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Re: [ft] Subpixel Hinting Patch


From: Moony
Subject: Re: [ft] Subpixel Hinting Patch
Date: Sun, 18 Apr 2010 20:08:12 -0500
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.1.9) Gecko/20100330 Fedora/3.0.4-1.fc12 Thunderbird/3.0.4

Ah yes, you have found the major weaknesses of the patch. :) I am still trying to determine why DejaVu doesn't seem to be hinted at all, or very poorly, and why Palatino doesn't render. On the other hand, Liberation fonts and most legacy MS fonts look great. Some Apple ones look good and others need work. I know that the DejaVu TT instructions normally look quite good, so the patch is obviously not taking something into account. If you can help me debug this that would be great!

I used patched versions of cairo and XFT on my system, which I believe are re-implementing the FIR filter code that David wrote into Freetype, but I don't think that should have any effect on the hinting, which is the only thing that this patch is intended to modify. It's probably worth noting that I still personally use "autohint slight hinting" in fontconfig on a handful of fonts that don't render right with this patch yet.

Thanks for checking out the patch.


My fontconfig setup suggests the use of the legacy filter for instructed
glyf fonts, and the default filter for autofit and light-autofit fonts.
I have cairo patched to follow that suggestion, so gtk apps, in particular,
do just that.  I beleive qt apps also do so, but i may be mis-remembering
that.  XFT, on the other hand, continues to do its own thing, which is what
the legacy filter was designed to emulate.

And my primary UI fonts are DejaVu Serif and DejaVu Sans Mono, sized such
that the stems are one pixel wide when instructed.

With you patch, the fonts look very poor.  It increases colour fringing
to the point of distraction.

I even tried with the default filter instead of the legacy filter.

I very much like the idea, but I had to go back.

I think the new hinting should be in addition to, and not instead of
the current code.

Part of the issue, of course, is that the fonts I care about and the
fonts you are testing against are almost disjoint sets.

So before going back I did run ftview an ftdiff on a few of the fonts
which came with the laptop.  (It arrived with XP.)

The most interesting result of that test was that PALA.ttf (Palatino
Linotype, version 1.40) was invisible with your patch.

-JimC




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