# Patent

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

## Abstract

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

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



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



DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
FIG. 2 illustrates a tile based rendering system that has been modified to support an untransformed display list. During the geometry processing phase a primitive/command fetch unit 200 retrieves command and primitive data from memory and passes this to a position data fetch unit 205 which fetches a position part of static geometry data from memory 210 and passes it to transform 1 unit 215. This transforms the primitive into screen space only i.e. it does not apply any lighting/attribute processing as would occur in the system of FIG. 1. The resulting screen space position data is passed to a culling unit 220 which culls any geometry in the same manner as the system of FIG. 1. Unlike the system of FIG. 1 the culling unit does not write the remaining geometry data to a transformed parameter buffer, instead It only passes the position data of the remaining geometry to a tiling unit 225.
In the system of FIG. 1, the tiling unit generates references to transformed geometry that has been stored in the transformed parameter buffer, in the new system the tiling unit generates references to the untransformed static geometry data which are written to the tiled geometry lists 230 as before. These references are in the form of pointers to the geometry data in the memory 210. This process continues until all the geometry within the scene has been processed.
During the rasterisation phase object lists for each tile are fetched by a tiled parameter fetch unit 240 which supplies the static geometry references (pointers) from the total geometry lists to untransformed geometry fetch unit 245 which fetches the untransformed static geometry data from memory 210 and passes it to the transform 2 unit 250. The transform 2 unit retransforms the retrieved data to screen space and applies any required lighting/attribute processing etc to the geometry. The transformed geometry is then passed to hidden surface removal unit (HSR) 255 which removes surfaces which will not contribute to the final scene as In the system of FIG. 1. The remaining stages 260 through to 280 all operate in the same manner as stages 150 through 170 (in FIG. 1) as described above. [John: should FIG. 2 also include a source of dynamic geometry?]
In a further optimisation it is possible to defer any lighting or attribute processing that is required after hidden surface removal has been performed. This means that this processing is only applied to that geometry which is visible within the final scene giving significant improvements in both throughput and power consumption. FIG. 3 illustrates a modification to the system that implements deferred lighting/attribute processing. Units 300 and 305 operate as described for units 240 and 245 of FIG. 2, unlike unit 250 in FIG. 2 the transform 2 unit 310 only transforms the position data before passing it onto the hidden surface removal unit 315. The visible primitives emitted by the hidden surface removal unit are then passed to transform 3 unit 320 where any lighting/attribute processing is performed. The operation of units 325 to 350 is the same as units 145 to 170 in FIG. 1.
It should be noted that each of the three transformation units mentioned above could all be implemented in a single “universal” unit similar to that described in our British Patent Application GB-A-2430513. Although the above approaches eliminate the need for a transformed parameter buffer they have the disadvantage of requiring the position data to be transformed in both phases and for the transformation to be repeated for every tile that any piece of geometry overlaps. FIG. 4 Illustrates a modification to the rasterisation phase of the untransformed display list system in which a cache is added in order to minimise the number of times the data is retransformed in the rasterisation phase. It should be noted that although FIG. 4 shows a modification with respect to a non deferred lighting/attribute processing system It is equally applicable to either. As in FIG. 2 the tiled parameter fetch unit 400 fetches the tiled object list references generated in the geometry processing phase from memory. The references are passed to a cache control unit 405 which checks to see if there is an entry In the transformed data cache memory 410 that corresponds to the object reference, if there is the cache control unit reads the data from the cache and passes it to the hidden surface removal unit 425. If there is no corresponding entry in the cache the cache control unit issues the reference to the untransformed geometry fetch unit 415 which fetch the data from memory and passes it to the transform 2 unit 420. The transform 2 unit transforms and applies any lighting/attribute process required to the geometry data and then passes it back to the cache control unit. The cache control unit then adds it to the transformed data cache memory for future reference before passing it to the hidden surface removal unit. The operation of units 425 to 450 is the same as units 145 to 170 in FIG. 1.
In order to eliminate the additional geometry processing pass used in the above approach the result of the position transform can be stored in a parameter buffer for use in the second pass. Although this results in the need for transformed parameter storage it may be consider a useful trade off compared against transforming the position data multiple times. It should also be noted that there are cases were an application will update the vertex data during a scene, this type of vertex data is often referred to as dynamic, in these circumstances the data must be transformed and copied to a parameter buffer as per a conventional tile based rendering device.
FIG. 5 illustrates a hybrid system that allows the use of both untransformed and transformed display lists. During the geometry processing phase a primitive/command fetch unit 500 retrieves command and primitive data from memory and passes this to the geometry fetch unit 505 which fetches both the dynamic geometry data 507 and static geometry data 510 from memory and passes it to the transform 1 unit 515.
For dynamic geometry the transform 1 unit transforms the position and applies any required lighting/attribute processing as per a traditional tile based rendering system, for static geometry only the position is transformed as previously described. The resulting data is passed to a culling unit 520 which culls any geometry that isn't visible using well known methods. The culling unit writes any remaining dynamic geometry and static position data to the transformed parameter buffer 535 and also passes the position data of the remaining geometry to the tiling unit 525 which generates a set of screen objects lists for each tile which are written to the tiled geometry lists 530. It should be noted that the tiled geometry lists indicate which geometry is dynamic and which is static. As in FIG. 2 the tiled parameter fetch unit 540 fetches the tiled object list references generated in the geometry processing phase from memory. The references are passed to the cache control unit 545 which checks to see if there is an entry in the transformed data cache memory 550 that corresponds to the object reference, if there is the cache control unit reads the data from the cache and passes it to the hidden surface removal unit 565. If there is no corresponding entry in the cache the cache control unit issues the reference to either the transformed parameter fetch unit 547 or the untransformed geometry fetch unit 555 based on the type indicated in the tiled reference lists. Transformed geometry is fetched by the transformed parameter fetch unit and passed back to the cache control unit and untransformed geometry is fetched by the untransformed geometry fetch unit and processed by transform unit 2 560 before being passed back to the cache control unit Both geometry types are then written to the cache by the control unit before being passed to the hidden surface removal unit All subsequent units 665 through to 590 operate as previously described for units 145 through 170 in FIG. 1.

