[SDL] per-surface alpha and hardware surfaces?

[Sean Ridenour]

On Sunday 14 December 2003 7:22 pm, Warren Schwader wrote:
> Here is a simple and short little Win32 program that loads an 800 x 600
> background image and draws another image on top (movable using the mouse.) 
> Then I use SetAlpha on the surface that I move with the mouse to fade it in
> and out.
>
> My question is:  Is per-surface alpha supposed to work when using hardware
> surfaces? (or maybe it is related to double-buffering? - not sure.)  Why
> doesn't the dSurface show?  Yes I could convert all of my surfaces to
> per-alpha surfaces (and that works fine) but I'd rather not have to go that
> far.
>
> The problem is that the call to:    SDL_SetAlpha(dSurface, SDL_SRCALPHA,
> alpha);  makes it so that the 2nd image (dSurface) doesn't show.  When this
> line is removed, everything is fine.

I don't think you can use color keying and alpha transparency at the same 
time. You've got to pick one or the other, as far as I know.

-Sean Ridenour

> #include "sdl.h"
> #include "sdl_image.h"
>
> SDL_Surface* mainSurface;
> SDL_Event  event;
>
> int main(int argc, char* argv[])
> {
>    SDL_Init(SDL_INIT_EVERYTHING);
>    mainSurface = SDL_SetVideoMode(800, 600, 32,
>                            SDL_SWSURFACE | SDL_FULLSCREEN | SDL_DOUBLEBUF);
>
>    SDL_Surface* backSurface1 = IMG_Load("back.png");// load an 800 x 600
> image SDL_Surface* backSurface = SDL_DisplayFormat(backSurface1);
>    SDL_FreeSurface(backSurface1);
>
>    / / load a 300 x270 image.  The transparent color is RGB 255, 0, 221
>    SDL_Surface* dSurface1 = IMG_Load("dialogimage.png");
>    SDL_Surface* dSurface = SDL_DisplayFormat(dSurface1);
>    SDL_FreeSurface(dSurface1);
>
>    SDL_SetColorKey(dSurface, SDL_SRCCOLORKEY,
>    SDL_MapRGB(mainSurface->format, 255, 0, 221));
>
>    int alpha = 0;
>    int alphaAdjust = 2;
>    SDL_Rect rect;
>    rect.x = 0;
>    rect.y = 100;
>    rect.w = 300;
>    rect.h = 270;
>
>    while (true)
>    {
>         SDL_PollEvent(&event);
>         if (event.type == SDL_KEYDOWN)
>              break;
>         if (event.type == SDL_MOUSEMOTION)
>         {
>              rect.x = event.motion.x;
>              rect.y = event.motion.y;
>              if (rect.x < 0) rect.x = 0;
>              if (rect.y < 0) rect.y = 0;
>              if (rect.x > 500) rect.x = 500;
>              if (rect.y > 330) rect.y = 330;
>         }
>
>         // blit the background to the screen surface and then update the
> screen SDL_FillRect(mainSurface, NULL, SDL_MapRGB(mainSurface->format, 0,
> 0, 0)); SDL_BlitSurface(backSurface, NULL, mainSurface, NULL);
>         SDL_SetAlpha(dSurface, SDL_SRCALPHA, alpha);
>         SDL_BlitSurface(dSurface, NULL, mainSurface, &rect);
>         SDL_UpdateRect(mainSurface, 0, 0, mainSurface->w, mainSurface->h);
>         SDL_Flip(mainSurface);
>
>         alpha += alphaAdjust;
>         if (alpha < 0)
>         {
>              alpha = 0;
>              alphaAdjust = 2;
>         }
>         if (alpha > 255)
>         {
>              alpha = 255;
>              alphaAdjust = -2;
>         }
>     }
>
>     SDL_FreeSurface(mainSurface);
>     SDL_FreeSurface(backSurface);
>     SDL_Quit();
>     return(0);
> }
>
> Thanks,
> -- Warren Schwader