What is the actual number of vertex uniform components for GLSL shader on ATI graphics card?

Question

I'm writing a GLSL vertex shader for an iMac with a AMD Radeon HD 6970M 2048 MB graphics card:

GL_MAX_VERTEX_ATTRIBS: 16
GL_MAX_VERTEX_UNIFORM_COMPONENTS: 4096
GL_VERSION: 2.1 ATI-7.12.9
GL_SHADING_LANGUAGE_VERSION: 1.20

In my shader I would like to have a large array of uniform mat4s:

uniform mat4 T[65]

but if I try to have 65 of these my shader (secretly) switches to Apple Software Renderer mode. If I instead use 64:

uniform mat4 T[64]

everything is fine.

Seems to be a problem with exceeding the maximum number of uniforms. But as I wrote above I'm getting 4096 for GL_MAX_VERTEX_UNIFORM_COMPONENTS so 4096/(4*4) = 256 not 64...

OpenGL.org wiki says

ATI/AMD note: The ATI max component values are wrong. They are the actual number of components divided by 4.

But reading this I would think that if I query GL_MAX_VERTEX_UNIFORM_COMPONENTS and get 4096 that I actually have 16,384. What seems to be the case is that GL_MAX_VERTEX_UNIFORM_COMPONENTS returns the actual number of components multiplied by 4. This would then give 1024/(4*4) = 64.

Can anyone confirm this?

Edit: My shader is simply:

#version 120

// 65 goes into software render mode
#define MAX_T 64
attribute vec4 indices;
uniform mat4 T[MAX_T];

void main()
{
  gl_Position =  T[int(indices[0])]*gl_Vertex;
}

Could you please post your shader code, or at least the definition of all used uniforms. And you're right isofar, that you need to divide the 1024 by 4 not multiply. The wiki entry is worded badly. — datenwolf
I posted the shader code above (I also noticed that before I had a one off in my counting, so I edited the numbers in the original question 63->64 and 64->65 accordingly). — Alec Jacobson
@datenwolf You should post this as an answer, as this seems to be his problem quite obviously. I guess he just uses some other small uniforms that "block" the last 64th matrix slot (builtin uniforms also count, I think). — Christian Rau
I'm curious: what happens if you made two mat4 arrays, of length 33? — Nicol Bolas
@Nicol 33 doesn't work because you end up with 66. On the other hand 32 does work. — Alec Jacobson

datenwolf datenwolf · Accepted Answer · 2011-12-21T14:48:44

You're right isofar, that you need to divide the 4096 by 4 not multiply. The wiki entry is was worded badly.

What is the actual number of vertex uniform components for GLSL shader on ATI graphics card?

1 Answers