Question

i need to get the color at a particular coordinate from a texture. There are 2 ways i can do this, by getting and looking at the raw png data, or by sampling my generated opengl texture. Is it possible to sample an opengl texture to get the color (RGBA) at a given UV or XY coord? If so, how?

Answer 1

Off the top of my head, your options are

1. Fetch the entire texture using glGetTexImage() and check the texel you’re interested in.
2. Draw the texel you’re interested in (eg. by rendering a GL_POINTS primitive), then grab the pixel where you rendered it from the framebuffer by using glReadPixels.
3. Keep a copy of the texture image handy and leave OpenGL out of it.

Options 1 and 2 are horribly inefficient (although you could speed 2 up somewhat by using pixel-buffer-objects and doing the copy asynchronously). So my favourite by FAR is option 3.

Edit: If you have the GL_APPLE_client_storage extension (ie. you’re on a Mac or iPhone) then that’s option 4 which is the winner by a long way.