further includes various drives and associated computer-readable media. A hard disk drive  1209  reads from and writes to a (typically fixed) magnetic hard disk  1211 . An additional (possible optional) magnetic disk drive  1213  reads from and writes to a removable “floppy” or other magnetic disk  1215 . An optical disk drive  1217  reads from and, in some configurations, writes to a removable optical disk  1219  such as a CD ROM or other optical media. Hard disk drive  1209  and optical disk drive  1217  are connected to system bus  1207  by a hard disk drive interface  1221  and an optical drive interface  1225 , respectively. The drives and their associated computer-readable media provide nonvolatile storage of computer-readable instructions, data structures, program modules, game programs and other data for personal computer system  1201 . In other configurations, other types of computer-readable media that can store data that is accessible by a computer (e.g., magnetic cassettes, flash memory cards, digital video disks, Bernoulli cartridges, random access memories (RAMs), read only memories (ROMs) and the like) may also be used. 
     A number of program modules including emulator  1303  may be stored on the hard disk  1211 , removable magnetic disk  1215 , optical disk  1219  and/or the ROM  1252  and/or the RAM  1254  of system memory  1205 . Such program modules may include an operating system providing graphics and sound APIs, one or more application programs, other program modules, program data and game data. A user may enter commands and information into personal computer system  1201  through input devices such as a keyboard  1227 , pointing device  1229 , microphones, joysticks, game controllers, satellite dishes, scanners, or the like. These and other input devices can be connected to processing unit  1203  through a serial port interface  1231  that is coupled to system bus  1207 , but may be connected by other interfaces, such as a parallel port, game port Fire wire bus or a universal serial bus (USB). A monitor  1233  or other type of display device is also connected to system bus  1207  via an interface, such as a video adapter  1235 . 
     System  1201  may also include a modem  1154  or other network interface means for establishing communications over a network  1152  such as the Internet. Modem  1154 , which may be internal or external, is connected to system bus  123  via serial port interface  1231 . A network interface  1156  may also be provided for allowing system  1201  to communicate with a remote computing device  1150  (e.g., another system  1201 ) via a local area network  1158  (or such communication may be via wide area network  1152  or other communications path such as dial-up or other communications means). System  1201  will typically include other peripheral output devices, such as printers and other standard peripheral devices. 
     In one example, video adapter  1235  may include a 3D graphics pipeline chip set providing fast 3D graphics rendering in response to 3D graphics commands issued based on a standard 3D graphics application programmer interface such as Microsoft&#39;s DirectX 7.0 or other version. A set of stereo loudspeakers  1237  is also connected to system bus  1207  via a sound generating interface such as a conventional “sound card” providing hardware and embedded software support for generating high quality stereophonic sound based on sound commands provided by bus  1207 . These hardware capabilities allow system  1201  to provide sufficient graphics and sound speed performance to play software stored in storage medium  62 . 
     All documents referred to above are hereby incorporated by reference into this specification 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 embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the scope of the appended claims.