Projection Matrix vs. Animation Speed

jbirac
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I am building a Mac OS X (Carbon) app to create script-based movies of a scaffold model (drawn using gluSphere and gluCylinder objects). The model is animated by manipulating the modelview – rather than the projection – matrix between each frame. The movie making process begins by setting an identity projection matrix (camera's line-of-sight), which looks effectively into the z-axis (protruding from the monitor); the projection matrix remains untouched throughout the entire movie. The animation is quite choppy for even the simplest models. But if the projection matrix is set just a tiny bit off (from identity), then the animation moves about 10x more smoothly.

I find the animation is choppy anytime the projection looks directly into any of the axes (+/- x, +/- y, +/- z); any other arbitrary projection results in a very smooth animation.

Does anybody have a clue as to why such a small deviation in the projection matrix improves the animation so dramatically?

Thanks in advance.

-- Jeff
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