On Thu, 4 Jul 2002, Tim Smith wrote:
>
> (on my system glxgears behaves quite oddly with pageflipping enabled even
> with only one; it has brief periods of seeming to animate slowly backwards,
> but shows no significant change in framerate - it is the only app that I've
> tried that behaves so strangely)
This is normal, and has nothing to do with any 3D acceleration, and
everything to do with human visual perception of successive still images.
Depending on the frequency of your display update and the frequency that
the actual drawing is done at, you get different "beats" - somewhat
similar to sound interference between two different frequencies of sound.
The normal frequency of glxgears being in the kHz range, the only thing
that really matters _visually_ is the interference of your monitor refresh
rate and the glxgears refresh rate. THAT is really the thing that makes
you visually see whether it goes backwards or forwards, or whether it just
looks like a blur. If the two frequencies are close multiples of each
others, it will look like the gears move slowly forwards or backwards.
Remember: what you are actualy _seeing_ is just a snapshot of whatever
gears position happened to be at the exact time when the display was
drawn. Your brain just interprets the successive still images as movement.
If you want to experiment with this, you can easily modulate the frequency
of glxgears by making the window smaller or bigger. Or you can partially
occlude it, and see how the gears move backwards or forwards or are just
blurry.
Linus
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