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[autofit] Return correct linear advance width values. 

This was quite a subtle bug which accidentally showed up with glyph
`afii10023' of arial.ttf (version 2.76).  This glyph is a composite;
the first component, `E', has an advance width of 1366 font units,
while the advance width of the composite itself (which looks like
uppercase `E' with dieresis) is 1367 font units.  I think this is
actually a bug in the font itself, because there is no reason that
this glyph has not the same width as uppercase `E' without the
dieresis.  Anyway, it helped identify this problem.

Using the TrueType hinter, the correct value (1367) of `afii10023'
was returned, but the autohinter mysteriously returned 1366.

Digging in the code showed that the autohinter recursively calls
FT_Load_Glyph to load the glyph, adding the FT_LOAD_NO_SCALE load
flag.  However, the `linearHoriAdvance' field is still returned as a
scaled value.  To avoid scaling twice, the old code in autofit reset
`linearHoriAdvance', using the `horiAdvance' field.  This seemed to
work since FT_LOAD_NO_SCALE was in use, but it failed actually,
because `horiAdvance' is defined as the distance of the first
subglyph's phantom points, which in turn are initialized using the
advance width of the first subglyph.  And as the given example
shows, these widths can differ.

* src/autofit/afloader.c (af_loader_load_g): Temporarily set
FT_LOAD_LINEAR_DESIGN while calling FT_Load_Glyph to get unscaled
values for the linear advance widths.
This commit is contained in:
Werner Lemberg 2012-03-11 15:20:32 +01:00
parent f1ec084292
commit 1036409692
2 changed files with 34 additions and 5 deletions
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31
ChangeLog
View File

@ -1,3 +1,34 @@
2012-03-11 Werner Lemberg <wl@gnu.org>
[autofit] Return correct linear advance width values.
This was quite a subtle bug which accidentally showed up with glyph
`afii10023' of arial.ttf (version 2.76). This glyph is a composite;
the first component, `E', has an advance width of 1366 font units,
while the advance width of the composite itself (which looks like
uppercase `E' with dieresis) is 1367 font units. I think this is
actually a bug in the font itself, because there is no reason that
this glyph has not the same width as uppercase `E' without the
dieresis. Anyway, it helped identify this problem.
Using the TrueType hinter, the correct value (1367) of `afii10023'
was returned, but the autohinter mysteriously returned 1366.
Digging in the code showed that the autohinter recursively calls
FT_Load_Glyph to load the glyph, adding the FT_LOAD_NO_SCALE load
flag. However, the `linearHoriAdvance' field is still returned as a
scaled value. To avoid scaling twice, the old code in autofit reset
`linearHoriAdvance', using the `horiAdvance' field. This seemed to
work since FT_LOAD_NO_SCALE was in use, but it failed actually,
because `horiAdvance' is defined as the distance of the first
subglyph's phantom points, which in turn are initialized using the
advance width of the first subglyph. And as the given example
shows, these widths can differ.
* src/autofit/afloader.c (af_loader_load_g): Temporarily set
FT_LOAD_LINEAR_DESIGN while calling FT_Load_Glyph to get unscaled
values for the linear advance widths.
2012-03-10 Werner Lemberg <wl@gnu.org>
[truetype] Fix SSW instruction.

8
src/autofit/afloader.c
View File

@ -104,9 +104,11 @@
AF_GlyphHints hints = &loader->hints;
FT_GlyphSlot slot = face->glyph;
FT_Slot_Internal internal = slot->internal;
FT_Int32 flags;
error = FT_Load_Glyph( face, glyph_index, load_flags );
flags = load_flags | FT_LOAD_LINEAR_DESIGN;
error = FT_Load_Glyph( face, glyph_index, flags );
if ( error )
goto Exit;
@ -124,10 +126,6 @@
FT_Vector_Transform( &loader->trans_delta, &inverse );
}
/* set linear metrics */
slot->linearHoriAdvance = slot->metrics.horiAdvance;
slot->linearVertAdvance = slot->metrics.vertAdvance;
switch ( slot->format )
{
case FT_GLYPH_FORMAT_OUTLINE: