Very odd problem with glActiveTexture..

OpenGL OpenGL: Advanced Coding
1

Im not even sure if this belongs in this section but here goes:

Im having a very weird issue with glActiveTexture. Code:


       glActiveTexture(GL_TEXTURE0);
        glEnable(GL_TEXTURE_2D);

        glBindTexture(GL_TEXTURE_2D, texture.TextureID);

        glBegin(GL_QUADS);
        glColor3f(r, g, b);

        glTexCoord2f(0.0f, 0.0f); glVertex3f(-1.0f, -1.0f, 0.0f);
        glTexCoord2f(1.0f, 0.0f); glVertex3f(1.0f, -1.0f, 0.0f);
        glTexCoord2f(1.0f, 1.0f); glVertex3f(1.0f, 1.0f, 0.0f);
        glTexCoord2f(0.0f, 1.0f); glVertex3f(-1.0f, 1.0f, 0.0f);

        glEnd();
        glBindTexture(GL_TEXTURE_2D, 0);
        glActiveTexture(GL_TEXTURE0);

Where r, g, b, and texture are passed in as arguments. If I use GL_TEXTURE0, the texture displays fine. If I use GL_TEXTURE1 through GL_TEXTURE3, the texture is a single washed out color (The color depends on the color of the texture). If i use anything beyond GL_TEXTURE4, the image is pure white.

Nothing else has changed in between tests. What gives?

2

The fixed function texture environment pipeline applies a math function to the value sampled from the each texture in order. That is, it does the texture environment operation from GL_TEXTURE0, then from GL_TEXTURE1, then from GL_TEXTURE2, etc. Each operation feeds into the next.

If you really want the flexibility to use arbitrary texture units however you want, you will need to use shaders.

If i use anything beyond GL_TEXTURE4, the image is pure white.

That’s probably because GL_MAX_TEXTURE_UNITS is 4 for your implementation. That means you can’t used fixed-function texture environment logic with more than 4 textures in a single pass.

3

That makes sense, however 4 is prohibitively low.

Ive read (on another thread here) that using shaders will give you access to the full range, but is there any way to check what that range is?

4

Ive read (on another thread here) that using shaders will give you access to the full range, but is there any way to check what that range is?

GL_MAX_TEXTURE_IMAGE_UNITS. That’s the maximum number across all shader stage. Each shader stage has its own limit, but this enum gets you the sum total of all shader stages.

In general, for DX9-class hardware, the limit is 8 for fragment shaders and 4 for vertex shaders. In DX10 and above, it’s 16 for each stage.

5

GL_MAX_TEXTURE_IMAGE_UNITS is actually the number of units available to the fragment stage.

GL_MAX_COMBINED_TEXTURE_IMAGE_UNITS is the sum across all stages.

6 
If I use GL_TEXTURE1 through GL_TEXTURE3, the texture is a single washed out color

you want to use
glMultiTexCoord2f

instead of glTexCoord2f

7

are you on NVIDIA? Check this out - I believe it’s still relevant: http://developer.nvidia.com/object/General_FAQ.html#t6

8

Yes i am, and that site helps tremendously.

Thanks for all the answers, it seems to make sense now.