fps & bones

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Original post by AlbertoT
hello

Normaly game programmers are concerned about the number of tris. I wonder wether also the number of bones can have a significant influence on fps?

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Yes.

In the animation system, a higher number of bones typically results in more matrix-matrix multiplies, more quaternion-matrix conversions, more quaternion interpolations etc, more inverse transposes if the bones have scaling and need lighting, higher memory usage etc - this all means extra CPU work - if the game is CPU bound rather than GPU bound (most are), then that will have a direct implication on the framerate.

In the vertex shader (aka vertex program) [assuming a PC/Xbox game], the amount of available constant registers is the biggest limiting factor. After things such as the world-view-projection transform and maybe a single vertex light have been taken into account, on the widest range of PC hardware there are only enough constants for a maximium of roughly 18 to 24 bones in the same draw call. If you need more bones, then you need another draw call, possibly passing over the same vertex data more than once - this can result in small render batches (bad for GPUs), extra bone upload overhead etc.

18-24 bones is a common limit imposed on artists/animators for the above reasons.

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Simon O''Connor
Game Programmer &
Microsoft DirectX MVP