Excurse - The Next Step Of The 3D Pipeline After T&L, The Triangle Setup
3D chips have been equipped with this feature for such a long time that hardly anybody remembers what it does anymore. However, it is obviously an integral part of the 3D pipeline, which is why it was implemented into the chips so early. Do you know what the triangle setup actually does?
After the transform and the lighting have been done we are still looking at a real 3D-scene, where each vertex has x,y and z coordinates. This 3D-scene has now to be changed to the 2D frame that we get displayed on our screen, which is made out of pixels. The triangle setup procedure has to be done polygon for polygon or better triangle for triangle. Some of the triangles of the 3D-scene might be covered by other triangles that are 'in front' of it, but at this stage it is unknown to the 3D chip which triangles are covered or partly covered and which aren't. So the triangle setup unit receives a triangle that's defined by three vertices. Each of these three vertices has a x,y and z coordinate which defines its place in the 3D-scene. The triangle setup 'fills' those triangles with pixels. Each of the pixels in the triangle receives the x and y coordinate for the place it takes on the screen and a z-coordinate which holds its depth information. Each of the pixels gets also light information and then the pixels of this very triangle are one by one sent to the pixel-rendering unit.
I am taking the time to explain this step, because it is integral to what comes next in the Radeon chip, the first part of the mysterious HyperZ, which makes Radeon so very special. It is also important to understand the term 'triangle size', as it has a big impact on the fill rate.
Let's look at a few facts now. First of all it depends on the triangle size as well as the screen resolution how many pixels represent the triangle.