[Pixman] Dithering patches, v2

[Basile Clement]

I forgot to mention two things:

 - Enabling dithering makes processing much slower, since it forces the
wide pipeline to be used (and some optimisations are disabled).  This
could be solved through the use of fast paths where needed (e.g. when
blitting images from an equal or lower bits-per-pixels format, or with
fused gradient+dithering paths).  In any case, the design of the
implementation allows setting the `dither` property of a destination
image in between calls to `pixman_image_composite` where dithering is
required, allowing users to pay the cost of the slower wide pipeline
only for those calls where dithering is actually required.

 - The dithering is not gamma corrected, which doesn't matter much for
8bpp formats -- but makes a noticeable difference for lower bpp formats,
producing images which are way too luminous (this is particularily
visible in the dithering demos when using 2bpp and 1bpp formats).  Since
the main use case for now is a8r8g8b8 gradient export in Inkscape (and
since the series is starting to be a bit large), I did not include gamma
correction in this series.  If needed, it can be added in subsequent
patches, and should probably be configurable since the proper curve to
use depend on the characteristic of the target device, which pixman may
not know about.

- Basile

On 4/9/19 11:29 PM, basile-pixman at clement.pm wrote:
> I am resubmitting for review and inclusion my series of patches on
> dithering from October, which I ended up not having the time to finish
> then.  There are only a couple changes to make the patches compatible
> with the recently added floating point formats, and I have also added a
> dithering matrix based on blue noise which provides more pleasant
> results than the Bayer matrix from the first series.
>
> Maarten, I would appreciate if you can take a second look (although as
> noted above not much has changed since your first review -- I don't
> think you had comments on the dithering part?).
>
> - Basile
>
>
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