[cairo] Struggling with subpixel text (last blocker for 1.8)

David Turner david at freetype.org
Thu Sep 25 18:27:14 PDT 2008

the problem is veyr probably that you have a FreeType installation that
support sub-pixel filtering, just like the default sources do.

for the record, here's how this works in FreeType currently:

- FreeType provides APIs that allow you to generate RGB decimated images
  as well as filter them with different algorithms. These APIs are available
  *all* builds of FreeType

- However, due to patents, the default build will ensure that all RGB
  images returned by the library obey the condition R=G=B, i.e. that the
  images are grayscale even when displayed through a "LCD" composite

- You can manually change one configuration to change this in the build
  to override this limitation.

The idea is that clients of the library should assume that they can get the
proper RGB glyphs and compose them through their favorite RGB-enabled
graphics library. Even if they do that, their code will not be infringing
with a
default build of FreeType since technically it's still drawing grayscale

I suggest that you rebuild FreeType on your system with the
FT_CONFIG_OPTION_SUBPIXEL_RENDERING option enabled and see if this changes

As for the state of the code, I'm sorry if it's not crystal clear; it's been
written a very long time ago :-)

Hope this helps

- David

2008/9/25 Carl Worth <cworth at cworth.org>

> So I stayed up all night trying to get the 1.8 release done.
> The test suite was in pretty bad shape as of the last 1.7.6 snapshot.
> There were dozens of failing tests, and the causes were all over the
> place:
> * Stale reference images needing to be updated
> * Known bugs that needed to be marked as XFAIL
> * New regressions that needed to be fixed
> It was pretty tedious work sorting through all that, and for the ones
> that were actual regressions we had to actually fix the bugs too. Thanks
> to Behdad, Chris, Emmanuel, and Adrian for helping out with that
> process.
> The final result I got to was a single test not passing, (I
> intentionally wasn't testing xlib to avoid X server/driver issues):
>        text-antialias-subixel
> And this one is a regression. It's giving me grayscale antialiasing
> instead of subpixel on image and other backends. The text passes fine
> back in 1.6.4.
> I bisected the failure down to this large commit:
>  commit 7f870035177da7aa63fa33dce837528f442d9ffe
>  Author: Sylvain Pasche <sylvain.pasche at gmail.com>
>  Date:   Tue Jan 22 23:51:32 2008 +0100
>    [cairo-ft] LCD filtering patch from David Turner
>    This is the patch David Turner wrote some times ago and can be found on
>    http://david.freetype.org/lcd/cairo-1.2.4-lcd-filter-1.patch. It was
>    somewhat modified for compatibility with cairo 1.5
> I spent a fair amount of time trying to debug this, but never succeeded.
> So now I'd like some suggestions on how to proceed. One option is to
> revert the patch, (it broke an existing test in the test suite after
> all). I've got a patch in my personal repository, (revert-lcd-filter
> branch), that does this and which seems to work:
>    http://cgit.freedesktop.org/~cworth/cairo/log/?h=revert-lcd-filter
> But meanwhile, I know that some users were particularly keen on being
> able to control, (via fontconfig), the various LCD filter options made
> available by freetype. And reverting this patch would remove that
> possibility.
> Behdad, do you have some guidance here? Can you fix the code so that the
> test suite works? Or shall we just go ahead with the revert and bring
> back the LCD filter functionality in 1.8.2 or so? I know you were hoping
> to wake up and see that the release had already happened. Sorry that
> didn't happen, but this is really the last issue left.
> -Carl (overdue for sleep)
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