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I am trying to work with textures in openGL to do parallel computation (openCL is not an option). I understand that the domain is 0.0 to 1.0 on each axis from the bottom left. However, is the coordinate of a pixel during rendering map to the pixel's upper left corner, or the middle of the pixel?

Ex. If I have a texture that is 100x100px. Is the coordinate of the upper left most pixel (0.0,1.0) or is it (0.005,0.995)? I assumed it was the former, but have been getting some unexpected results. I am wondering if I am accessing data on "pixel boundaries" and openGL is interpolating to a value that is averaging the two neighboring pixels.

Dan Collins
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    You can [turn off filtering](http://www.opengl.org/wiki/Sampler_Object#Filtering) by using `GL_NEAREST`. – Nicol Bolas Feb 06 '13 at 21:46
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    Turning off filtering doesn't solve the fencepost problem. – datenwolf Feb 06 '13 at 22:40
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    I did answer this question already here: http://stackoverflow.com/a/5879551/524368 – datenwolf Feb 06 '13 at 22:41
  • possible duplicate of [OpenGL Texture Coordinates in Pixel Space](http://stackoverflow.com/questions/5879403/opengl-texture-coordinates-in-pixel-space) – datenwolf Feb 06 '13 at 22:42
  • I am now using GL_NEAREST and was able to follow the fencepost submission in the other thread. My results are much better now. Thanks! – Dan Collins Feb 07 '13 at 02:41

2 Answers2

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OpenGL pixels/fragments are conceptually 1x1 squares centered on half-integer pixels. The OpenGL 4.5 specification states:

A fragment is located by its lower left corner, which lies on integer grid coordinates. Rasterization operations also refer to a fragment’s center, which is offset by (1/2,1/2) from its lower left corner (and so lies on half-integer coordinates).

So for 100x100 px the horizontal distance between texels is 1/100 = 0.01. Therefore the distance between the left edge and the first texel center is 0.01/2 = 0.005.

The first vertical texel center from the lower left corner is also 0.005 so from the top the first texel center is 1.0 - 0.005 = 0.995.

That's why you get those numbers.

You can see my post here for more detail (tex coordinate stuff toward the end).

Community
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wcochran
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The center of the pixel is actually the latter, i.e. (0.005,0.995).

As Nicol Bolas mentioned in the comments, you can prevent opengl from interpolatingh across pixels by changing the filtering method to GL_NEAREST when you create the texture.

Hannesh
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