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        Polygon Count



        GameCube
        Playstation 2
        Xbox
        Dreamcast                       Glossary

























 Let's help clear up some of the confusion that centers around the Dreamcast's  polygonal rate. When SEGA first introduced the Dreamcast back in November  1998, they indicated that the machine could do 3 million polygons per second,  which is a sustainable rate that could be gotten through software running on  the machine at that time. 

 I shall direct you to an article from IEEE Micro, of which these quotes came from:

 
The CPU was clearly an important part of the Dreamcast specification, and selection of the device was a lengthy and carefully considered process. Factors considered included performance, cost, power requirements, and delivery schedule. There wasn't an off-the-shelf processor that could meet all requirements, but Hitachi's SH-4 processor, which was still in development, could adapt to deliver the 3D geometry calculation performance necessary. The final form has an internal floating-point unit of 1.4 Gflops, which can calculate the geometry and lighting of more than 10 million polygons per second. Among the features of the SH-4 CPU is the store queue mechanism that helps send polygon data to the rendering engine at close to maximum bus bandwidth.1 The final device is implemented using a 0.25-micron, five-layer-metal process.

The system ASIC combines a PowerVR rendering core with a system bus controller, implemented using a 0.25-micron, five-layer-metal process. Imagination Technologies (formerly VideoLogic) provided the core logical design and Sega supplied the system bus. NEC provided the ASIC design technologies and chip layout, including qualification for 100-MHz operation. Fill rates are a maximum of 3.2 Gpixels per second for scenes comprising purely opaque polygons, falling to 100 million pixels per second when transparent polygons are used at the maximum hardware sort depth of 60. Overall rendering engine throughput is 7 million polygons per second, but in Dreamcast, geometry data storage becomes the limiting factor before pixel engine throughput.

 You're only as fast as your slowest component, so the DC is rated at 7 million  polygons per second maximum sustainable rate, and in a game situation, would  most likely be rated around 5 to 6 million polygons per second depending on  how good a top developer would be at squeezing performance out of the  system. I consider a rate lower than 7 mpps, simply because other game code  has an effect on the polygon rate. The more complex the game AI is, the lower  the polygon rate that the machine can achieve.

 Note, the above quote contains some information, which could be easily  misunderstood, as the above article states:

Fill rates are a maximum of 3.2 Gpixels per second for scenes comprising purely opaque polygons, falling to 100 million pixels per second when transparent polygons are used at the maximum hardware sort depth of 60.
 No 3D game today even comes close to having an opaque overdraw of 60  times! It's more like 2 to 3 times of overdraw, so the comparative pixel rate  would be 100 million to 300 million pixels per second maximum. I indicate  comparative, as that means how an "infinite plane" architecture would be  compared to a traditional architecture that renders every polygon in a scene.
 

 Here is a very interesting comment:

Overall rendering engine throughput is 7 million polygons per second, but in Dreamcast, geometry data storage becomes the limiting factor before pixel engine throughput

 Lets see if the Dreamcast can render more polygons than it can store, I will use 6 million  polygons per second for this example:
6,000,000 (PPS) / 30 (FPS) = 200,000 Polygons per scene
200,000 x 40 Bytes (size of polygon) = 8MB
 Since the Dreamcast only has 8MB of video memory, there goes all of it to polygons.

 0 MB left for textures, and even with VQ compression that is not very  much. At 3 mpps per second, there is 4 MB available for textures, and that is  much better. Just shows you, that there is not much point in creating a game  engine on the DC that does more than 3 million polygons per second. Anyway  90 percent of the developers out there cannot even get over a million polygons  per second on the Dreamcast.