## Claims

1. A method for rendering computer graphics images in a tile based rendering system, the method comprising the steps of:
in a geometry processing phase,
retrieving untransformed position data from stored untransformed static geometry data in a memory;
transforming the retrieved untransformed position data to screen space;
compiling for screen space position data a list of corresponding pointers to the static geometry data in the memory from which the position data was retrieved for each of a plurality of rectangular areas; and,
in a rasterization phase,
for each of the plurality of rectangular areas, retrieving pointer data from each respective list of corresponding pointers;
retrieving the untransformed position data from the stored untransformed static geometry data in the memory, the retrieved data being data pointed to by the retrieved pointer data for each respective list;
transforming the retrieved untransformed data to screen space;
applying hidden surface removal and attribute processing to the thus transformed position data to obtain resulting data; and
supplying the resulting data to a buffer for display.

2. The method according to claim 1 further including the step of lighting or attribute processing after applying the hidden surface removal.

3. The method according to claim 1, wherein the step of transforming the position data to the screen space includes the step of storing the transformed data in a cache, and the step of retrieving the position data from the geometry data in the memory corresponding to the pointer data includes the step of checking whether data to be retrieved is present in the cache.

4. The method according to claim 1 further including the step of writing the transformed position data to a parameter buffer such that subsequent transformation is not required while processing a rectangular area.

5. The method according to claim 1 further comprising the steps of:
retrieving untransformed position data from dynamic geometry data in a second memory; and
transforming the retrieved position data to screen space position data and image data,
wherein the compiling step further comprises the steps of compiling with the screen space position data the list of pointers to the dynamic geometry data , and storing the transformed dynamic geometry data, and
the step of retrieving the untransformed position data from the untransformed static geometry data in the memory corresponding to the pointer data further comprises the step of retrieving the transformed position data from the dynamic geometry data.

6. A tile based rendering system with reduced parameter memory comprising:
a geometry processing section comprising means for retrieving untransformed position data from stored untransformed static geometry data in a memory; means for transforming the retrieved untransformed position data to screen space; and means for compiling for screen space position data a list of corresponding pointers to the static geometry data in the memory from which the position data was retrieved for each of a plurality of rectangular areas; and
a rasterization section comprising, for each of the plurality of rectangular areas, means for retrieving pointer data from each respective list of corresponding pointers; means for retrieving the untransformed position data from the stored untransformed static geometry data in the memory, the retrieved data being data pointed to by the retrieved pointer data for each respective list; means for transforming the retrieved untransformed data to screen space; means for applying hidden surface removal and attribute processing to the thus transformed position data to obtain resulting data; and means for supplying the resulting data to a buffer for display.

7. The rendering system according to claim 6, wherein the means for transforming the position data to the screen space includes means for storing the transformed data in a cache, and the means for retrieving the position data from the geometry data in the memory corresponding to the pointer data includes means for checking where data to be retrieved is present in the cache.

8. The rendering system according to claim 6 further including means for writing the transformed position data to a parameter buffer such that subsequent transformation is not required while processing a rectangular area.

9. The rendering system according to claim 6 further comprising:
means for retrieving untransformed position data from dynamic geometry data in a second memory; and
means for transforming the retrieved position data to the screen space position data and image data,
wherein the compiling means includes means for compiling with the screen space position data the list of pointers to the dynamic geometry data, and means for storing transformed dynamic geometry data, and
the means for retrieving the untransformed position data from the untransformed static geometry data in the memory corresponding to the pointer data further includes means for retrieving the transformed position data from the dynamic geometry data.

10. The rendering system according to claim 6 further including means for applying lighting or attribute processing after applying the hidden surface removal.