You need to tell SDL what OpenGL kind and version before you create a
window. For example if you want OpenGLES 2.0 you would write:
SDL_Init(SDL_INIT_VIDEO);
SDL_GL_SetAttribute(SDL_GL_CONTEXT_PROFILE_MASK, SDL_GL_CONTEXT_PROFILE_ES);
SDL_GL_SetAttribute(SDL_GL_CONTEXT_MAJOR_VERSION, 2);
SDL_GL_SetAttribute(SDL_GL_CONTEXT_MINOR_VERSION, 0);
If you want normal OpenGL 4.0 you would write:
SDL_GL_SetAttribute(SDL_GL_CONTEXT_PROFILE_MASK,
SDL_GL_CONTEXT_PROFILE_CORE);
SDL_GL_SetAttribute(SDL_GL_CONTEXT_MAJOR_VERSION, 4);
SDL_GL_SetAttribute(SDL_GL_CONTEXT_MINOR_VERSION, 0);
Then you can create the SDL and load the appropriate OpenGL function
prototypes using:
glClear := SDL_GL_GetProcAddress('glClear');
It's up to you to know which functions are part of which which OpenGL spec,
and you should need to load the functions dynamically to stubs:
var glClear: procedure(mask: GLbitfield); cdecl;
Don't forget, if you want a specific values for stencil bits, pixel order,
multisampling levels and other stuff, you need to set those values before
you create a window. Check the return values of SDL_CreateWindow and
SDL_GL_CreateContext to see if anything above fails, for example the target
device doesn't support OpenGL 4.0.
SDL_GL_SetAttribute(SDL_GL_MULTISAMPLESAMPLES, 8); // 8x anti-aliasing,
must be called before SDL_CreateWindow
SDL_GL_SetAttribute(SDL_GL_STENCIL_SIZE, 16); // 16 bit stencil buffer,
must be called before SDL_CreateWindow
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