OpenGL Texture Coordinates in Pixel Space

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This has been asked a few times, but I don’t have the links at hand, so a quick and rough explanation. Let’s say the texture is 8 pixels wide:

 | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 |
 ^   ^   ^   ^   ^   ^   ^   ^   ^
0.0  |   |   |   |   |   |   |  1.0
 |   |   |   |   |   |   |   |   |
0/8 1/8 2/8 3/8 4/8 5/8 6/8 7/8 8/8

The digits denote the texture’s pixels, the bars the edges of the texture and in case of nearest filtering the border between pixels. You however want to hit the pixels’ centers. So you’re interested in the texture coordinates

(0/8 + 1/8)/2 = 1 / (2 * 8)

(1/8 + 2/8)/2 = 3 / (2 * 8)

…

(7/8 + 8/8)/2 = 15 / (2 * 8)

Or more generally for pixel i in a N wide texture the proper texture coordinate is

(2i + 1)/(2N)

However if you want to perfectly align your texture with the screen pixels, remember that what you specify as coordinates are not a quad’s pixels, but edges, which, depending on projection may align with screen pixel edges, not centers, thus may require other texture coordinates.

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