Accessing uniform variables from a Cg shader in OpenGL

Question

I am trying to implement a simple PC program with OpenGL, using mandatorily Cg shaders (no Unity whatsoever).

I have found some tips on this page http://bobobobo.wordpress.com/2008/10/05/cg-1/ and kept the Cgprogram, Cgcontext and Cgprofile types in my own project.

Problem is: How can you access uniform variables in the C++ code? I have tried the glGetUniformLocation way, but that won't work because the first parameter must be the program's ID of GLuint type and I have CGprogram type.

I have already searched by myself (including the Nvidia Cg 'tutorial') but I can't seem to find both Cg and OpenGL discussed on a page without having to deal with Unity.

edit: I can now access the variables, but in my case I'm trying to change a texture, and it doesn't bind.

Answer 1

cgGetNamedParameter

Example code:

CGprogram cgprog;
cgprog = cgCreateProgram(...);
cgGLLoadProgram(cgprog);
CGparameter modelviewproj, lightpos;
modelviewproj = cgGetNamedParameter(cgprog, "modelviewproj");
lightpos = cgGetNamedParameter(cgprog, "lightpos");

Sample code paraphrased from the book Advanced Game Development with Programmable Graphics Hardware by Watt Policarpo.

Answer 2

I have finally managed to find a way to access uniform texture variables.

First (after preparing the GL window) you have to initialize the Cg stuff:

myCgContext = cgCreateContext();
myCgVertexProfile = cgGLGetLatestProfile(CG_GL_VERTEX);
cgGLSetOptimalOptions(myCgVertexProfile);
myCgFragmentProfile = cgGLGetLatestProfile(CG_GL_FRAGMENT);
cgGLSetOptimalOptions(myCgFragmentProfile);

myCgVertexProgram =
    cgCreateProgramFromFile(myCgContext, CG_SOURCE,"../vertexshader.cg", myCgVertexProfile,"VS_Main",NULL);

myCgFragmentProgram =
    cgCreateProgramFromFile(myCgContext, CG_SOURCE,"../fragmentshader.cg", myCgFragmentProfile,"FS_Main",NULL);

cgGLLoadProgram(myCgVertexProgram);
cgGLLoadProgram(myCgFragmentProgram);

Then, in your display function, you have to do what you usually do, but put the following lines exactly BEFORE the actual draw (glDrawArrays in my case):

CGparameter loctex = cgGetNamedParameter(myCgFragmentProgram, "tex"); // in my fragment shader I have declared: "uniform sampler2D tex;"
cgGLSetTextureParameter(loctex, IDtex[1]); // IDtex is my array of texture IDs
cgGLEnableTextureParameter(loctex);

You can find more examples of accessing Cg variables in the NVIDIA OpenGL SDK projects (but that's difficult...); otherwise I have found great simple examples of using Cg with OpenGL here (scroll down & check right column): http://perso.limsi.fr/jacquemi/RGA-TD/index.html