How to normalize image coordinates for texture space in OpenGL?

Question

Say I have an image of size 320x240. Now, sampling from an sampler2D with integer image coordinates ux, uy I must normalize for te

Answer 1

Texture coordinates (and pixel coordinates) go from 0 to 1 on the edges of the pixels no matter how many pixels.

A 4 pixel wide texture

   0          0.5          1    <- texture coordinate
   |           |           |
   V           V           v 
   +-----+-----+-----+-----+
   |     |     |     |     |
   |     |     |     |     |   <- texels
   +-----+-----+-----+-----+

A 5 pixel wide texture

   0             0.5             1    <- texture coordinate
   |              |              |
   V              V              v 
   +-----+-----+-----+-----+-----+
   |     |     |     |     |     |
   |     |     |     |     |     |   <- texels
   +-----+-----+-----+-----+-----+

A 6 pixel wide texture

   0                0.5                1    <- texture coordinate
   |                 |                 |
   V                 V                 V 
   +-----+-----+-----+-----+-----+-----+
   |     |     |     |     |     |     |
   |     |     |     |     |     |     |   <- texels
   +-----+-----+-----+-----+-----+-----+

A 1 pixel wide texture

   0 0.5 1   <- texture coordinate 
   |  |  |
   V  V  V 
   +-----+
   |     |
   |     |   <- texels
   +-----+

If you use u = integerTextureCoordinate / width for each texture coordinate you'd get these coordinates

   0    0.25  0.5   0.75       <- u = intU / width;
   |     |     |     |     
   V     V     V     V     
   +-----+-----+-----+-----+
   |     |     |     |     |
   |     |     |     |     |   <- texels
   +-----+-----+-----+-----+

Those coordinates point directly between texels.

But, the texture coords you want if you want to address specific texels are like this

     0.125 0.375 0.625 0.875   
      |     |     |     |   
      V     V     V     V  
   +-----+-----+-----+-----+
   |     |     |     |     |
   |     |     |     |     |   <- texels
   +-----+-----+-----+-----+

Which you get from

   u = (integerTextureCoord + .5) / width