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Code Issues 4 Releases Wiki Activity

[autofit] Improve scoring algorithm for identifying stems. 

Problem reported by Karsten Lücke <karsten.luecke@kltf.de>.

The new algorithm takes care of the width of stems: If the distance
between two segments is larger than the largest stem width, the
demerits quickly increase for larger distances.  This improves
hinting of slanted fonts (especially if the inner parts of serifs
have non-horizontal `shoulders'), avoiding false stem links.

* src/autofit/aflatin.c (af_latin_hints_link_segments): Use largest
stem width (if available) to compute better demerits for distances
between stems.
(af_latin_hints_detect_features): Pass stem width array and array
size.
(af_latin_metrics_init_widths): Updated to use original algorithm.
(af_latin_hints_apply): Updated to use new algorithm.

* src/autofit/aflatin.h: Updated.
* src/autofit/afcjk.c: Updated.
This commit is contained in:
Werner Lemberg 2014-04-05 16:27:19 +02:00
parent 87360a78f3
commit 71f53e122b
4 changed files with 110 additions and 17 deletions
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23
ChangeLog
View File

@ -1,3 +1,26 @@
2014-04-05 Werner Lemberg <wl@gnu.org>
[autofit] Improve scoring algorithm for identifying stems.
Problem reported by Karsten Lücke <karsten.luecke@kltf.de>.
The new algorithm takes care of the width of stems: If the distance
between two segments is larger than the largest stem width, the
demerits quickly increase for larger distances. This improves
hinting of slanted fonts (especially if the inner parts of serifs
have non-horizontal `shoulders'), avoiding false stem links.
* src/autofit/aflatin.c (af_latin_hints_link_segments): Use largest
stem width (if available) to compute better demerits for distances
between stems.
(af_latin_hints_detect_features): Pass stem width array and array
size.
(af_latin_metrics_init_widths): Updated to use original algorithm.
(af_latin_hints_apply): Updated to use new algorithm.
* src/autofit/aflatin.h: Updated.
* src/autofit/afcjk.c: Updated.
2014-04-03 Werner Lemberg <wl@gnu.org> 2014-04-03 Werner Lemberg <wl@gnu.org>
Don't require `gzip' module for `sfnt'. Don't require `gzip' module for `sfnt'.

4
src/autofit/afcjk.c
View File

@ -4,7 +4,7 @@
/* */ /* */
/* Auto-fitter hinting routines for CJK writing system (body). */ /* Auto-fitter hinting routines for CJK writing system (body). */
/* */ /* */
/* Copyright 2006-2013 by */ /* Copyright 2006-2014 by */
/* David Turner, Robert Wilhelm, and Werner Lemberg. */ /* David Turner, Robert Wilhelm, and Werner Lemberg. */
/* */ /* */
/* This file is part of the FreeType project, and may only be used, */ /* This file is part of the FreeType project, and may only be used, */
@ -178,6 +178,8 @@
goto Exit; goto Exit;
af_latin_hints_link_segments( hints, af_latin_hints_link_segments( hints,
0,
NULL,
(AF_Dimension)dim ); (AF_Dimension)dim );
seg = axhints->segments; seg = axhints->segments;

