What's the difference between TMU and openGL's GL_TEXTUREn?

Question

I can't quite understand what's the difference. I know TMU is a texture mapping unit on GPU, and in opengl, we can have many texture units.I used to think they're the same, that if I got n TMU, then I can have n GL_TEXTURE to use, but I found that this may not be true. Recently, I was working on an android game, targetting a platform using the Mali 400MP GPU.According to the document, it has only one TMU, I thought that I can use only one texture at a time.But suprisingly, I can use at least 4 textures without trouble.Why is this? Is the hardware or driver level doing something like swap different textures in/out automatically for me? If so, is it supposed to cause a lot of cache miss?

Answer 1

I'm not the ultimate hardware architecture expert, particularly not for Mali. But I'll give it a shot anyway, based on my understanding.

The TMU is a hardware unit for texture sampling. It does not get assigned to a OpenGL texture unit on a permanent basis. Any time a shader executes a texture sampling operation, I expect this specific operation to be assigned to one of the TMUs. The TMU then does the requested sampling, delivers the result back to the shader, and is available for the next sampling operation.

So there is no relationship between the number of TMUs and the number of supported OpenGL texture units. The number of OpenGL texture units that can be supported is determined by the state tracking part of the hardware.

The number of TMUs has an effect on performance. The more TMUs are available, the more texture sampling operations can be executed within a given time. So if you use a lot of texture sampling in your shaders, your code will profit from having more TMUs. It doesn't matter if you sample many times from the same texture, or from many different textures.