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Wednesday, September 28, 2005

Halting the gears


It's general knowledge that glxgears can be used to get a quick peek of a graphics card's performance. glxgears is fill-limited, that is, the frames-per-second (FPS) depends on the size of the window. I describe here a small exercise with glxgears: Halting the gears.

Increasing the window size decreases the FPS. However, the perceived sense of rotation (clockwise or anti-clockwise) of the gears appears to switch as you increase the window size. That is, as you drag the corner of the window and make it larger, the gears will appear to be rotating clockwise and then at a point they'll apear to be moving anti-clockwise and then again they'll appear to moving clockwise at a point... and so on. This is akin to the rotation of a fan wherein you can sometimes feel the blades moving in a direction opposite to the blades' actual direction of motion. So, after making x no of rotations, if a point p on the periphery of a gear lands ahead of it's original place, the perceived sense of rotation is the same as its actual rotation direciton. However, if the point p falls short of its original place, the gear will be perceived as rotation in the opposite direction.

The Exercise: Drag the corner of the window to increase the size and bring it to the point such that the perceived sense of rotation of the gears is just about to change. At this juncture, it'll appear that the gears aren't even moving -- as if they're halted. It's a bit difficult to manouvre the size to a perfect standstill, but it's worth the pains!
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