94
src/autofit/aflatin.c
View File

@ -171,7 +171,15 @@
if ( error ) if ( error )
goto Exit; goto Exit;
/*
* We assume that the glyphs selected for the stem width
* computation are `featureless' enough so that the linking
* algorithm works fine without adjustments of its scoring
* function.
*/
af_latin_hints_link_segments( hints, af_latin_hints_link_segments( hints,
0,
NULL,
(AF_Dimension)dim ); (AF_Dimension)dim );
seg = axhints->segments; seg = axhints->segments;
@ -1338,19 +1346,28 @@
} }
/* Link segments to form stems and serifs. */ /* Link segments to form stems and serifs. If `width_count' and */
/* `widths' are non-zero, use them to fine-tune the scoring function. */
FT_LOCAL_DEF( void ) FT_LOCAL_DEF( void )
af_latin_hints_link_segments( AF_GlyphHints hints, af_latin_hints_link_segments( AF_GlyphHints hints,
FT_UInt width_count,
AF_WidthRec* widths,
AF_Dimension dim ) AF_Dimension dim )
{ {
AF_AxisHints axis = &hints->axis[dim]; AF_AxisHints axis = &hints->axis[dim];
AF_Segment segments = axis->segments; AF_Segment segments = axis->segments;
AF_Segment segment_limit = segments + axis->num_segments; AF_Segment segment_limit = segments + axis->num_segments;
FT_Pos len_threshold, len_score; FT_Pos len_threshold, len_score, dist_score, max_width;
AF_Segment seg1, seg2; AF_Segment seg1, seg2;
if ( width_count )
max_width = widths[width_count - 1].org;
else
max_width = 0;
/* a heuristic value to set up a minimum value for overlapping */
len_threshold = AF_LATIN_CONSTANT( hints->metrics, 8 ); len_threshold = AF_LATIN_CONSTANT( hints->metrics, 8 );
if ( len_threshold == 0 ) if ( len_threshold == 0 )
len_threshold = 1; len_threshold = 1;
@ -1358,6 +1375,11 @@
/* a heuristic value to weight lengths */ /* a heuristic value to weight lengths */
len_score = AF_LATIN_CONSTANT( hints->metrics, 6000 ); len_score = AF_LATIN_CONSTANT( hints->metrics, 6000 );
/* a heuristic value to weight distances (no call to */
/* AF_LATIN_CONSTANT needed, since we work on multiples */
/* of the stem width) */
dist_score = 3000;
/* now compare each segment to the others */ /* now compare each segment to the others */
for ( seg1 = segments; seg1 < segment_limit; seg1++ ) for ( seg1 = segments; seg1 < segment_limit; seg1++ )
{ {
@ -1377,10 +1399,9 @@
if ( seg1->dir + seg2->dir == 0 && pos2 > pos1 ) if ( seg1->dir + seg2->dir == 0 && pos2 > pos1 )
{ {
/* compute distance between the two segments */ /* compute distance between the two segments */
FT_Pos dist = pos2 - pos1; FT_Pos min = seg1->min_coord;
FT_Pos min = seg1->min_coord; FT_Pos max = seg1->max_coord;
FT_Pos max = seg1->max_coord; FT_Pos len;
FT_Pos len, score;
if ( min < seg2->min_coord ) if ( min < seg2->min_coord )
@ -1394,12 +1415,43 @@
len = max - min; len = max - min;
if ( len >= len_threshold ) if ( len >= len_threshold )
{ {
/* the score is the sum of two values indicating the */ /*
/* `quality' of a fit, measured along the segments' main */ * The score is the sum of two demerits indicating the
/* axis (`len_score / len') and orthogonal to it (`dist'): */ * `badness' of a fit, measured along the segments' main axis
/* smaller overlappings cause a higher score, and segments */ * and orthogonal to it, respectively.
/* with a greater distance cause a higher score also */ *
score = dist + len_score / len; * o The less overlapping along the main axis, the worse it
* is, causing a larger demerit.
*
* o The nearer the orthogonal distance to a stem width, the
* better it is, causing a smaller demerit. For simplicity,
* however, we only increase the demerit for values that
* exceed the largest stem width.
*/
FT_Pos dist = pos2 - pos1;
FT_Pos dist_demerit, score;
if ( max_width )
{
/* distance demerits are based on multiples of `max_width'; */
/* we scale by 1024 for getting more precision */
FT_Pos delta = ( dist << 10 ) / max_width - ( 1 << 10 );
if ( delta > 10000 )
dist_demerit = 32000;
else if ( delta > 0 )
dist_demerit = delta * delta / dist_score;
else
dist_demerit = 0;
}
else
dist_demerit = dist; /* default if no widths available */
score = dist_demerit + len_score / len;
/* and we search for the smallest score */ /* and we search for the smallest score */
if ( score < seg1->score ) if ( score < seg1->score )
@ -1733,6 +1785,8 @@
FT_LOCAL_DEF( FT_Error ) FT_LOCAL_DEF( FT_Error )
af_latin_hints_detect_features( AF_GlyphHints hints, af_latin_hints_detect_features( AF_GlyphHints hints,
FT_UInt width_count,
AF_WidthRec* widths,
AF_Dimension dim ) AF_Dimension dim )
{ {
FT_Error error; FT_Error error;
@ -1741,7 +1795,7 @@
error = af_latin_hints_compute_segments( hints, dim ); error = af_latin_hints_compute_segments( hints, dim );
if ( !error ) if ( !error )
{ {
af_latin_hints_link_segments( hints, dim ); af_latin_hints_link_segments( hints, width_count, widths, dim );
error = af_latin_hints_compute_edges( hints, dim ); error = af_latin_hints_compute_edges( hints, dim );
} }
@ -2694,6 +2748,8 @@
FT_Error error; FT_Error error;
int dim; int dim;
AF_LatinAxis axis;
error = af_glyph_hints_reload( hints, outline ); error = af_glyph_hints_reload( hints, outline );
if ( error ) if ( error )
@ -2707,14 +2763,22 @@
if ( AF_HINTS_DO_HORIZONTAL( hints ) ) if ( AF_HINTS_DO_HORIZONTAL( hints ) )
#endif #endif
{ {
error = af_latin_hints_detect_features( hints, AF_DIMENSION_HORZ ); axis = &metrics->axis[AF_DIMENSION_HORZ];
error = af_latin_hints_detect_features( hints,
axis->width_count,
axis->widths,
AF_DIMENSION_HORZ );
if ( error ) if ( error )
goto Exit; goto Exit;
} }
if ( AF_HINTS_DO_VERTICAL( hints ) ) if ( AF_HINTS_DO_VERTICAL( hints ) )
{ {
error = af_latin_hints_detect_features( hints, AF_DIMENSION_VERT ); axis = &metrics->axis[AF_DIMENSION_VERT];
error = af_latin_hints_detect_features( hints,
axis->width_count,
axis->widths,
AF_DIMENSION_VERT );
if ( error ) if ( error )
goto Exit; goto Exit;

