无法在OSX 10.11（MBP，GeForce 750M）上使用SDL2创建有效的OpenGL 3.3上下文

Question

I have the following (working) setup code for OpenGL using SDL2:

int main(int argc, char* argv[])
{
  SDL_SetMainReady();
  if (SDL_Init(SDL_INIT_VIDEO) != 0) return 1);

  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_BUFFER_SIZE, 32);
  SDL_GL_SetAttribute(SDL_GL_DOUBLEBUFFER, 1);
  SDL_GL_SetAttribute(SDL_GL_DEPTH_SIZE, 16);
  SDL_GL_SetAttribute(SDL_GL_ACCELERATED_VISUAL, 1);
  SDL_GL_SetAttribute(SDL_GL_CONTEXT_FLAGS, SDL_GL_CONTEXT_FORWARD_COMPATIBLE_FLAG);

  SDL_GL_SetAttribute(SDL_GL_CONTEXT_MAJOR_VERSION, 2);
  SDL_GL_SetAttribute(SDL_GL_CONTEXT_MINOR_VERSION, 1);

   auto sdl_window = SDL_CreateWindow("GL Experiment",
     SDL_WINDOWPOS_CENTERED, // x position.
     SDL_WINDOWPOS_CENTERED, // y position.
     640, // Width.
     640, // Height.
     SDL_WINDOW_HIDDEN | SDL_WINDOW_OPENGL);
   if (sdl_window == nullptr) return 2;
   auto sdl_gl_context = SDL_GL_CreateContext(sdl_window);
   if (sdl_gl_context == nullptr) return 3;

   ...

However, if I change the SDL_GL_CONTEXT_VERSION... lines to:

// Use a core profile setup.
SDL_GL_SetAttribute(SDL_GL_CONTEXT_MAJOR_VERSION, 3);
SDL_GL_SetAttribute(SDL_GL_CONTEXT_MINOR_VERSION, 3);
SDL_GL_SetAttribute(SDL_GL_CONTEXT_PROFILE_MASK, SDL_GL_CONTEXT_PROFILE_CORE);

Then it breaks in an odd way:

• The program does not crash / exit with an error.
• glClear appears to still work (the window has whatever background colour is chosen).
• No rendering appears (I am using the OpenGL SuperBible 'GLTools' code to do the rendering, which should only be using Core Profile calls, and works with the 2.1 context)

• There are some oddities in the attributes the context reports:

   SDL_GL_RED_SIZE: 0 SDL_GL_GREEN_SIZE: 0 SDL_GL_BLUE_SIZE: 0 SDL_GL_ALPHA_SIZE: 0 SDL_GL_BUFFER_SIZE: 4 SDL_GL_DOUBLEBUFFER: 1 SDL_GL_DEPTH_SIZE: 0 SDL_GL_STENCIL_SIZE: 0 SDL_GL_ACCUM_RED_SIZE: 0 SDL_GL_ACCUM_GREEN_SIZE: 0 SDL_GL_ACCUM_BLUE_SIZE: 0 SDL_GL_ACCUM_ALPHA_SIZE: 0 SDL_GL_STEREO: 0 SDL_GL_MULTISAMPLEBUFFERS: 0 SDL_GL_MULTISAMPLESAMPLES: 0 SDL_GL_ACCELERATED_VISUAL: 1 SDL_GL_CONTEXT_MAJOR_VERSION: 3 SDL_GL_CONTEXT_MINOR_VERSION: 3 SDL_GL_CONTEXT_FLAGS: 2 SDL_GL_CONTEXT_PROFILE_MASK: 1 SDL_GL_SHARE_WITH_CURRENT_CONTEXT: 0 SDL_GL_FRAMEBUFFER_SRGB_CAPABLE: 0 

So the context claims to have 0 bit colour bit depth etc. This seems inconsistent with the rendering of the initial glClear call though, which happily renders in colour.

I've checked a few OpenGL functions and they appear to resolve to real function addresses (just using OSX's <opengl/gl3.h> to get these, but I've checked and they're the same addresses that loading the function pointers through SDL yields.

Update

The output from glGetString is:

    GL_VENDOR: NVIDIA Corporation
    GL_RENDERER: NVIDIA GeForce GT 750M OpenGL Engine
    GL_VERSION: 4.1 NVIDIA-10.6.47 310.42.05f01
    GL_SHADING_LANGUAGE_VERSION: 4.10

This is when 3.3/Core is requested, so clearly it gives a 4.1 context. When 2.1 is requested it's the same but correctly shows GL_VERSION: 2.1 ... .

Answer 1

It turns out the OpenGL context has been created successfully, ie the above code works just fine in terms of creating the context.

The issue therefore appears to be twofold:

1. The OpenGL SuperBible (5th edition) drawing code must contain a bug in the form of Compatibility Profile code that prevents the actual rendering from working.
2. Either SDL 2.0.4, or the underlying OSX methods it uses, appear to contain a bug when reporting on the bit depth etc of the context created.

Update

Having spent some time getting the OpenGl SuperBible examples in earlier chapters working with a 3.3 Core context, I discover in a later chapter that they deliberately target a Compatibility profile so the book will work on earlier systems. This explains point 1 above, and it is a shame they do not provide this information up front when describing how to get started!