Abstract:
A method and an apparatus are provided for combining multiple independent tile-based graphic cores. An incoming geometry stream is split into a plurality of streams and sent to respective tile based graphics processing cores. Each one generates a separate tiled geometry list as described. These may be combined into a master tiling unit or, alternatively, markers may be inserted into the tiled geometry lists which are used in the rasterization phase to switch between tiling lists from different geometry processing cores.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 12/315,263, filed on Dec. 1, 2008 now U.S. Pat. No. 8,310,487, which claims priority from GB App. No. 0723536.9, filed on Nov. 30, 2007, both of which are incorporated by reference in their entirety herein for all purposes. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to a three-dimensional computer graphics rendering system and in particular to a method and an apparatus associated with combining multiple independent tile based graphics cores for the purpose of increasing geometry processing performance. 
     It is desirable to offer computer graphics processing cores at many different performance points e.g. from basic hand-held applications through to sophisticatedly dedicated graphic computers. However, the complexity of modern computer graphics makes it difficult to do this in either a timely or cost effective manner. As such, it is desirable to have a method of combining multiple independent cores such that performance may be increased without developing a whole new core. 
     Tile based rendering systems are well-known. These subdivide an image into a plurality of rectangular blocks or tiles.  FIG. 1  illustrates an example of a tile based rendering system. A primitive/command fetch unit  101  retrieves command and primitive data from memory and passes the command and the primitive data to a geometry processing unit  102 . The geometry processing unit  102  transforms the primitive and command data into screen space using well-known methods. This data is then supplied to a tiling unit  103  which inserts object data from the screen space geometry into object lists for each of a set of defined rectangular regions or tiles. An object list for each tile contains primitives that exist wholly or partially in that tile. The list exists for every tile on the screen, although some object lists may have no data in them. These object lists are fetched by a tile parameter fetch unit  105  which supplies the object lists tile by tile to a hidden surface removal unit (HSR)  106 . The hidden surface removal unit (HSR)  106  removes surfaces which will not contribute to the final scene (usually because they are obscured by another surface). The HSR unit  106  processes each primitive in the tile and passes only data for visible pixels to a testing and shading unit (TSU)  108 . The TSU takes the data from the HSR and uses the data 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  110 . The alpha test/fogging/alpha blending unit  110  can apply degrees of transparency/opacity to the surfaces again using well-known techniques. Alpha blending is performed using an on chip tile buffer  112  thereby eliminating the requirement to access external memory for this operation. Once each tile has been completed, the pixel processing unit  114  performs any necessary backend processing such as packing and anti-alias filtering before writing the result data to a rendered scene buffer  116 , ready for display. 
     In British Patent No. GB2343598 there is described a process of scaling rasterization performance within a tile based rendering environment by separating geometry processing and tiling operations into a separate processor that supplies multiple rasterization cores. This method does not take into account the issues of scaling geometry processing and in particular tiling throughput across multiple parallel tile based cores. 
     It is commonly known that 3D hardware devices must preferably preserve the ordering of primitives with respect to the order in which they were submitted by a supplying application. For example  FIG. 2  illustrates 4 triangles T 1  ( 200 ), T 2  ( 210 ), T 3  ( 220 ) and T 4  ( 230 ) that are present by the application in the order T 1 , T 2 , T 3 , T 4  and overlap the four tiles Tile  0  ( 240 ), Tile  1  ( 250 ), Tile  2  ( 260 ) and Tile  3  ( 270 ) as shown. In order to preserve the original order of the triangles in the tile lists the triangles would be referenced in each tile list as follows.
 
TABLE-US-00001 Tile 0 Tile 1 Tile 2 Tile 3 T1 T2 T3 T3 T2 T3 T4 T3
 
     In order to evenly distribute load across geometry and tiling processors, the input data needs to be split across the processors either on a round-robin basis or based on the load on individual processors. However, as each processor is generating object tile lists locally, the preservation of the order in which objects are inserted into tiles requires that the order in which the processors write to the per tile object lists be controlled. This control would normally require communication between each of the GPC&#39;s (Graphics Processing Cores) present, meaning that their design would need to be changed when scaling the number of cores present. 
