具有0个颜色位和0个深度位的SDL2 OpenGL 4.2上下文

Question

I've been trying to start a new SDL + GLEW + OpenGL project, and the setup has been difficult (MinGW-w64-32 on Windows 8 64-bit with an Intel i7-4700MQ CPU and NVidia GTX 745M GPU).

If I set the GL attributes to be used for context creation to use OpenGL version 4.2, the color and depth bit sizes get set to 0. However, if I request a 2.1 context (which is also the default), I can get the requested bit depths (8 bits for each color, 24 bits for depth). In either case, however, glClearColor has no effect (just a black background).

In both cases, the result of a few glGetString calls is the same- a 4.2 context, suggesting SDL's output is far from correct.

The entirety of the code can be found here , it's mostly boilerplate for a larger project for now. The relevant sections would most likely be

if(SDL_Init(SDL_INIT_EVERYTHING) != 0) {
    std::cerr << "Error initializing SDL.\n";
    exit(1);
}

SDL_GL_SetAttribute(SDL_GL_ACCELERATED_VISUAL, 1);
SDL_GL_SetAttribute(SDL_GL_CONTEXT_MAJOR_VERSION, 2);
SDL_GL_SetAttribute(SDL_GL_CONTEXT_MINOR_VERSION, 1);
//SDL_GL_SetAttribute(SDL_GL_CONTEXT_FLAGS, SDL_GL_CONTEXT_FORWARD_COMPATIBLE_FLAG);
SDL_GL_SetAttribute(SDL_GL_RED_SIZE, 8);
SDL_GL_SetAttribute(SDL_GL_GREEN_SIZE, 8);
SDL_GL_SetAttribute(SDL_GL_BLUE_SIZE, 8);
SDL_GL_SetAttribute(SDL_GL_ALPHA_SIZE, 8);
SDL_GL_SetAttribute(SDL_GL_DEPTH_SIZE, 24);
SDL_GL_SetAttribute(SDL_GL_DOUBLEBUFFER, 1);

SDL_Window* window = SDL_CreateWindow("RenderSystem", SDL_WINDOWPOS_CENTERED, SDL_WINDOWPOS_CENTERED, 640, 480, SDL_WINDOW_OPENGL|SDL_WINDOW_RESIZABLE);

if(!window) {
    std::cerr << "Window creation failed.\n";
    SDL_Quit();
    exit(2);
}

SDL_GLContext context = SDL_GL_CreateContext(window);

if(!context) {
    std::cerr << "OpenGL Context creation failed.\n";
    SDL_DestroyWindow(window);
    SDL_Quit();
    exit(3);
}

SDL_GL_MakeCurrent(window, context);

and

glClearColor(1.0f, 1.0f, 1.0f, 1.0f);

SDL_Event evt;
bool run = true;

while(run) {

    SDL_PollEvent(&evt);

    switch(evt.type) {
        case SDL_KEYDOWN:
            if(evt.key.keysym.sym == SDLK_ESCAPE) {
                run = false;
            }
            break;

        case SDL_QUIT:
            run = false;
            break;
    }

    SDL_GL_SwapWindow(window);

}

Answer 1

I'm using SDL's built-in function SDL_GL_GetAttribute which returns the values that SDL uses to create the context, not the actual context attributes (as I understand it).

This is incorrect, I took a look at the SDL implementation of SDL_GL_GetAttribute (...) (see src/video/SDL_video.c ) and it does what I described. You cannot query the values on a core profile context because they are not defined for the default framebuffer.

Here is where the problem comes from:

int
SDL_GL_GetAttribute(SDL_GLattr attr, int *value)
{
  // ...

  switch (attr) {
  case SDL_GL_RED_SIZE:
    attrib = GL_RED_BITS;
    break;
  case SDL_GL_BLUE_SIZE:
    attrib = GL_BLUE_BITS;
    break;
  case SDL_GL_GREEN_SIZE:
    attrib = GL_GREEN_BITS;
    break;
  case SDL_GL_ALPHA_SIZE:
    attrib = GL_ALPHA_BITS;
    break;
  }

  // ...

  glGetIntegervFunc(attrib, (GLint *) value);
  error = glGetErrorFunc();
}

That code actually generates a GL_INVALID_ENUM error on a core profile, and the return value of SDL_GL_GetAttribute (...) should be non-zero as a result.

If you must get meaningful values from SDL_GL_GetAttribute (...) for bit depths, then that means you must use a compatibility profile. SDL2 does not extract this information from the pixel format it selected (smarter frameworks like GLFW do this), but it naively tries to query it from GL.

Answer 2

If not fail me memory, SDL_GL_DEPTH_SIZE has to be the sum of all color channels:

using four color channels:

SDL_GL_SetAttribute (SDL_GL_DEPTH_SIZE, 32);

If you were using 3 color channels would then:

SDL_GL_SetAttribute (SDL_GL_DEPTH_SIZE, 24);

Already had some problems with it, this might be the problem. Sorry for my english.