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bug#33885: 27.0.50; PUA character makes emacs crash
From: |
Werner LEMBERG |
Subject: |
bug#33885: 27.0.50; PUA character makes emacs crash |
Date: |
Fri, 28 Dec 2018 18:03:11 +0100 (CET) |
>> (2) Improve the heuristics to get sensible global ascent and
>> descent values of a font. In particular, use the FreeType
>> library to access a TrueType's or OpenType's `OS/2' table and
>> compare its vertical metrics values with the `hhea' data to
>> eliminate bugs in the font – sign problems with `descent' do
>> happen especially in older fonts that were either designed for
>> Apple or for Windows, and the fields for the `other' OS were
>> filled out arbitrarily, more or less.
>
> I'd appreciate more details on how this could be done. I'm no
> expert on fonts nor on FreeType, and last I checked we have no one
> else.
A possible workflow.
(0) Check that we have an SFNT font (which means either TrueType or
OpenType):
if (FT_IS_SFNT(ft_face))
...
(1) Use
TT_OS2* os2_table;
os2_table = (TT_OS2)FT_Get_Sfnt_Table(ft_face, FT_SFNT_OS2);
to extract the `OS/2' table data from an existing `FT_Face' object
for a given font. We are interested in the fields
fsSelection
sTypoAscender
sTypoDescender
sTypoLineGap
usWinAscent
usWinDescent
of FreeType's `TT_OS2' structure and in
ascent
descent
of xft's `XftFont' structure; the latter is based on the
`ascender' and `descender' fields of an OpenType font's `hhea'
table (with `descent' having the opposite sign of of `descender').
[Since `XftFont' doesn't provide `hhea's `lineGap' value it
probably makes sense to use `FT_Get_Sfnt_Table' also to access
this value.]
(2) Check that
sTypoAscender - sTypoDescender > 0
ascent + descent > 0
usWinAscent + usWinDescent > 0
and filter out the value pairs that don't fit. For example, I've
seen fonts where `sTypo*' values were all zero.
(3) Check that
-sign(descent) == sign(sTypoDescender) == -sign(usWinDescent)
If this holds, use `usWin*' or `sTypo*' values, depending on bit 7
of `fsSelection'. Maybe use `ascent/descent' if on a Mac.
If this doesn't hold, try to sanitize the not-fitting value.
Werner
- bug#33885: 27.0.50; PUA character makes emacs crash, Werner LEMBERG, 2018/12/27
- bug#33885: 27.0.50; PUA character makes emacs crash, Eli Zaretskii, 2018/12/27
- bug#33885: 27.0.50; PUA character makes emacs crash, Eli Zaretskii, 2018/12/27
- bug#33885: 27.0.50; PUA character makes emacs crash, Werner LEMBERG, 2018/12/28
- bug#33885: 27.0.50; PUA character makes emacs crash, Eli Zaretskii, 2018/12/28
- bug#33885: 27.0.50; PUA character makes emacs crash, Werner LEMBERG, 2018/12/28
- bug#33885: 27.0.50; PUA character makes emacs crash, Eli Zaretskii, 2018/12/28
- bug#33885: 27.0.50; PUA character makes emacs crash, Werner LEMBERG, 2018/12/28
- bug#33885: 27.0.50; PUA character makes emacs crash, Eli Zaretskii, 2018/12/28
- bug#33885: 27.0.50; PUA character makes emacs crash,
Werner LEMBERG <=
- bug#33885: 27.0.50; PUA character makes emacs crash, Werner LEMBERG, 2018/12/28
- bug#33885: 27.0.50; PUA character makes emacs crash, Eli Zaretskii, 2018/12/28