[SDL] SDL_DisplayFormatAlpha() and 16-bit surfaces
screwtape at froup.com
Tue Aug 6 01:19:00 PDT 2002
On Tuesday, August 6, 2002, at 05:21 , Adam Gates wrote:
> Chris Thielen wrote:
>> On Mon, 2002-08-05 at 22:49, Tim Allen wrote:
>>> It would be nice if there were some way of taking a surface with
>>> colour-key alpha, and transforming it to a surface in the display
>>> format with colour-key alpha, rather than a surface in the display
>>> format with 8-bits of alpha which are either 0 or 255 for any given
>>> pixel - I'm sure colour-key alpha, especially with RLE encoding,
>>> would be faster than doing proper alpha-blending. If you're blitting
>>> to a 16-bit surface, I'm sure doing the 32->16 bit conversion is
>>> wasteful, too.
>> Yea, that's why I'm wondering this, I'd like to avoid 32->16 bit
>> conversions I'm doing right now. It's not too big a deal because the
>> majority of the screen isn't updated every frame, and not all the
>> sprites need alpha, so not all are 32-bit, but it's still a waste to
>> convert it from 32 to 16 on every blit, I agree.
>> -- chris (chris at luethy.net)
> Are you concerned about this because the screen surface is 16-bit?
> But the screen surface doesn't have alpha, does it?
> If it doesn't then 16-bit RGBA -> 16-bit RGB is just as costly in
But a surface with colour-key alpha is stored as RGB, not RGBA. So if
you take a surface with colour-key alpha and run it through
DisplayFormatAlpha, then you have the RGBA -> RGB conversion as well as
the 32-bit -> 16-bit conversion.
All I really want is 16-bit RGB -> 16-bit RGB (ignoring pixels of a
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