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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;
}
463
A uniform is a global Shader variable declared with the "uniform" storage qualifier. These act as parameters that the user of a shader program can pass to that program. Their values are stored in a program object.
The vertex shader will be executed roughly once for every vertex in the stream. A vertex shader is (usually) invariant with its input. That is, within a single Drawing Command, two vertex shader invocations that get the exact same input attributes will return binary identical results.
So how does the Vertex Shader receives data? It receives data by making use of the Vertex-Fetch stage, in-qualifier variables and glVertexAttrib().
gl_Position is a special variable that holds the position of the vertex in clip space. Since a vertex shader's main output is the position in clip space, it must always set gl_Position. This vertex shader just transforms each vertex position (by the VP matrix).
You're right isofar, that you need to divide the 4096 by 4 not multiply. The wiki entry is was worded badly.
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