Question

Using Radeon 3870HD I’ve run into strange behavior with AMD’s drivers (updated yesterday to newest version available).

First of all I’d like to note that whole code ran without problem on NVIDIA GeForce 540M and that glGetUniformLocation isn’t failing all the time.

So my problem is that I get from glGetUniformLocation strange values with one of my shader programs used in app, while the other shader program doesn’t have such flaw. I switch that shaders between frames so I’m sure it isn’t temporary and is related to that shader. By strange values I mean something like 17-26, while I have only 9 uniforms present. I’m using my interface for shaders that afterwards queries for type of GLSL variable with just obtained variable and as side effect I also query its name. For all those 17-26 locations I got returned the name wasn’t set and same for type. Now I’ve got idea to debug into interface which is separate library and change those values to something I’d expect: 0-8. Using debugger I changed these and indeed they returned proper variable name in that shader and also type was correct.

My question is how possibly could the code that is working always with NVIDIA and also with that other shader on Radeon behave differently for another shader, that is treated the same way, fail?

I include related part of interface for this:

//this fails to return correct value
m_location = glGetUniformLocation(m_program.getGlID(), m_name.c_str());

printGLError();
if(m_location == -1){
    std::cerr << "ERROR: Uniform " << m_name <<  " doesn't exist in program" << std::endl;
    return FAILURE;
}

GLsizei charSize = m_name.size()+1, size = 0, length = 0;
GLenum type = 0;
GLchar* name = new GLchar[charSize];
name[charSize-1] = '\0';
glGetActiveUniform(m_program.getGlID(), m_location, charSize, &length, &size, &type, name); 

delete name; name = 0;

if(!TypeResolver::resolve(type, m_type))
    return FAILURE;

m_prepared = true;
m_applied = false;

Answer 1

The index you pass to glGetActiveUniform is not supposed to be a uniform location. Uniform locations are only used with glUniform calls; nothing else.

The index you pass to glGetActiveUniform is just a index between 0 and the value returned by glGetProgram(GL_ACTIVE_UNIFORMS). It is used to ask what uniforms exist and to inspect the properties of those uniforms.

Your code works on NVIDIA only because you got lucky. The OpenGL specification doesn’t guarantee that the order of uniform locations is the same as the order of active uniform indices. AMD’s drivers don’t work that way, so your code doesn’t work.