     SUMMARY OF THE INVENTION 
     Preferred embodiments of the present invention provide a method and an apparatus that allow a tile based rendering system to scale geometry processing and tiling performance in a linear fashion. This is accomplished by the use of a hierarchical list structure that allows chunks of incoming geometry to be processed and tiled locally within a core and for resulting region lists from each core to be linked efficiently together in an order that corresponds to the original input geometry order. Further, the mechanism employed allows multiple cores to be used in parallel with little or no required modification to each of those cores. 
     Preferably, embodiments of the invention provide a method and an apparatus for combining multiple independent tile based graphic cores in which an incoming geometry stream is split into a plurality of geometry streams, one per tile based graphics processing core. Each core and separate tiled geometry lists for each triangle the core processes are then combined using either a master tiling unit which takes data from the geometry processing cores to generate a master tile list for each tile preserving the input geometry order, or during rasterization processing having markers within the tiled geometry lists to have the rasterization core switch between lists. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred embodiments of the invention will now be described in detail by way of examples with reference to the accompanying drawings in which: 
         FIG. 1  illustrates a prior art tile based rendering system as discussed above; 
         FIG. 2  illustrates an example set of four triangles overlapping four tiles as described above; 
         FIG. 3  illustrates the splitting of a control stream across multiple cores; 
         FIG. 4  shows the data structure proposed for the tile reference lists in an embodiment of the invention; 
         FIG. 5  shows a proposed hierarchical tile list data structure in an embodiment of the invention; 
         FIG. 6  illustrates the splitting of control stream across multiple core at a courser granularity in an embodiment of the invention; 
         FIG. 7  illustrates an example implementation of a system embodying the invention; 
         FIG. 8  illustrates a modification to the example system of  FIG. 7 ; 
         FIG. 9  illustrates the splitting of a controls team across multiple cores using Pipe Interleave Markers; 
         FIG. 10  shows a proposed tile list data structure in an embodiment of the invention that uses Pipe Interleave Markers; 
         FIG. 11  illustrates the algorithm used by a rasterization core to process multiple tile lists that are linked together using Pipe Interleave Markers; 
         FIG. 12  illustrates an example geometry processing system using Pipeline Interleave Marks; and 
         FIG. 13  illustrates an example of the front end of a tile based rasterization system capable of generating the process tile lists by a PIM based geometry system. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIG. 3  illustrates a simplified version of the proposed system using a master tiling unit to combine the geometry lists. In order to process incoming geometry stream  300  across multiple cores, the geometry stream  300  must first be split into separate streams, one per core by a stream splitter  310  which divides the data stream for processing across (in this example) the two geometry processing cores (GPC)  340  and  350 . In this example, the incoming stream is distributed across the two cores in a simple round-robin basis. However, it is also possible to split the stream across the processing cores based on the processing load on each core. It is important to note that the stream splitter distributes control state as well as primitives to each geometry processing core. The control state contains information that instructs the hardware how to process the geometry data. For example, a matrix may be used to transform the geometry in a well known manner or details may be included regarding how texturing will be applied to the geometry. For example, the control stream for GPC 0   320  contains control state 1 associated with triangles T 1  through T 3 , and the control stream for GPC 1   330  contains state 1 for T 1  through T 3  and state  2  for T 4 . 
     Each of the GPC&#39;s  340  and  350  generates a separate tiled geometry list  360  and  370  for each triangle that it processes. For each tile that is updated for each triangle, the GPC&#39;s insert a reference into a respective one of the tile reference lists (TRL)  365  and  375 . The TRL&#39;s and the per GPC tile geometry lists form the data structures illustrated in  FIG. 4 , and are used by a master tiling unit  380  to produce a master tile list  390 . 
     The TRL  400  for GPC 0  contains references for triangles T 1  and T 3  that are processed through that core. For example, T 1  is present in tile  0 . 0  only, and thus a reference and a pointer to the corresponding tiled geometry list  410  are included in the TRL, followed by references for T 3  in all four tiles. Similarly the TRL for GPC 1   420  contains references for each tile overlapped by T 2  and T 4  in the corresponding tiled geometry list for GPC 1   430 . It should be noted that the tiled triangle lists include an “End” marker after each triangle is indicated at  430 . 
