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 textureCoords[]=
{
    .1f,  .1f,
    .3f,  .1f,
    .1f,  .3f,
    .3f,  .3f
};

Then I want to repeat that sprite N times to a triangle strip (or quad) defined by:

GLfloat vertices[]=
{
   -100.f, -100.f,
    100.f, -100.f,
   -100.f,  100.f,
    100.f,  100.f
};

I know it has something to do with GL_REPEAT and textureCoords going passed the range [0,1]. This however, doesn't work: (trying to repeat N = 10)

GLfloat textureCoords[]=
{
    10.1f,  10.1f,
    10.3f,  10.1f,
    10.1f,  10.3f,
    10.3f,  10.3f
};

We're seeing our full texture atlas repeated...

How would I do this the right way?

Answer 1

It can't be done the way it's described in the question. OpenGL's texture coordinate modes only apply for the entire texture.

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.

None of the texture wrap modes support the kind of operation as described in the question, i.e. they all map to the full [0,1] range, not some arbitrary subset. 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.

You basically have two choices: Either create a new texture that only has the sprite you need from the existing texture or write a GLSL vertex program to map the texture coordinates appropriately.