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Texture coordinate problem

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I have a simple 2x2 texture in nearest mode. If I fetch texel at uv position (0.4999, 0.) like this:

texture2D(tex, vec2(0.4999, 0.))

it returns the texel at position ij (1,0). I was expecting the following mapping:
u in [0-0.5[ => i=0
u in [0.5-1.0] => i=1

I get this strange result on a ATI(HD5870). Not on nvidia.

Do you have an explanation?
Thank you.

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I can't say why 0.4999 get's rounded to 0.5, but I guess it's not unbelievable that it could happen.

What are you doing that you need to have such exact floating point calculation? If you want per-pixel lookup I'd use a samplerRect (direct pixel access), instead of using floating point.

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