     The master tiling unit (MTU)  380  in  FIG. 3  reads the TRL&#39;s in the same round-robin order that the primitives are distributed across the GPC&#39;s (in this example), taking the tile references for one triangle from each TRL before moving to the next. The MTU  380  takes the tile references and generates a master tile list  390 , and this list has the data structure illustrated in  FIG. 5 . With a normal tile based rendering system, each tile in region headers  500  points to a corresponding tile list within the top level tile lists  510 . It should be noted that these lists preserve the original presentation order of the triangles and each list has a “Terminate” master. The top level tile lists contain links to the referenced triangle lists within each tile  520  and  530  as generated by the GPC&#39;s and discussed above. 
     As mentioned above, each triangle in each GPC tiled list is followed by an “End” marker. These markers are used by rasterization hardware in order to instruct it to move from the GPC tile lists back to the high level tile list. The marker is used so that groups of triangles can be processed on each GPC instead of single triangles. This is important as it minimizes the amount of memory associated with the high level tile lists and allows greater decoupling of GPC&#39;s in case that the vertex processing on some triangles takes more time than others. 
       FIG. 6  illustrates the splitting of the incoming primitive stream from an application across multiple GPC&#39;s where blocks of 1000 triangles are pushed down to each GPC. The incoming data stream  600  contains four primitives, prim  1 ,  2 ,  3  and  4 . Each primitive contains 4, 50, 2000, 1500 triangles respectively. The stream splitter  610  splits the stream into four blocks for processing across the two GPC&#39;s ( 650  and  660 ) as illustrated. Blocks  620  and  630  are passed to GPC 0  and blocks  640  and  650  are passed to GPC 1 . Prim  1  and Prim  2  are both sent to GPC 0  along with first portions of Prim  3  and Prim  4 . The remaining portions of Prim  3  and Prim  4  are sent to GPC 1 . The purpose of the split is to attempt to balance the load between the two GPCs. In blocks  620  and  630 , data from Prim  3  is also split between the two blocks, both of which are processed by GPC 0 . This process produces similar block sizes. The TRL and top level data structures are unchanged with the exception that instead of pointing to a single triangle, the per tile references point to groups of triangles from each block within each tile. 
       FIG. 7  illustrates an example of implementing a system that uses two geometry processing and tiling cores. The primitive and command fetch unit  700  reads the incoming control stream and passes it to the stream splitter unit  705  which splits the stream for processing across the two (or more) cores as described above. The splitter passes pointers to the primitives to be fetched for the separate cores, specifically the FIFO&#39;s  712  and  714  at the input to the “local” primitive fetch units  715  and  716 . The FIFO&#39;s are required to help decouple the stream splitting processing from the time taken by each core in order to process each batch of the primitives. The local primitive fetch units read pointers from the FIFO&#39;s  712  and  714 . The local primitive fetch units then read the actual geometry data from memory and pass it to the geometry processing units  720  and  721 , which process the geometry and pass it to the local tiling units  725  and  726 . The tiling units tile the processed geometry generated by local tiled lists  730  and  731 , and pass TRL&#39;s for these lists into tile reference FIFO&#39;s  740  and  741  which buffer the previously described TRL&#39;s while waiting to be consumed by the master tiling unit (MTU)  750 . It should be noted that these FIFO&#39;s can be contained either in external memory or on chip, allowing significant flexibility in the amount of buffering between the GPC&#39;s and the master tiling unit. The use of a FIFO/buffer allows the GPC&#39;s to be decoupled from the operation of the MTU, and this minimizes stalls in case that the MTU spends a significant amount of time generating the master tile lists. The MTU uses the TRL data from the FIFO&#39;s to generate the master region lists  760 , which form the data structure together with the local tile lists as described above. 
