Texture coordinates in OpenGL

Question

Sorry for a really noob-level question...

I want to apply a specific piece of the texture (not the entire texture) to a quad. The texture is a 256x64 image and I'd like to be able to specify the relevant piece by stating the pixel coordinates of its upper-left and bottom-right corners ( [0,0] being the upper left corner of the whole image and [256,64] being the bottom right).

Any ideas on how to do that?

Thanks.

Answer 1

The fractional answer is correct, but if you want to use integer texture coordinates (for example in a VBO) you can use the GL_TEXTURE matrix to change your texture coordinate system:

        glMatrixMode(GL_TEXTURE)
        glLoadIdentity()
        glScalef(1f/256f, 1f/64f, 1f)

After that your texture coordinate units would be pixels. Another scaling strategy would be to scale so each tile is 1x1 in the final units.

Answer 2

imagine you want to use the 20x20 texel starting at 10,10, you'd use the following coordinates:

[10.f/256.f,10.f/64.f]
[30.f/256.f,10.f/64.f]
[30.f/256.f,30.f/64.f]
[10.f/256.f,30.f/64.f]

Answer 3

You use float-based texture coordinates, where they range from (0.0f, 0.0f) to (1.0f, 1.0f). In your case, you take your pixel values and divide them by either 256-1 or 64-1, depending on which dimension you are dealing with. (This assumes you really meant to say that your image goes to [255,63] and not [256,64], meaning that your coordinates are 0-based and not 1-based.)

Answer 4

Let's look at the following, simplified example of 1D texture of 8 pixels width (the pixel centers are at the digits):

 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 |

The corresponding texture coordinates are:

 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 |
0/8 1/8 2/8 3/8 4/8 5/8 6/8 7/8 8/8

Now if you want the subtexture from texels 3 to 7 the texture coordinates to use are 2/8 to 7/8.

You should also take note about something else: Integer texture coordinates don't address texel centers, but the exact mid between them; this may cause a blurred appearance in certain situations.