[cairo-bugs] [Bug 33711] Alpha Value Normalization from 255 to 128

bugzilla-daemon at freedesktop.org bugzilla-daemon at freedesktop.org
Sun Jan 30 16:06:18 PST 2011


--- Comment #1 from Bill Spitzak <spitzak at gmail.com> 2011-01-30 16:06:15 PST ---
I think he is saying that the hardware has 8 bits of alpha but basically 
treats all numbers <128 as zero and all numbers >=128 as 255.

Cairo can just produce the final image in it's own image memory and send 
it to the hardware as an opaque image. If for some reason they want 
Cairo's output to composite atop something else using the hardware, the 
best I can suggest is dithering using error diffusion of the alpha 
channel. Each pixel in the image buffer from cairo is either replaced 
with the unpremultiplied value with alpha==255, or with black with alpha==0.

On 01/30/2011 02:43 AM, bugzilla-daemon at freedesktop.org wrote:
> https://bugs.freedesktop.org/show_bug.cgi?id=33711
>             Summary: Alpha Value Normalization from 255 to 128
>             Product: cairo
>             Version: 1.8.8
>            Platform: Other
>          OS/Version: Linux (All)
>              Status: NEW
>            Severity: blocker
>            Priority: medium
>           Component: image backend
>          AssignedTo: cworth at cworth.org
>          ReportedBy: ssseintr at gmail.com
>           QAContact: cairo-bugs at cairographics.org
> Dear Cairo Team,
> Thanks for the excellent graphics engine. My self Vicky and developing embedded
> applications for consumer electronics.
> I'm using cairo in one platform it is linux based. The problem is the hardware
> in which I'm working restricts the Alpha value to 128(in ARGB32). So both cairo
> drawing&  native drawing makes the hardware acceleration impossible. I'm using
> cairo's image surface, not any native backend's like x11 or directfb.
> Can u pls show me the way where can i restrict the alpha value to 128. So for
> my platform, 0-full transparent&  128-Opaque.
> Thanks in advance.
> With Regards,
> Vicky,.

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