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rick_appleton

OpenGL OpenGL different rendering on different PCs

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I'm trying to make something of a UnitTest library for my renderers, and I've just tried running it on a different PC. Unfortunately, that PC gives different results when rendering. - The first issue I noticed is that the same clear color gives a different pixel value on the two PCs. - The second issue I found is that triangles are rasterized differently. I have a simple triangle, with no matrices (projection, or modelview), no lighting, no texturing, and on one pc one of the edges is extended exactly 1 pixel. I can provide examples if needed, but surely this kind of thing shouldn't be possible? In this case, both cards are NVidia, although the OSs are different (Vista vs XP). Are there any settings which might influence something like this? The triangle has a vertical side, a horizontal side, and the third side is exactly 45 deg. So I don't think alias settings should effect this. Edit: I've tried it on a third pc now with an integrated Intel graphics chip, and it returns the same as the first pc.

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I cannot comment on how actual implementations may be different, but the specification allows for differences here. The specification describes a set of rules for "perfect" rasterization, but also allows for deviations from this "perfect" result. For a triangle, since you mention it, the specification states exactly what pixels are rasterized, and allows for one pixel deviation from this defined result. The result you're experiencing is within acceptable limits as far as the standard goes (well, assuming one is correct and one is one-off, so to speak).

This deviations is there to allow for optimizations that may not produce exactly this "perfect" result but will improve performance. If the driver versions are different, there could be some subtle difference in rasterization, or the steps leading to it, between your platforms.

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It's probably just drivers. Nvidia updates drivers all the time and there is so much work that goes on in a video card that anything could change from each driver update to each video card. Are you really that worried about it though?

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