     Using a simple round-robin scheme means that filling up either of the split stream FIFO&#39;s  712  or  714  due to one GPC takes significantly longer than the other GPC that the stream splitter will stall. As such, in the case of significant imbalance in processing time, these FIFO&#39;s may need to be significantly larger in order to prevent any of the GPC pipelines from being idle.  FIG. 8  depicts an alternative embodiment in which the splitter sends groups of primitives to each core based on how busy that core is. The processing load of each core is monitored. The operation of the system is identical to one described above with the exception that the stream splitter  805  is fed with information from the geometry processing units  820  and  821  which indicates how busy they are, for example the fullness of input buffering. The stream splitter uses this information to direct groups of primitives to the GPC which is least heavily loaded. As the order which primitives will be submitted to the cores is now non-deterministic, the stream splitter must generate a core reference sequence for the MTU so that it can pull the TRL&#39;s from the TRL FIFO&#39;s in the correct order. The reference sequence is written into a Service Order FIFO  870  by the stream splitter which the MTU reads in order to determine which TRL FIFO is to be read next. 
       FIG. 9  illustrates a system that allows the geometry to be processed across multiple cores using “Pipe Interleave Markers” instead of a master tiling unit. Like the master tiling unit based system, the incoming geometry stream data is split by the stream splitter  910  and distributed to the GPCs  940 ,  950  as described above. Each GPC generates its own tiled geometry lists  960  and  970 .  FIG. 10  illustrates the structure of the tile geometry lists. Each GPC generates its own region headers  1000  and  1020  which point to the tiled geometry lists  1010  and  1030 . Like the normal tile based rendering system, the geometry passes through each core. At the end of each geometry block, a GPC inserts a “Pipe Interleave Market” (PIM)  1040  which is used during the rasterization process to enable traversal of the lists in the correct order by a single core. 
     The flow chart in  FIG. 11  illustrates how the rasterization uses the PIM markers to traverse the lists. At the start of processing each tile, the contents of the region headers generated by each core are loaded into a core list pointer array at  1100 . This results in each entry within the array containing a pointer to the region list generated for each core of the region being processed. Processing of the region lists starts assuming that the first block of primitive data was processed by the first GPC i.e. GPC0 by setting an index into the array to 0 at  1105 . The pointer value is then tested at  1110  to see if it is zero. If it is zero, it means the list is either empty or has already been processed to completion. The array index is incremented at  1115  and the test performed at  1110  is repeated. This process is repeated until a list that contains geometry is found where point data is fetched at  1120  from the tiled geometry list using the point indexed by the array index. At  1125 , the fetched data is tested to determine if it is a PIM. If it is a PIM, then the current list pointer is updated to point to the next data in the tiled geometry list and written back into the core list pointer array at  1130 . The array index is then set to the value specified within the PIM at  1135 , and the processing jumps back to  1110 . If the test at  1125  does not detect a PIM, the fetched data is tested to see if it is an End market at  1140 . If an end is detected, then processing of the current tile has completed and the hardware will move onto processing the next tile. If the data is not an end marker, then it is a geometry or state reference and is processed at  1145  as necessary. The list pointer is then updated at  1150  and processing returns to  1120  to fetch the next entries within the tiled geometry list. 
       FIG. 12  illustrates an example of an implementation of a PIM based system that uses two geometry processing and tiling cores. The primitive and command fetch unit  1200  reads the incoming control stream and passes it to the stream splitter unit  1205  which splits the stream for processing across the two (or more) cores as described above. The splitter passes pointers to the primitives to be fetched to the separate cores, specifically the FIFO&#39;s  1212  and  1214  at the input to the “local” primitive fetch units  1215  and  1216 . The FIFO&#39;s are required to help decouple the stream splitting processing from the time taken by each core to process each batch of primitives. The local primitive fetch units read the actual geometry data from memory and pass it to the geometry processing units  1220  and  1221 , which process the geometry and pass it to the tiling units  1225  and  1226 . The tiling units tile the processed geometry generated by the per core tile lists  1230  and  1231 . 
       FIG. 13  illustrates the front end of the rasterization core capable of traversing tile lists generated by multiple GPC&#39;s using PIMs. The region header fetch unit  1310  reads the region headers from the screen space tiled geometry lists  1300  generated by each GPC and writes the resulting lists pointers into the core list pointer array  1320  as described in  FIG. 11 . The tiled geometry list fetch unit  1330  then fetches and processes the per tile control lists as described in  FIG. 11 , and passes resulting geometry to the hidden surface removal unit  1340 . All of the processings at the hidden surface removal unit  1340  are the same as described for a normal tile based rendering system.