Thanks so much. It works perfectly now.
May I suggest the following modification in pixman.h so that it is clear for
all users ?
/*
* Important note: pixman colors are alpha-premultiplied
* and must conform: red <= alpha ; green <= alpha ; blue <= alpha
* So that for instance pure white with 0x80 alpha should
* be r=0x80, g=0x80, b=0x80, a=0x80
*/
struct pixman_color
{
uint16_t red;
uint16_t green;
uint16_t blue;
uint16_t alpha;
};
Thanks
> Le 12 juin 2014 à 17:01, soren.sandm...@gmail.com (Søren Sandmann) a écrit :
>
> Eric Nicolas <erik.nico...@gmail.com> writes:
>
>> Hello, Thanks for your answer.
>>
>> Below is a small test program (works on little endian machines).
>>
>> As is the result is already strange to me :
>> in: r=20 g=20 b=80 a=FF
>> out: r=90 g=90 b=C0 a=FF
>> Where as I expected the fill color #FFFFFFFF x the mask 0x80 = #FFFFFF80
>> and so an output of #9090FFFF.
>>
>> However, if I set the fill color to #FFFFFF80, the result seems really
>> wrong :
>> in: r=20 g=20 b=80 a=FF
>> out: r=98 g=98 b=E0 a=FF
>>
>> I would expect fill x mask => #FFFFFF40 and thus out: #606060C0FF.
>>
>> I especially do not understand how a lower alpha input color can end up in
>> a lighter output on the target image.
>>
>> What I am doing wrong here ?
>
> The thing you are missing is that Pixman works with premultiplied
> alpha. That is, if the rgb values are R, G, and B, and the alpha value
> is A, then pixman will assume that pixels are stored as
>
> R * A, G * A, B * A, A
>
> and it will also generate pixels in this format. The reason for doing
> this is that it avoids divisions when compositing.
>
> When you set the fill value to 0xffffff80 pixman will interprete it as
>
> (1.0 / 0.5), (1.0 / 0.5), (1.0 / 0.5), 0.5
>
> = (2, 2, 2, 0.5)
>
> ie., you get out-of-range color values, which is why the result will be
> lighter. Normally, the alpha value in premultiplied pixels should not be
> bigger than the color values, though it can occasionally be useful.
>
> Premultiplied alpha also means that when applying an alpha mask, it must
> be applied to all four channels, so in this case you get
>
> 0xffffff80 x 0x80 = 0x80808040
>
> when applying the mask.
>
>
> Søren
_______________________________________________
Pixman mailing list
Pixman@lists.freedesktop.org
http://lists.freedesktop.org/mailman/listinfo/pixman
