Fill rate
From DmWiki
Fill rate is the number of pixels that can be drawn by a graphics card per second. Fill rate will go down when using complex shaders. It is important to budget fill rate when designing a game. The requirement of interactive rendering puts a limit on the amount of fill that can be used each frame, and overflowing this limit will cause the frame rate to drop.
Fill rate problems are aggravated by overdraw. Overdraw occurs when an object is drawn to the frame buffer, and then another object (such as a wall) is drawn on top of it, covering it up. The time spent drawing the first object was wasted because it isn't visible. Minimizing overdraw is the aim of visibility systems like portals and PVS.
Detecting fill rate limitation
There is a simple trick to detect if graphics are fill rate-limited. To do this, run the game in windowed mode. Also, if not done already, put an FPS (frames per second) counter somewhere in the application window. Now, render a view typical for the game or one desired to benchmark; keep the FPS rate in mind. Now, gradually resize the window down to a size of a few pixels while observing the frame rate. If the FPS skyrockets, the game is fill rate-limited! Try to optimize throughput by using hardware buffers or sort primitives before rendering. Otherwise, if the game runs too slow there is probably too much workload on the CPU (CPU-limited) or the geometry may be too complex (geometry-limited).
See also
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