Theoretical Peak fill-rate
Most companies that use the word fill-rate are actually referring to the Theoretical Peak fill-rate. Let me start by explaining how they get to this number.
To find the Theoretical Peak fill-rate you take the clock frequency of the 3D accelerator chip and you multiply it with the number of texturing units. A texturing unit is a unit that is capable of producing one dual textured on-screen pixel. But all of this will become clear with some examples :
The 3Dfx Banshee processor usually runs at 100 MHz and it only has a single texturing unit (meaning that per clock it can calculate one on-screen bilinear filtered pixel). According to the definition I just gave the Banshee processor has a Theoretical Peak fill-rate of 100 Mpixels (or Mtexels, refer to the discussion about Pixels vs Texels for more details).
The NVIDIA RIVA TNT processor has two texturing units and usually runs at 90 MHz. Again according to the definition it has a Theoretical Peak fill-rate of 2 x 90 = 180 MPixels (or Texels).
The 3Dfx Voodoo2 processor also runs at 90 MHz and has 2 texturing units so it also has a Theoretical Peak fill-rate of 180 Mpixels (or Texels).
Now I do have to warn you... its isn't as simple as this. Both the TNT and Voodoo2 only have that fill-rate under very specific conditions. The 3Dfx Voodoo2 ( and 3) processors have a limitation : their texturing units have to work on the same on-screen pixel. Basically that means that if an on-screen pixel is single textured the second texturing unit will not be able to do much (since it has to work on that same pixel as the first unit but there is only work for that one unit). The result of this is that Voodoo2 only has a Theoretical Peak fill-rate of 180 Mpixels *IF* the two texture units can work in parallel on the same screen pixel. So when a game uses dual texturing (this means two textures applied to the same polygon, and thus to the same on-screen pixel) Voodoo2 will have a Theoretical Peak fill-rate of 180 Mpixels, however when the game is single textured the Voodoo2 only has a Theoretical Peak fill-rate of 90 Mpixels. For this reason you often see clarifications about the fill-rate : a certain fill-rate under certain conditions.
The same is true for NVIDIA's RIVA TNT. The TNT processor can either work on two separate pixels or in parallel on the same pixel but not the two mixed. Basically this means that before you start a game you need to select the correct mode. If a game uses both single and dual textured polygons the TNT will see a drop in fill-rate since at certain moments one of the texturing units will be idle, just like happens with 3Dfx Voodoo2.
So Quick recap : The Theoretical Peak fill-rate is found by multiplying the clock frequency of the 3D accelerator with the number of texturing units, on top of that there should be a clarification about the necessary conditions under which the units can work in parallel.
A good example of this kind of fill-rate information is this quote:
"...90 Mpixels/sec sustained fill rate for dual textured bi-linear filtered polygons, with LOD MIP-mapping, Z-buffering,alpha-blending and fogging enabled..."I personally would like to see it like this :
"...90 Mpixels/sec Theoretical Peak fill-rate for dual textured Bi-linear filtered polygons..."The addition of the words "Theoretical" and "Peak" is necessary because these quoted numbers will never be reached in a real world situations.
