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[autofit] Fix double division in stem darkening.

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The old code used to divide the darkening amount by em_ratio twice,
leading to unnecessarily bold stems on certain fonts with higher
units per em (e.g. Inter). This patch fixes it.

The return value of af_loader_compute_darkening was also changed to
use 16.16 fixed point to get rid of a redundant truncation operation.
This should slightly improve the precision, although it's still
bottlenecked by the emboldening function, which uses 26.6 fixed point.

* src/autofit/afloader.[ch]
(af_loader_compute_darkening): Return FT_Fixed.
(af_loader_embolden_glyph_in_slot): Revise calculations.
This commit is contained in:
Tatsuyuki Ishi 2020-12-18 22:10:30 -05:00 committed by Alexei Podtelezhnikov
parent 7bdf386e75
commit 0d5f1dd37c
3 changed files with 30 additions and 16 deletions
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17
ChangeLog
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@ -1,3 +1,20 @@
2020-12-18 Tatsuyuki Ishi <ishitatsuyuki@gmail.com>
[autofit] Fix double division in stem darkening.
The old code used to divide the darkening amount by em_ratio twice,
leading to unnecessarily bold stems on certain fonts with higher
units per em (e.g. Inter). This patch fixes it.
The return value of af_loader_compute_darkening was also changed to
use 16.16 fixed point to get rid of a redundant truncation operation.
This should slightly improve the precision, although it's still
bottlenecked by the emboldening function, which uses 26.6 fixed point.
* src/autofit/afloader.[ch]
(af_loader_compute_darkening): Return FT_Fixed.
(af_loader_embolden_glyph_in_slot): Revise calculations.
2020-12-17 Alexei Podtelezhnikov <apodtele@gmail.com>
* include/freetype/ftmodapi.h (FT_FACE_DRIVER_NAME): New public macro.

27
src/autofit/afloader.c
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@ -105,7 +105,6 @@
globals->stem_darkening_for_ppem;
FT_Fixed em_size = af_intToFixed( face->units_per_EM );
FT_Fixed em_ratio = FT_DivFix( af_intToFixed( 1000 ), em_size );
FT_Matrix scale_down_matrix = { 0x10000L, 0, 0, 0x10000L };
@ -142,12 +141,11 @@
darken_by_font_units_x =
af_intToFixed( af_loader_compute_darkening( loader,
face,
stdVW ) );
darken_x = FT_DivFix( FT_MulFix( darken_by_font_units_x,
size_metrics->x_scale ),
em_ratio );
af_loader_compute_darkening( loader,
face,
stdVW ) ;
darken_x = FT_MulFix( darken_by_font_units_x,
size_metrics->x_scale );
globals->standard_vertical_width = stdVW;
globals->stem_darkening_for_ppem = size_metrics->x_ppem;
@ -161,12 +159,11 @@
darken_by_font_units_y =
af_intToFixed( af_loader_compute_darkening( loader,
face,
stdHW ) );
darken_y = FT_DivFix( FT_MulFix( darken_by_font_units_y,
size_metrics->y_scale ),
em_ratio );
af_loader_compute_darkening( loader,
face,
stdHW ) ;
darken_y = FT_MulFix( darken_by_font_units_y,
size_metrics->y_scale );
globals->standard_horizontal_width = stdHW;
globals->stem_darkening_for_ppem = size_metrics->x_ppem;
@ -594,7 +591,7 @@
*
* XXX: Currently a crude adaption of the original algorithm. Do better?
*/
FT_LOCAL_DEF( FT_Int32 )
FT_LOCAL_DEF( FT_Fixed )
af_loader_compute_darkening( AF_Loader loader,
FT_Face face,
FT_Pos standard_width )
@ -713,7 +710,7 @@
}
/* Convert darken_amount from per 1000 em to true character space. */
return af_fixedToInt( FT_DivFix( darken_amount, em_ratio ) );
return FT_DivFix( darken_amount, em_ratio );
}

2
src/autofit/afloader.h
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@ -75,7 +75,7 @@ FT_BEGIN_HEADER
FT_UInt gindex,
FT_Int32 load_flags );
FT_LOCAL_DEF( FT_Int32 )
FT_LOCAL_DEF( FT_Fixed )
af_loader_compute_darkening( AF_Loader loader,
FT_Face face,
FT_Pos standard_width );