What is GL_MAX_COMBINED_TEXTURE_IMAGE_UNITS?

Question

I am a beginner in OpenGL. I am learning about textures in OpenGL. What I don't understand is how to determine how many texture units are in the GPU. I heard someone said that you can see how many texture units by writing the following code.

int total_units;
glGetIntegerv(GL_MAX_COMBINED_TEXTURE_IMAGE_UNITS, &total_units);
std::cout << total_units << '\n';  // the result is 192

Is there 192 texture units in my GPU? In documentation, it says:

GL_MAX_COMBINED_TEXTURE_IMAGE_UNITS params returns one value, the maximum supported texture image units that can be used to access texture maps from the vertex shader and the fragment processor combined. If both the vertex shader and the fragment processing stage access the same texture image unit, then that counts as using two texture image units against this limit. The value must be at least 48. See glActiveTexture.

So I wanted to know how many texture units can be used to access texture maps from the vertex and fragment shaders. So I wrote and run the following code.

int vertex_units, fragment_units;
glGetIntegerv(GL_MAX_VERTEX_TEXTURE_IMAGE_UNITS, &vertex_units);
std::cout << vertex_units << "\n";   // the result is 32
glGetInteferv(GL_MAX_TEXTURE_IMAGE_UNITS, &fragment_units);
std::cout << fragment_units << "\n"; // the result is also 32

So 32+32=64. But why does GL_MAX_COMBINED_TEXTURE_IMAGE_UNITS shows me the result of 192? I think I am missing something. What do I need to calculate to get 192?

And also, in OpenGL, why are there only GL_TEXTURE0 to GL_TEXTURE31 macros? I think these macros are for each shaders. Am I right?

Answer 1

What do I need to calculate to get 192?

You need to recognize the fact that there are more than 2 shader stages. OpenGL 4.5 has six programmable stages: vertex, tessellation control, tessellation evaluation, geometry, fragment, and compute. Each of them can access some number of textures, and the "combined" value specifies how many can be accessed by all stages in total.

why are there only GL_TEXTURE0 to GL_TEXTURE31 macros?

The proper way to set the active texture unit is with the value GL_TEXTURE0 + i, where i is the texture unit index. All of the macros 1 to 31 are monotonically increasing in value, so GL_TEXTURE0 + 5 is the same value as GL_TEXTURE5.

So to set the active texture unit, use glActiveTexture(GL_TEXTURE0 + i);.

Basically, OpenGL only defines 32 macros because they never needed to define more than 1. The other 31 are just for the sake of convenience.