sandmann at daimi.au.dk
Fri Feb 6 18:12:23 PST 2009
Simon Thum <simon.thum at gmx.de> writes:
> The issues I mean are:
> > Higher Quality Image Filtering
> > Add better filtering, especially for downscaling transformations.
> > Color Management/Gamma Correction
> > Find out what color management support is needed in cairo, XRender, and pixman.
> I'd just like t point out there is a close relation often overlooked.
> Basically, if you scale down using 'super sampling' (I didn't find it in
> pixman, but it's a likely choice. Also called a box filter.) quality
> suffers more the more you scale down.
Super sampling would be a likely choice, yes, but sadly pixman doesn't
even do that. The best thing it has to offer is a bilinear filter. For
downscaling with a scale factor larger than 2, that is so ugly that
it's not even funny. 
> However, when you implement super sampling with respect to gamma, the
> quality loss drops to nearly zero. I'd say typically you need no more
> strange "higher quality" filters when that's done.
Even a simple non-gamma-correct supersampling filter would be good
enough for many purposes. As I recall, GdkPixbuf, which has provided
acceptable quality in GNOME for many years, does essentially this.
I do think a high-quality filter such as Lanczos would be a useful
thing to have, but getting to even passable quality is more important.
> I extensively researched that during my diploma, and if you can point me
> to the right spots in pixman, I may be able to draw up a patch.
The right spot in pixman is in ACCESS(fbFetchTransformed)() in
pixman-transformed.c, where a new supersampling fetcher would be put
I agree that gamma correct filtering would look even better, but
whether that's interesting on its own instead of as a side effect of a
general SRGB surface, I don't know. It seems a little iffy to me to
have a PIXMAN_BOX_FILTER_GAMMA_CORRECT, instead of a general SRGB
surface that could be filtered with a normal PIXMAN_FILTER_BOX.
> As for gamma correction in general, It's an extensive work and change.
> Basically, you need to check every single pixel-level computation
> whether it is gamma-invariant, if not, make it so. I devised a small
> calculus allowing to judge that matter. Maybe I can find it again, if
> there's interest.
> That said, it would certainly be worth to do it. However I'm far from
> having the time to do so.
I did an experiment a while back - the result is in the gamma branch of
If you install it, all cairo rendering, including filtering, will be
gamma correct, and if you run a Render based compositing manager, you
will get gamma correct alpha blended windows.
I don't intend it as anything other than a demonstration, since gamma
correction should probably be considered along with color management
in general. And I did it in the dumbest way possible, so it's way too
slow to be practical.
 This is not the fully story, because pixman and Render also allow
general filter kernels to be uploaded. This is not exactly fast,
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