OpenGL, disabling a texture unit, glActiveTexture and glBindTexture

Question

How do I turn off a texture unit, or at least prevent its state changing when I bind a texture? I'm using shaders so there's no glDisable for this I don't think. The problem is that the chain of events might look something like this:

Create texture 1 (implies binding it)
Use texture 1 with texture unit 1
Create texture 2 (implies binding it)
Use texture 2 with texture unit 2

, but given glActiveTexture semantics, it seems this isn't possible, because the create of texture 2 will become associated with the state of texture unit 1, as that was the last unit I called glActiveTexture on. i.e. you have to write:

Create texture 1
Create texture 2
Use texture 1 with texture unit 1
Use texture 2 with texture unit 2

I've simplified the example of course, but the fact that creating and binding a texture can incidentally affect the currently active texture unit even when you are only binding the texture as part of the creation process is something that makes me somewhat uncomfortable. Unless of course I've made an error here and there's something I can do to disable state changes in the current glActiveTexture?

Thanks for any assistance you can give me here.

Nicol Bolas Nicol Bolas · Accepted Answer · 2011-09-25T19:49:58

This is pretty much something you just have to learn to live with in OpenGL. GL functions only affect the current state. So to modify an object, you must bind it to the current state, and modify it.

In general however, you shouldn't have a problem. There is no reason to create textures in the same place where you're binding them for use. The code that actually walks your scene and binds textures for rendering should never be creating textures. The rendering code should establish all necessary state for each rendering (unless it knows that all necessary state was previously established in this rendering call). So it should be binding all of the textures that each object needs. So previously created textures will be evicted.

And in general, I would suggest unbinding textures after creation (ie: glBindTexture(..., 0)). This prevents them from sticking around.

And remember: when you bind a texture, you also unbind whatever texture was currently bound. So the texture functions will only affect the new object.

However, if you want to rely on an EXT extension, there is EXT_direct_state_access. It is supported by NVIDIA and AMD, so it's fairly widely available. It allows you to modify objects without binding them, so you can create a texture without binding it.

OpenGL, disabling a texture unit, glActiveTexture and glBindTexture

1 Answers