Why won't CG shaders work with GL 3.2?

问题

I've tried everything to get OpenGL 3.2 to render with CG shaders in my game engine but I have had no luck. So I decided to make a bare minimal project but still shaders won't work. In theory my test project should just render a red triangle but it is white because the shader is not doing anything.

I'll post the code here:

#include <stdio.h>
#include <stdlib.h>
#include <vector>
#include <string>
#include <GL/glew.h>
#include <Cg/cg.h>
#include <Cg/cgGL.h>
#include <SDL2/SDL.h>

int main()
{
    SDL_Window *mainwindow;
    SDL_GLContext maincontext;

    SDL_Init(SDL_INIT_VIDEO);

    SDL_GL_SetAttribute(SDL_GL_CONTEXT_MAJOR_VERSION, 3);
    SDL_GL_SetAttribute(SDL_GL_CONTEXT_MINOR_VERSION, 2);
    SDL_GL_SetAttribute(SDL_GL_DOUBLEBUFFER, 1);
    SDL_GL_SetAttribute(SDL_GL_DEPTH_SIZE, 24);

    mainwindow = SDL_CreateWindow("Test", SDL_WINDOWPOS_CENTERED, SDL_WINDOWPOS_CENTERED, 512, 512, SDL_WINDOW_OPENGL | SDL_WINDOW_SHOWN);

    maincontext = SDL_GL_CreateContext(mainwindow);

    glewExperimental = GL_TRUE;
    glewInit();

    _CGcontext* cgcontext;
    cgcontext = cgCreateContext();
    cgGLRegisterStates(cgcontext);

    CGerror error;
    CGeffect effect;
    const char* string;
    std::string shader;

    shader =
            "struct VS_INPUT"
            "{"
            "   float3 pos              : ATTR0;"
            "};"

            "struct FS_INPUT"
            "{"
            "   float4 pos                  : POSITION;"
            "   float2 tex                  : TEXCOORD0;"
            "};"

            "struct FS_OUTPUT"
            "{"
            "   float4 color                : COLOR;"
            "};"

            "FS_INPUT VS( VS_INPUT In )"
            "{"
            "   FS_INPUT Out;"
            "   Out.pos = float4( In.pos, 1.0f );"
            "   Out.tex = float2( 0.0f, 0.0f );"
            "   return Out;"
            "}"

            "FS_OUTPUT FS( FS_INPUT In )"
            "{"
            "   FS_OUTPUT Out;"
            "   Out.color = float4(1.0f, 0.0f, 0.0f, 1.0f);"
            "   return Out;"
            "}"

            "technique t0"
            "{"
            "   pass p0"
            "   {"
            "      VertexProgram = compile gp4vp VS();"
            "      FragmentProgram = compile gp4fp FS();"
            "   }"
            "}";

    effect = cgCreateEffect(cgcontext, shader.c_str(), NULL);
    error = cgGetError();
    if(error)
    {
        string = cgGetLastListing(cgcontext);
        fprintf(stderr, "Shader compiler: %s\n", string);
    }

    glClearColor ( 0.0, 0.0, 1.0, 1.0 );
    glClear ( GL_COLOR_BUFFER_BIT );

    float* vert = new float[9];

    vert[0] = 0.0; vert[1] = 0.5; vert[2] =-1.0;
    vert[3] =-1.0; vert[4] =-0.5; vert[5] =-1.0;
    vert[6] = 1.0; vert[7] =-0.5; vert[8]= -1.0;

    unsigned int m_vaoID;
    unsigned int m_vboID;

    glGenVertexArrays(1, &m_vaoID);
    glBindVertexArray(m_vaoID);

    glGenBuffers(1, &m_vboID);

    glBindBuffer(GL_ARRAY_BUFFER, m_vboID);
    glBufferData(GL_ARRAY_BUFFER, 9 * sizeof(GLfloat), vert, GL_STATIC_DRAW);

    glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 0, 0);
    glEnableVertexAttribArray(0);

    CGtechnique tech = cgGetFirstTechnique( effect );
    CGpass pass = cgGetFirstPass(tech);
    while (pass)
    {
        cgSetPassState(pass);
        glDrawArrays(GL_TRIANGLES, 0, 3);
        cgResetPassState(pass);
        pass = cgGetNextPass(pass);
    }

    glDisableVertexAttribArray( 0 );

    glBindVertexArray(0);

    delete[] vert;

    glBindBuffer(GL_ARRAY_BUFFER, 0);
    glDeleteBuffers(1, &m_vboID);
    glDeleteVertexArrays(1, &m_vaoID);

    SDL_GL_SwapWindow(mainwindow);
    SDL_Delay(2000);

    SDL_GL_DeleteContext(maincontext);
    SDL_DestroyWindow(mainwindow);
    SDL_Quit();

    return 0;
}

What am I doing wrong?

回答1:

I compiled the code and got the same result. So I added a CG error handler to get a bit more of information:

void errorHandler(CGcontext context, CGerror error, void * appdata) {
    fprintf(stderr, "%s\n", cgGetErrorString(error));
}
...
cgSetErrorHandler(&errorHandler, NULL);

When cgSetPassState and cgResetPassState were called I got the following error message:

Technique did not pass validation.

Not really very informative, of course. So I used GLIntercept to trace all OpenGL calls to a log file.

This time, when glewInit was called I got the following error message in the log file:

glGetString(GL_EXTENSIONS)=NULL glGetError() = GL_INVALID_ENUM

According OpenGL documentation, glGetString must not be called with GL_EXTENSIONS, was deprecated in 3.0, and glGetStringi must be used instead.

Finally, I found the issue in the GLEW library: http://sourceforge.net/p/glew/bugs/120/

I removed GLEW dependency and tested with gl3.h (and more recent glcorearb.h). I got the same error, but this time when cgGLRegisterStates was called.

I also tried CG trace.dll, just to get the same error (7939 = 0x1F03 = GL_EXTENSIONS):

glGetString
  {
  input:
    name = 7939
  output:
    return = NULL
  }

Then, I tested OpenGL 3.1 (SDL_GL_SetAttribute(SDL_GL_CONTEXT_MINOR_VERSION, 1);), and found that it was working fine:

glGetString(GL_EXTENSIONS)="GL_AMD_multi_draw_indirec..."

That is, the 3.1 context was compatible with previous OpenGL versions, but 3.2 not.

After a bit of Internet digging I found that you can create this type of compatible OpenGL context with SDL, just adding this line to the code:

SDL_GL_SetAttribute(SDL_GL_CONTEXT_PROFILE_MASK, SDL_GL_CONTEXT_PROFILE_COMPATIBILITY);

IMHO, CG Toolkit needs this type of compatibility profile.

回答2:

"Cg 3.1 context does not yet support forward-compatible OpenGL contexts!"

Source: http://3dgep.com/introduction-to-shader-programming-with-cg-3-1/

As the Cg project seems to be abandoned, it's also not likely to happen.

来源：https://stackoverflow.com/questions/14038001/why-wont-cg-shaders-work-with-gl-3-2