How to use GL_REPEAT to repeat only a selection of a texture atlas? (OpenGL)

Question

How can I repeat a selection of a texture atlas?

For example, my sprite (selection) is within the texture coordinates:

GLfloat textureC         


        

Answer 1

I'm not sure you can do that. I think OpenGL's texture coordinate modes only apply for the entire texture. When using an atlas, you're using "sub-textures", so that your texture coordinates never come close to 0 and 1, the normal limits where wrapping and clamping occurs.

There might be extensions to deal with this, I haven't checked.

EDIT: Normally, to repeat a texture, you'd draw a polygon that is "larger" than your texture implies. For instance, if you had a square texture that you wanted to repeat a number of times (say six) over a bigger area, you'd draw a rectangle that's six times as wide as it is tall. Then you'd set the texture coordinates to (0,0)-(6,1), and the texture mode to "repeat". When interpolating across the polygon, the texture coordinate that goes beyond 1 will, due to repeat being enabled, "wrap around" in the texture, causing the texture to be mapped six times across the rectangle.

This is a bit crude to explain without images.

Anyway, when you're texturing using just a part of the texture, there's no way to specify that larger texture coordinate in a way that makes OpenGL repeat it inside only the sub-rectangle.