6
src/autofit/aflatin.h
View File

@ -5,7 +5,7 @@
/* Auto-fitter hinting routines for latin writing system */ /* Auto-fitter hinting routines for latin writing system */
/* (specification). */ /* (specification). */
/* */ /* */
/* Copyright 2003-2007, 2009, 2011-2013 by */ /* Copyright 2003-2007, 2009, 2011-2014 by */
/* David Turner, Robert Wilhelm, and Werner Lemberg. */ /* David Turner, Robert Wilhelm, and Werner Lemberg. */
/* */ /* */
/* This file is part of the FreeType project, and may only be used, */ /* This file is part of the FreeType project, and may only be used, */
@ -169,6 +169,8 @@ FT_BEGIN_HEADER
FT_LOCAL( void ) FT_LOCAL( void )
af_latin_hints_link_segments( AF_GlyphHints hints, af_latin_hints_link_segments( AF_GlyphHints hints,
FT_UInt width_count,
AF_WidthRec* widths,
AF_Dimension dim ); AF_Dimension dim );
FT_LOCAL( FT_Error ) FT_LOCAL( FT_Error )
@ -177,6 +179,8 @@ FT_BEGIN_HEADER
FT_LOCAL( FT_Error ) FT_LOCAL( FT_Error )
af_latin_hints_detect_features( AF_GlyphHints hints, af_latin_hints_detect_features( AF_GlyphHints hints,
FT_UInt width_count,
AF_WidthRec* widths,
AF_Dimension dim ); AF_Dimension dim );
/* */ /* */