Question

I am using SDL 1.2 for a project. It renders things just fine, but I want to do some small pixel shader effects. All of the examples for this show using OpenGl driver for SDL’s video subsystem.

So, I start the video subsystem with opengl as the driver, and tell SDL_SetVideoMode() to use SDL_OPENGL. When I go to run the program, it now starts crashing on the SetVideoMode() call, which worked fine without forcing OpenGl).

I went back and ran the program without forcing OpenGl and dumped out SDL_VideoDriverName() and it says I am using the “directx” driver.

My question is two-pronged: what is wrong that it doesn’t like the opengl driver, and how to I get SDL to use opengl without problems here? Or, how do I get the SDL surface into DirectX to apply pixel shader effects?

I would prefer to use OpenGl as it would be easier to port code to other platforms.

As an example, I have added this code that breaks when I try to use the OpenGl system:

#define WIN32_LEAN_AND_MEAN
#include <windows.h>

#include <SDL.h>

INT WINAPI WinMain( HINSTANCE hInst, HINSTANCE, LPSTR strCmdLine, INT )
{
    SDL_putenv("SDL_VIDEODRIVER=opengl");
    SDL_Init( SDL_INIT_EVERYTHING );
    SDL_VideoInit("opengl",0);

    SDL_GL_SetAttribute( SDL_GL_DOUBLEBUFFER, 1 );  // crashes here
    SDL_Surface *mWindow = SDL_SetVideoMode(1024,768,32,SDL_HWSURFACE|SDL_HWPALETTE|SDL_DOUBLEBUF|SDL_OPENGL);

    SDL_Quit();

    return 0; 
}

Answer 1

SDL without the OpenGL option uses DirectX to obtain direct access to a 3D drawing surface. Using OpenGL triggers a whole different codepath in SDL. And using OpenGL with SDL you no longer can use the SDL surface for direct access to the pixels It’s very likely your program crashes, because you’re still trying to directly access the surface.

Anyway, if you want to use pixel shaders, you no longer must use direct pixel buffer access, as provided by plain SDL. You have to do everything through OpenGL then.

Update

Some of the parameters you give to SDL are mutually exclusive. Also the driver name given to SDL_VideoInit makes no sense if used together with OpenGL (it’s only relevant together with DirectDraw to select a specific output device).

Also, because you already did call SDL_Init(SDL_INIT_EVERYTHING) the call to SDL_VideoInit is redundant and maybe harmfull actually.

See this for a fully working OpenGL example:

http://sdl.beuc.net/sdl.wiki/Using_OpenGL_With_SDL