varLib: add --drop-implied-oncurves option

For the test, I used the Tests/varLib/data/Build.designspace as starting point, modified the 'a' glyph so that 1 on-curve point (the first one) becomes impliable for all the masters.

Lib/fontTools/varLib/__init__.py

@@ -24,7 +24,7 @@
 from fontTools.misc.textTools import Tag, tostr
 from fontTools.ttLib import TTFont, newTable
 from fontTools.ttLib.tables._f_v_a_r import Axis, NamedInstance
-from fontTools.ttLib.tables._g_l_y_f import GlyphCoordinates
+from fontTools.ttLib.tables._g_l_y_f import GlyphCoordinates, dropImpliedOnCurvePoints
 from fontTools.ttLib.tables.ttProgram import Program
 from fontTools.ttLib.tables.TupleVariation import TupleVariation
 from fontTools.ttLib.tables import otTables as ot
@@ -40,7 +40,7 @@
 from fontTools.colorLib.builder import buildColrV1
 from fontTools.colorLib.unbuilder import unbuildColrV1
 from functools import partial
-from collections import OrderedDict, namedtuple
+from collections import OrderedDict, defaultdict, namedtuple
 import os.path
 import logging
 from copy import deepcopy
@@ -965,13 +965,54 @@ def set_default_weight_width_slant(font, location):
             font["post"].italicAngle = italicAngle
 
 
+def drop_implied_oncurve_points(*masters: TTFont) -> int:
+    """Drop impliable on-curve points from all the simple glyphs in masters.
+
+    In TrueType glyf outlines, on-curve points can be implied when they are located
+    exactly at the midpoint of the line connecting two consecutive off-curve points.
+
+    The input masters' glyf tables are assumed to contain same-named glyphs that are
+    interpolatable. Oncurve points are only dropped if they can be implied for all
+    the masters. The fonts are modified in-place.
+
+    Args:
+        masters: The TTFont(s) to modify
+
+    Returns:
+        The total number of points that were dropped if any.
+
+    Reference:
+    https://developer.apple.com/fonts/TrueType-Reference-Manual/RM01/Chap1.html
+    """
+
+    count = 0
+    glyph_masters = defaultdict(list)
+    # multiple DS source may point to the same TTFont object and we want to
+    # avoid processing the same glyph twice as they are modified in-place
+    for font in {id(m): m for m in masters}.values():
+        glyf = font["glyf"]
+        for glyphName in glyf.keys():
+            glyph_masters[glyphName].append(glyf[glyphName])
+    count = 0
+    for glyphName, glyphs in glyph_masters.items():
+        try:
+            dropped = dropImpliedOnCurvePoints(*glyphs)
+        except ValueError as e:
+            # we don't fail for incompatible glyphs in _add_gvar so we shouldn't here
+            log.warning("Failed to drop implied oncurves for %r: %s", glyphName, e)
+        else:
+            count += len(dropped)
+    return count
+
+
 def build_many(
     designspace: DesignSpaceDocument,
     master_finder=lambda s: s,
     exclude=[],
     optimize=True,
     skip_vf=lambda vf_name: False,
     colr_layer_reuse=True,
+    drop_implied_oncurves=False,
 ):
     """
     Build variable fonts from a designspace file, version 5 which can define
@@ -1015,6 +1056,7 @@ def build_many(
                 exclude=exclude,
                 optimize=optimize,
                 colr_layer_reuse=colr_layer_reuse,
+                drop_implied_oncurves=drop_implied_oncurves,
             )[0]
             if doBuildStatFromDSv5:
                 buildVFStatTable(vf, designspace, name)
@@ -1028,6 +1070,7 @@ def build(
     exclude=[],
     optimize=True,
     colr_layer_reuse=True,
+    drop_implied_oncurves=False,
 ):
     """
     Build variation font from a designspace file.
@@ -1055,6 +1098,14 @@ def build(
         except AttributeError:
             master_ttfs.append(None)  # in-memory fonts have no path
 
+    if drop_implied_oncurves and "glyf" in master_fonts[ds.base_idx]:
+        drop_count = drop_implied_oncurve_points(*master_fonts)
+        log.info(
+            "Dropped %s on-curve points from simple glyphs in the 'glyf' table",
+            drop_count,
+        )
+
+
     # Copy the base master to work from it
     vf = deepcopy(master_fonts[ds.base_idx])
 
@@ -1228,6 +1279,14 @@ def main(args=None):
         action="store_false",
         help="do not rebuild variable COLR table to optimize COLR layer reuse",
     )
+    parser.add_argument(
+        "--drop-implied-oncurves",
+        action="store_true",
+        help=(
+            "drop on-curve points that can be implied when exactly in the middle of "
+            "two off-curve points (only applies to TrueType fonts)"
+        ),
+    )
     parser.add_argument(
         "--master-finder",
         default="master_ttf_interpolatable/{stem}.ttf",
@@ -1312,6 +1371,7 @@ def main(args=None):
         exclude=options.exclude,
         optimize=options.optimize,
         colr_layer_reuse=options.colr_layer_reuse,
+        drop_implied_oncurves=options.drop_implied_oncurves,
     )
 
     for vf_name, vf in vfs.items():

Tests/varLib/data/DropOnCurves.designspace

@@ -0,0 +1,20 @@
+<?xml version='1.0' encoding='utf-8'?>
+<designspace format="3">
+    <axes>
+        <axis default="400" maximum="1000" minimum="400" name="weight" tag="wght" />
+    </axes>
+    <sources>
+        <source familyname="Test Family" filename="master_ufo/TestFamily-Master1.ttx" name="master_1" stylename="Master1">
+            <location>
+                <dimension name="weight" xvalue="400" />
+            </location>
+        </source>
+        <source familyname="Test Family" filename="master_ufo/TestFamily-Master2.ttx" name="master_2" stylename="Master2">
+            <location>
+                <dimension name="weight" xvalue="1000" />
+            </location>
+        </source>
+    </sources>
+    <instances>
+    </instances>
+</designspace>