[SDL] Alpha blending, how to

David Olofson david at olofson.net
Sun Sep 17 17:27:50 PDT 2006


On Sunday 17 September 2006 20:03, Ismael Barros wrote:
> Hi,
> 
> I'm pretty new to SDL, and I don't know if it's possible to do this,
> but I wanted to blit a 3-color surface, with a transparent color, an
> opaque color and a semi-transparent (alpha=128) color.
> 
> The surface I'm trying to blit is a window created with tiles
> extracted from this palete:
> http://img356.imageshack.us/img356/5391/windowbiggr4.png
> 
> Black color (0, 0, 0) must be transparent, the blue of the border
> must be opaque and the other blue (deeper) must have alpha=128
> 
> The effect I'm trying to achieve is this:
> http://img516.imageshack.us/img516/2704/scnv9.png
> 
> I also can split the window in two surfaces, the opaque border and
> the content, but I wanted to know if there is a cleaner way to do
> it. 
> 
> Any idea?

I'm not quite sure what the problem is... If you use alpha, you just 
use the alpha channel to specify how transparent each pixel should be 
- including totally transparent pixels. So, just erase the black 
pixels in GIMP, PhotoShop or whatever you're using, enable 
SDL_SRCALPHA after loading the surface, and that's it!

You can get antialiazing (which is basically semi-transparent pixels 
around the otherwise jagged edges) as en extra bonus while you're at 
it. Just let GIMP/PhotoShop/... do it's usual stuff. (Most such 
applications require that you work in RGB mode to get AA.)

(You *can* use colorkey together with [full surface] alpha blending, 
which is useful in some special cases - but I'm not totally sure you 
can use colorkey with an alpha channel. Why would you, anyway? Alpha 
is the more powerful method handling transparency, so if you're using 
that, why bother using colorkey?)


//David Olofson - Programmer, Composer, Open Source Advocate

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