[Pixman] sRGB processing for pixman
sandmann at cs.au.dk
Fri Jun 15 12:40:25 PDT 2012
"Antti S. Lankila" <alankila at bel.fi> writes:
> On 09.06.2012 14:16, Antti S. Lankila wrote:
>> Currently the color space processing is just a cheap integer hack
>> with gamma 2.0. It is sufficient for testing purposes, but not
>> production quality. I'm leaning towards tabularizing the official
>> sRGB-to-linear function and its inverse in a future patch.
> Just to avoid any future confusion, I found the time to write the
> tabularized conversion. A simple screenshot that shows sRGB (top) and
> non-sRGB (bottom) processing:
> The URL still works, although I recommend making it
> /~alankila/pixman/0001etc. in the future.
Comments on the patch:
- I think the code that generated the tables should be included, and the
tables generated as part of the build process.
- We can't break source level compatibility. Instead of adding a new
field in the formats, how about simply adding
which is #defined to (PIXMAN_TYPE_ARGB | (1 << 5))? That still makes
it possible to determine quickly whether the format is srgb encoded,
but doesn't break source compatibility.
- expand/contract seems like the wrong place to do the conversion. It
makes more sense to me to do it directly in pixman-access.c for the
format in question. Conceptually, everything pixman does internally is
with linearly encoded pixels, and so expanding the output of an 8-bit
fetcher is not subject to gamma conversions.
Instead, gamma conversions belong as the first thing after fetching
and the last thing before storing.
That also means the 8 bit fetcher for this new format would be done by
(a) fetching wide, gamma-converted pixels, and then (b) contracting
them to 8 bit without gamma conversion. This loses a lot of
information of course, but that's why the format is considered "wide".
- I'm not convinced that we need that pixman_image_get_space()
accessor. There is already a pixman_image_get_format() that can be
used to get the information.
- The format should be added to the test suite, at the very least the
- There are formatting issues. Please see COPYING.
But other than that, I think this is a useful feature and should
basically go in as proposed. There was some discussion on IRC regard
full color space support. While this may some day happen, I don't think
gamma corrected compositing should block on it.
(If you come up with a new patch, please just mail it to the list instead of
sending a link).
More information about the Pixman