[SDL] fastest way to draw transparent box

[Loren Osborn]

actually, if it is a SDL_SWSURFACE, the wastest thing to do if you want
a gray-alpha box is probably:

for 50%

  Uint32 mask = (surface->format->Rmask & (surface->format->Rmask >> 1))
        | (surface->format->Gmask & (surface->format->Gmask >> 1))
        | (surface->format->Bmask & (surface->format->Bmask >> 1));

then for each pixel:

      pixel = (pixel >> 1) & mask;


for 75%

  Uint32 mask1 =
          (surface->format->Rmask & (surface->format->Rmask >> 1))
        | (surface->format->Gmask & (surface->format->Gmask >> 1))
        | (surface->format->Bmask & (surface->format->Bmask >> 1));
  Uint32 mask2 =
          (surface->format->Rmask & (surface->format->Rmask >> 2))
        | (surface->format->Gmask & (surface->format->Gmask >> 2))
        | (surface->format->Bmask & (surface->format->Bmask >> 2));

then for each pixel:

      pixel = ((pixel >> 1) & mask1)+((pixel >> 2) & mask2);

For an SDL_HWSURFACE, the blitting a surface with a surface alpha is
probably fastest...

Cheers,

-Loren

On Tue, 2002-10-01 at 23:20, patrick at 2dgame-tutorial.com wrote:
> Yes it is, because alphablending is often hardware-supported on video-cards. Make sure however 
> that no conversion has to be done by making the surface' format equal to the screens' Call 
> SDL_CreateRGBsurface wisely...
> 
> Patrick.
> 
> Chris Thielen chris at luethy.net schreef: 
> 
> >hey sdlers,
> >	i want to draw a simple transparent grey box, at like, 25%-50%
> >transparency on my main screen surface and was wondering what the
> >fastest way to do this is? the program runs on a 16bpp display, is
> >creating a surface, filling it grey, and setting the alpha the fastest
> >way to do it? wouldn't this be slow to blit 40-50 times per second? is
> >there a faster way to do it, maybe do the pixel calculations myself
> >since it's just a solid color i want?
> >
> >-- chris (chris at luethy.net)
> >
> >
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> >SDL at libsdl.org
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