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Semi-Transparent Rectangles

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I'm trying to be able to create a rectangle with an alpha value which I can then draw to my screen using SDL. I tried doing this
SDL_FillRect(screen, rect, SDL_MapRGBA(screen->format, 255, 0, 0, 150));

where screen is the surface set as the window and rect is the destination rectangle, but it just draws a fully opaque rectangle in the position. My bits per pixel is 32. Does anyone know how I would be able to do this.

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Quote:
Original post by thejahooli
I'm trying to be able to create a rectangle with an alpha value which I can then draw to my screen using SDL. I tried doing this

*** Source Snippet Removed ***

where screen is the surface set as the window and rect is the destination rectangle, but it just draws a fully opaque rectangle in the position. My bits per pixel is 32. Does anyone know how I would be able to do this.

I'm not sure about the most recent versions of SDL, but as I recall, you have to make your own function to do this. Something like this should work:


//Returns 0 on success, -1 on failure. Call SDL_GetError to get the error messages.
int SDL_FillAlphaRect(SDL_Surface *destination, SDL_Rect *rect, SDL_Color color)
{
if(destination == NULL)
{
SDL_SetError("SDL_FillAlphaRect: 'destination' is NULL");
return -1;
}
if(rect == NULL)
{
SDL_SetError("SDL_FillAlphaRect: 'rect' is NULL");
return -1;
}

SDL_Surface *temporarySurface = SDL_CreateRGBSurface(SDL_SWSURFACE, rect->w, rect->h, destination->format->BitsPerPixel,
destination->format->Rmask, destination->format->Gmask,
destination->format->Bmask, destination->format->Amask);
if(temporarySurface == NULL)
return -1; //We don't want to overwrite CreateRGBSurface's error message, so we just return.

if(SDL_FillRect(screen, NULL, SDL_MapRGB(temporarySurface->format, color.r, color.g, color.b)) == -1)
return -1;


if(SDL_SetAlpha(temporarySurface, SDL_SRCALPHA, color.a) == -1)
return -1;

if(SDL_BlitSurface(temporarySurface, NULL, destination, rect) == -1)
return -1;

SDL_FreeSurface(temporarySurface);

return 0;
}





Warning: Since I'm creating and then destroying a surface every function call, it wont be super fast, but it should work for casual usage (a few GUI windows, as opposed to a couple hundred partical effects or something). Also, I didn't bother testing the function (just typed it straight into the post), but it *should* work.

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