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Sample neighbor texels

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I have 2 questions about GLSL texture sampling.

1) If I sample a texture with texture2DRect( diffuseTex , gl_TexCoord[0].xy ) ;
where xy are non-normalized coordinates (i.e. from (0,0) to (800,600),
where does a coordinate of (0,0) sample? Is this the bottom left cornor of a texel or the center or...?

2) What offsets do I need if I want to sample the right and left neighbor of the current texel in the fragment shader.

RightNeighborSample = texture2DRect( diffuseTex , gl_TexCoord[0].xy + rightOffset );
LeftNeighborSample = texture2DRect( diffuseTex , gl_TexCoord[0].xy + leftOffset );
Which values do I need for rightOffset? vec2(1,0) or vec2(0.5, 0), or ...?


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Well, thanks. But actually that did not answer my questions.

First of all I am using texture2DRect(), which uses NON normalized coordiantes (I wrote it in my posting). Second I want to know where the sampling point in the texels is.

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For regular textures, (0,0) is the lower left corner of the lower left texel, so I would imagine that it is the same for rectangular textures as well.

If each texel is already lined up with a pixel boundary, then gl_TexCoord[0].xy should already be selecting the center of a texel, so you'll just want to add 1.0 in each direction that you want to get the neighbor texel.

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