Re: Alpha blit question

[Paul Bleisch <bleisch@chromatic.com>]

I would say that you would want to support both because both will
be supported in upcoming hardware.  Looking at the PC industry,
which is a good baseline IMO, cards in 1998/1999 timeframe will
support both channeled alpha blits (alpha per pixel) and uniform
alpha blits (one alpha component per surface) because Microsoft
will be sticking this in DirectDraw and GDI2k.  Also, there are
times when you want to use perpixel and there are times when you
want to use persurface alpha even if it is harder/slower in 
software.  You may also want to think about whether you need to
do anything special to "support" premultiplied alpha *and* non-
premultiplied alpha.



Word,
Paul


At 11:10 AM 4/14/98 -0700, you wrote:
>When implementing a source alpha blit, is it better in general to use
>a separate alpha surface (faster in software) or extract the alpha 
>information from each pixel (faster in hardware) ?
>
>Hardware accelerated 2-D alpha blending is very rare at the moment.
>
>Also, the semantics of the source alpha value are:
>
>	As alpha goes up, the amount of the source blended goes down.
>
>right?
>(I thought real alpha was a measure of opacity, not transparency)
>
>See ya!
>	-Sam Lantinga				(slouken@devolution.com)
>
>--
>Author of Simple DirectMedia Layer -
>	http://www.devolution.com/~slouken/SDL/
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Paul Bleisch
bleisch@chromatic.com