[SDL] RGBA->RGBA blitting again

David Olofson david at olofson.net
Thu Nov 27 05:04:00 PST 2003


On Thursday 27 November 2003 13.30, MagerValp wrote:
> I'm writing a small font library that prints text to a transparent
> overlay, which is then displayed on the screen when a key is held
> down. The code works fine as long as the font is stored in a
> colorkeyed surface, but when I copy it to an RGBA surface with
> SDL_DisplayFormatAlpha for faster blitting, the text itself becomes
> transparent.

Yes... That's because RGBA->RGBA blits don't apply the source alpha 
the way you'd expect. (Performance reasons, I guess.)


> I searched the archives, and I guess this is an old
> problem, but this limits me to slow blits with 1-bit alpha and no
> anti-aliased fonts. You can see my test code here:
>
>   http://www.cling.gu.se/~cl3polof/testalpha.c
>   http://www.cling.gu.se/~cl3polof/10x12yellow.bmp

Nice little font, BTW. Reminds me of great old Atari ST and Amiga 
games. :-)


> Is there a workaround for this?

The easy solution, if you don't have too much text on the screen, is 
to just render directly to the screen. This is how it's done in Kobo 
Deluxe, and the text "overlays" only have a slight effect on the 
frame rate on very low end hardware, as far as I've seen.

If you need massive amounts of text, you're probably better off 
rendering the overlay surfaces with your own blitting code, that 
applies/combines alpha the way you want. If there's no overlapping, 
you could just copy the full pixels from the font surface. Limited 
overlapping could be handled by cheating; just or the source and 
destination alpha together. Note that if you change the overlay 
surfaces frequently, the rendering and RLE decoding/encoding overhead 
will become significant.


BTW, don't ask for a 32 bpp display unless you absolutely need it to 
be exactly 32 bpp. If you run that code in windowed mode, or on a 
target that can't switch to 32 bpp, you'll get a shadow rendering 
buffer that is converted and blitted to the screen when you flip. Not 
insanely expensive if you're doing full screen s/w rendering anyway, 
but for more normal applications it eliminates any chances of using 
h/w accelerated blits.


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

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