How to determine an OpenGL texture's size in texels?

Question

I know this is such a trivial question, yet I haven't found any information that seems to help, starting from glGet()'s documentation. I'm using my textures as sprites, so it's crucial that I keep track of their sizes in texels/pixels.

When I'm creating my textures with glTexImage2D() I'm required to pass the size of the pixel data array to it. But does OpenGL store it anywhere, or do I have to keep track of them in my code? (This feels awkward, as client code would still be able to overwrite the texture with glTexImage2D() / glTexSubImage2D() / glCopyTexImage2D() etc. calls while not changing the size accordingly, thereby resulting in visual distortions.)

Answer 1

If you really want to rely on GL, use

glGetTexLevelParameteriv(GL_TEXTURE_2D, 0, GL_TEXTURE_WIDTH, &param)

That said, I'm not sure what client code means in your context, but if you're expecting to write middleware that allows your customers to manipulate directly your own state, you're in trouble.

Answer 2

As Bahbar says, glGetTexLevelParameteri can be used to query the GL_TEXTURE_WIDTH. However, I strongly advise against using this as a general method. For one thing, you can stall the pipeline with glGet* calls, as the driver has to get the GPU to finish what it's doing, figure out what you asked, and return the answer. The result is a massive performance hit.

So you really are better off managing state like this in your own code. Presumably you have a Sprite class (or struct or equivalent), with the texture handle. It's nothing to dedicate a few more bytes to keep track of the width and height, and it will save you from having to hit the driver or GPU to get the information.

As for glTeSubImage*, well that's a policy decision as to when and how you update the texture.