Width-correct individual characters when copying them

Fira Code punctuation characters (a) vary in horizontal advance among
themselves and (b) may not match the horizontal advance or glyph width
of the target font. This change fixes the horizontal advance when
copying individual characters.

Depending on the width of the copied characters (relative to the input
font) and the setting of --scale-character-glyphs-threshold, it will
also either scale the copied glyphs horizontally, or simply center them
inside the now correctly sized hbox.

Note that this only applies to individual characters (and thus is a
no-op if the user hasn't turned on individual character copy with the
--copy-character-glyphs option); properly scaling ligatures is still a
work in progress.

Makefile

@@ -1,3 +1,10 @@
+# Alternate options for ligaturize.py.
+# Run `fontforge -lang=py ligaturize.py --help` for details.
+LIGATURIZE_OPTS=
+# LIGATURIZE_OPTS+=" --copy-character-glyphs"
+# LIGATURIZE_OPTS+=" --scale-character-glyphs-threshold=0.0"
+# LIGATURIZE_OPTS+=" --scale-character-glyphs-threshold=2.0"
+
 TTF_SRCS=$(wildcard input-fonts/*.ttf)
 OTF_SRCS=$(wildcard input-fonts/*.otf)
 
@@ -7,4 +14,5 @@ OTF_OUTS=$(patsubst input-fonts/%.otf,output-fonts/Liga%.ttf,${OTF_SRCS})
 all: ${TTF_OUTS} ${OTF_OUTS}
 
 output-fonts/Liga%.ttf: input-fonts/%.* ligatures.py ligaturize.py
-	fontforge -lang=py ligaturize.py "$<" "$@" 2>&1 | fgrep -v 'This contextual rule applies no lookups.'
+	fontforge -lang=py ligaturize.py $(LIGATURIZE_OPTS) "$<" "$@" 2>&1 \
+	| fgrep -v 'This contextual rule applies no lookups.'

ligaturize.py

@@ -10,6 +10,7 @@
 # See ligatures.py for a list of all the ligatures that will be copied.
 
 import fontforge
+import psMat
 import os
 from os import path
 import sys
@@ -72,6 +73,10 @@ class LigatureCreator(object):
         self.opts = opts
         self._lig_counter = 0
 
+        # Scale firacode to correct em height.
+        self.firacode.em = self.font.em
+        self.emwidth = self.font['m'].width
+
     def copy_ligature_from_source(self, ligature_name):
         try:
             self.firacode.selection.none()
@@ -81,7 +86,43 @@ class LigatureCreator(object):
         except ValueError:
             return False
 
+    def correct_character_width(self, glyph):
+        """Width-correct copied individual characters (not ligatures!).
+
+        This will correct the horizontal advance of characters to match the em
+        width of the output font, and (depending on the width of the glyph, the
+        em width of the output font, and the value of the command line option
+        --scale-character-glyphs-threshold) optionally horizontally scale it.
+
+        Glyphs that are not horizontally scaled, but which still need horizontal
+        advance correction, will be centered instead.
+        """
+
+        if glyph.width == self.emwidth:
+            # No correction needed.
+            return
+
+        widthdelta = float(abs(glyph.width - self.emwidth)) / self.emwidth
+        if widthdelta >= self.opts.scale_character_glyphs_threshold:
+            # Character is too wide/narrow compared to output font; scale it.
+            scale = float(self.emwidth) / glyph.width
+            glyph.transform(psMat.scale(scale, 1.0))
+        else:
+            # Do not scale; just center copied characters in their hbox.
+            # Fix horizontal advance first, to recalculate the bearings.
+            glyph.width = self.emwidth
+            # Correct bearings to center the glyph.
+            glyph.left_side_bearing = (glyph.left_side_bearing + glyph.right_side_bearing) / 2
+            glyph.right_side_bearing = glyph.left_side_bearing
+
+        # Final adjustment of horizontal advance to correct for rounding
+        # errors when scaling/centering -- otherwise small errors can result
+        # in visible misalignment near the end of long lines.
+        glyph.width = self.emwidth
+
+
     def copy_character_glyphs(self, chars):
+        """Copy individual (non-ligature) characters from the ligature font."""
         if not self.opts.copy_character_glyphs:
             return
         print("Copying %d character glyphs from %s..." % (
@@ -94,6 +135,7 @@ class LigatureCreator(object):
             self.font.selection.none()
             self.font.selection.select(char)
             self.font.paste()
+            self.correct_character_width(self.font[char])
 
     def add_ligature(self, input_chars, firacode_ligature_name):
         if firacode_ligature_name is None:
@@ -203,6 +245,15 @@ def parse_args():
              " font as well. This will result in punctuation that matches the"
              " ligatures more closely, but may not fit in as well with the rest"
              " of the font.")
+    parser.add_argument("--scale-character-glyphs-threshold",
+        type=float, default=0.1, metavar='THRESHOLD',
+        help="When copying character glyphs, if they differ in width from the"
+             " width of the input font by at least this much, scale them"
+             " horizontally to match the input font even if this noticeably"
+             " changes their aspect ratio. The default (0.1) means to scale if"
+             " they are at least 10%% wider or narrower. A value of 0 will scale"
+             " all copied character glyphs; a value of 2 effectively disables"
+             " character glyph scaling.")
     return parser.parse_args()
 
 args = parse_args()
@@ -217,7 +268,6 @@ else:
 
 print('Reading ligatures from %s' % ligature_font_path)
 firacode = fontforge.open(ligature_font_path)
-firacode.em = font.em
 
 creator = LigatureCreator(font, firacode, args)
 ligature_length = lambda lig: len(lig['chars'])