C++ Semi-Transparent Window SDL

Question

I wish to have a semi-transparent SDL background (nothing to do with sub-surfaces or images), such that instead of having a black background it is actually transparent, but the other things I draw are not. My current code is a slightly modified copy of Code::Blocks' SDL project, similar to how various applications have rounded borders or odd shapes besides rectangles.

#ifdef __cplusplus
    #include <cstdlib>
#else
    #include <stdlib.h>
#endif
#ifdef __APPLE__
#include <SDL/SDL.h>
#else
#include <SDL.h>
#endif

int main ( int argc, char** argv )
{
    putenv("SDL_VIDEO_WINDOW_POS");
    putenv("SDL_VIDEO_CENTERED=1");

    // initialize SDL video
    if ( SDL_Init( SDL_INIT_VIDEO ) < 0 )
    {
        printf( "Unable to init SDL: %s\n", SDL_GetError() );
        return 1;
    }

    // make sure SDL cleans up before exit
    atexit(SDL_Quit);

    // create a new window
    SDL_Surface* screen = SDL_SetVideoMode(640, 480, 16,
                                           SDL_HWSURFACE|SDL_DOUBLEBUF|SDL_NOFRAME);
    if ( !screen )
    {
        printf("Unable to set 640x480 video: %s\n", SDL_GetError());
        return 1;
    }

    // load an image
    SDL_Surface* bmp = SDL_LoadBMP("cb.bmp");
    if (!bmp)
    {
        printf("Unable to load bitmap: %s\n", SDL_GetError());
        return 1;
    }

    // centre the bitmap on screen
    SDL_Rect dstrect;
    dstrect.x = (screen->w - bmp->w) / 2;
    dstrect.y = (screen->h - bmp->h) / 2;

    // program main loop
    bool done = false;
    while (!done)
    {
        // message processing loop
        SDL_Event event;
        while (SDL_PollEvent(&event))
        {
            // check for messages
            switch (event.type)
            {
                // exit if the window is closed
            case SDL_QUIT:
                done = true;
                break;

                // check for keypresses
            case SDL_KEYDOWN:
                {
                    // exit if ESCAPE is pressed
                    if (event.key.keysym.sym == SDLK_ESCAPE)
                        done = true;
                    break;
                }
            } // end switch
        } // end of message processing

        // DRAWING STARTS HERE

        // clear screen
        SDL_FillRect(screen, 0, SDL_MapRGB(screen->format, 0, 0, 0));

        // draw bitmap
        SDL_BlitSurface(bmp, 0, screen, &dstrect);

        // DRAWING ENDS HERE

        // finally, update the screen :)
        SDL_Flip(screen);
    } // end main loop

    // free loaded bitmap
    SDL_FreeSurface(bmp);

    // all is well ;)
    printf("Exited cleanly\n");
    return 0;
}

Answer 1

I think what you're trying to do is in fact a shaped window (parts of the window are transparent depending on a mask that you provide). It seems there's no way to do that with SDL 1.2, however there is a SDL_SetWindowShape function just for this in SDL 1.3 for which you can find a pre-release snapshot here but it's not even in beta yet so I suggest waiting until it's officialy released :)

Answer 2

this is a link to a pretty neat article about development of an older application for Mac OS 9, which did not have support for shaped windows, either. It's actually a neat article in general about software development.

But the idea seems pretty smart, and I wonder if you might be able to get it working here, too. Instead of trying to make a transparent background, they actually take a screen-shot of the computer right where their window is going to go, and then use that screen shot for their background. When the user drags the window around on the screen, they continue to update the background with new screen-shots. I think this might be more complicated than you were hoping for, but it's certainly an interesting idea.