Question

I’m trying to integrate a Qt library with an SDL application. I’d like to convert a QPixmap to an SDL_Surface, and then display that surface. How can I do this? I’ve not been able to find any good examples.

I’ve managed the following code so far:

Uint32 rmask = 0x000000ff;
Uint32 gmask = 0x0000ff00;
Uint32 bmask = 0x00ff0000;
Uint32 amask = 0xff000000;

SDL_FillRect(screen, NULL, SDL_MapRGBA(screen->format, 255, 255, 255, 255));

const QImage *qsurf = ...;

SDL_Surface *surf = SDL_CreateRGBSurfaceFrom((void*)qsurf->constBits(), qsurf->width(), qsurf->height(), 32, qsurf->width() * 4, rmask, gmask, bmask, amask);
SDL_BlitSurface(surf, NULL, screen, NULL);
SDL_FreeSurface(surf);
SDL_Flip(screen);

This works, but the only problem is that each time my QImage-based surface is painted, the underlying area is not cleared and the transparent parts “fade” into solid over the course of a few frames.

I do have SDL_FillRect which I would imagine clears the screen but it doesn’t seem to be doing so. screen is the primary SDL surface.

Answer 1

My original problem of overpainting was because my source image was actually not getting cleared properly. Oops. Once I fixed that, I simply had my masks wrong; not having it click in my mind how SDL was using these. The final, working code is as follows:

Here’s the function to convert a QImage to an SDL_Surface:

/*!
 * Converts a QImage to an SDL_Surface.
 * The source image is converted to ARGB32 format if it is not already.
 * The caller is responsible for deallocating the returned pointer.
 */
SDL_Surface* QImage_toSDLSurface(const QImage &sourceImage)
{
    // Ensure that the source image is in the correct pixel format
    QImage image = sourceImage;
    if (image.format() != QImage::Format_ARGB32)
        image = image.convertToFormat(QImage::Format_ARGB32);

    // QImage stores each pixel in ARGB format
    // Mask appropriately for the endianness
#if SDL_BYTEORDER == SDL_BIG_ENDIAN
    Uint32 amask = 0x000000ff;
    Uint32 rmask = 0x0000ff00;
    Uint32 gmask = 0x00ff0000;
    Uint32 bmask = 0xff000000;
#else
    Uint32 amask = 0xff000000;
    Uint32 rmask = 0x00ff0000;
    Uint32 gmask = 0x0000ff00;
    Uint32 bmask = 0x000000ff;
#endif

    return SDL_CreateRGBSurfaceFrom((void*)image.constBits(),
        image.width(), image.height(), image.depth(), image.bytesPerLine(),
        rmask, gmask, bmask, amask);
}

And the core of my drawing function:

// screen == SDL_GetVideoSurface()
// finalComposite == my QImage that SDL will convert and display
SDL_FillRect(screen, NULL, SDL_MapRGBA(screen->format, 255, 255, 255, 255));
SDL_Surface *surf = QImage_toSDLSurface(finalComposite);
SDL_BlitSurface(surf, NULL, screen, NULL);
SDL_FreeSurface(surf);
SDL_Flip(screen);