Re: [Qemu-devel] Re: [PATCH] Use opaque alpha channel to support Xgl

[Anthony Liguori <aliguori@us.ibm.com>]

Christian Walther wrote:
> Anthony Liguori wrote:
>
>> I really don't like this patch as it seems like a hack but I couldn't
>> figure out a way to differentiate in SDL between a 24-bit depth with
>> a 32-bit pixel width (which is a common, non-alpha format) and a true
>> 32 bit depth with an alpha channel.
>
> Check SDL_Surface->format.Amask (see 
> http://www.libsdl.org/cgi/docwiki.cgi/SDL_5fPixelFormat).
Here's the pixel format under normal X (24 bit depth):

BitsPerPixel 32
BytesPerPixel 4
Rloss Gloss, Bloss, Aloss {0, 0, 0, 8}
Rshift, Gshift, Bshift, Ashift {16, 8, 0, 0}
Rmask, Gmask, Bmask, Amask {ff0000, ff00, ff, 0}
Colorkey: 0
Alpha: 255

Here's the pixel format under Xgl:

BitsPerPixel 32
BytesPerPixel 4
Rloss Gloss, Bloss, Aloss {0, 0, 0, 8}
Rshift, Gshift, Bshift, Ashift {16, 8, 0, 0}
Rmask, Gmask, Bmask, Amask {ff0000, ff00, ff, 0}
Colorkey: 0
Alpha: 255

Notice that they are identical.  They probably shouldn't be (what's SDL 
doing not setting an Amask when there is an alpha channel...).
>> Hopefully, this will be fixed in future versions of SDL.
>
> I'm not convinced there's something to fix in SDL rather than Qemu 
> (but I'm not familiar with the issue), but if there is, please post 
> this on the SDL mailing list (sdl@libsdl.org, 
> http://news.gmane.org/gmane.comp.lib.sdl).
I will.

Regards,

Anthony Liguori
>  -Christian
>
>
>
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