[cairo] X11 image data rendering problems (Xrender ?)

[Julien Danjou]

At 1206310014 time_t, Bertram Felgenhauer wrote:
> Imlib2's ARGB32 format is, however, incompatible with cairo's, because
> cairo expects pre-multiplied alpha images. (This is mentioned in passing
> at
>   http://cairographics.org/manual/cairo-Image-Surfaces.html#cairo-format-t
> but I don't know whether the docs explain this in detail.)

Ah, that was a close one. And I did see this sentence in the API, but
did not know that the word 'pre-multiplied' was so important.

I did not use your solution, because it seems that blending does not act
the same way at each run, but I now compute pre-multiplied values
myself, and it works much better than before:

 dataimg = p_new(unsigned char, w * h * 4);

 for(i = 0; i < size; i++, dataimg += 4)
 {
     dataimg[3] = (data[i] >> 24) & 0xff;           /* A */
     /* cairo wants pre-multiplied alpha */
     alpha = dataimg[3] / 255.0;
     dataimg[2] = ((data[i] >> 16) & 0xff) * alpha; /* R */
     dataimg[1] = ((data[i] >>  8) & 0xff) * alpha; /* G */
     dataimg[0] = (data[i]         & 0xff) * alpha; /* B */
 }

Thanks a lot Bertram,
And keep doing such good work guys,

Cheers,
-- 
Julien Danjou
// ᐰ <julien at danjou.info>   http://julien.danjou.info
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