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Re: [ft-devel] gamma correction and stem darkening

From: Derek B. Noonburg
Subject: Re: [ft-devel] gamma correction and stem darkening
Date: Sat, 16 Jan 2016 22:54:06 -0800 (PST)

On 17 Jan, Werner LEMBERG wrote:
>> Unfortunately, I don't think it will be possible to detect the
>> tricky fonts.
> I think it *is* possible.  The method, as implemented by Toshiya-san,
> compares various SFNT table checksums.  Among them is the `cvs '
> table, and modifying this table is essentially impossible without
> interpreting and changing the bytecode.  Ditto for `fpgm'.  I dare to
> say that even a subsetted font with a single glyph contains the
> original `cvs ' and `fpgm' tables unmodified, which should enable
> FreeType to discover its trickyness.
> Do you have counterexamples to this assumption?

Very interesting... I wasn't aware that FreeType used that technique.  I
just looked through the code and found tt_check_trickyness_sfnt_ids -- I
assume that's the code you're referring to.  I didn't trace through the
code; do I need to do anything special to enable those checksum tests?

I have at least one PDF file with Chinese TrueType fonts that render
correctly with the native hinter and incorrectly without it.  I'll
extract those fonts and see what the cvt and fpgm tables look like.

I'll also be happy to make the fonts available for testing.

- Derek

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