[varLib.mutator] Improve CharString rounding

Since CharString point coordinates are relative, rounding each point independently can accumulate rounding errors causing to the last point to not match the first one, which leads to rendering artifacts. This is a hack to round the absolute point coordinates, but using pens to convert the relative coordinates to absolute ones and back. This is a hack since it drops hinting operators, and it can’t preserve the original operators, in addition to being slower. There is probably a more clever way to doing this.
fonttools · Apr 13, 2024 · 5a347b7 · 5a347b7
1 parent 8caf16c
commit 5a347b7
Show file tree

Hide file tree

Showing 3 changed files with 25 additions and 12 deletions.
diff --git a/Lib/fontTools/pens/t2CharStringPen.py b/Lib/fontTools/pens/t2CharStringPen.py
@@ -47,7 +47,7 @@ def _closePath(self):
     def _endPath(self):
         pass
 
-    def getCharString(self, private=None, globalSubrs=None, optimize=True):
+    def getProgram(self, optimize=True):
         commands = self._commands
         if optimize:
             maxstack = 48 if not self._CFF2 else 513
@@ -62,6 +62,10 @@ def getCharString(self, private=None, globalSubrs=None, optimize=True):
             program.insert(0, otRound(self._width))
         if not self._CFF2:
             program.append("endchar")
+        return program
+
+    def getCharString(self, private=None, globalSubrs=None, optimize=True):
+        program = self.getProgram(optimize=optimize)
         charString = T2CharString(
             program=program, private=private, globalSubrs=globalSubrs
         )

diff --git a/Lib/fontTools/varLib/mutator.py b/Lib/fontTools/varLib/mutator.py
@@ -69,6 +69,8 @@ def interpolate_cff2_PrivateDict(topDict, interpolateFromDeltas):
 
 
 def interpolate_cff2_charstrings(topDict, interpolateFromDeltas, glyphOrder):
+    from fontTools.pens.t2CharStringPen import T2CharStringPen
+
     charstrings = topDict.CharStrings
     for gname in glyphOrder:
         # Interpolate charstring
@@ -100,7 +102,7 @@ def interpolate_cff2_charstrings(topDict, interpolateFromDeltas, glyphOrder):
                     next_ti = tuplei + num_regions
                     deltas = charstring.program[tuplei:next_ti]
                     delta = interpolateFromDeltas(vsindex, deltas)
-                    charstring.program[argi] += otRound(delta)
+                    charstring.program[argi] += delta
                     tuplei = next_ti
                     argi += 1
                 new_program.extend(charstring.program[last_i:end_args])
@@ -109,6 +111,12 @@ def interpolate_cff2_charstrings(topDict, interpolateFromDeltas, glyphOrder):
             new_program.extend(charstring.program[last_i:])
             charstring.program = new_program
 
+        # Rebuild charstring program by passing it through T2CharStringPen which
+        # will have the side effect of rounding coordinates.
+        t2Pen = T2CharStringPen(width=None, glyphSet=None)
+        charstring.draw(t2Pen)
+        charstring.program = t2Pen.getProgram(optimize=True)
+
 
 def interpolate_cff2_metrics(varfont, topDict, glyphOrder, loc):
     """Unlike TrueType glyphs, neither advance width nor bounding box

diff --git a/Tests/varLib/data/test_results/InterpolateTestCFF2VF.ttx b/Tests/varLib/data/test_results/InterpolateTestCFF2VF.ttx
@@ -34,35 +34,33 @@
           84 hmoveto
           432 660 -432 hlineto
           48 -628 rmoveto
-          102 176 64 106 rlineto
-          4 hlineto
-          62 -106 100 -176 rlineto
+          102 176 64 106 4 0 62 -106 100 -176 rlineto
           -342 42 rmoveto
           536 vlineto
           154 -270 rlineto
           22 26 rmoveto
-          -56 92 -94 168 rlineto
-          302 hlineto
-          -94 -168 -54 -92 rlineto
+          -56 92 -94 168 302 0 -94 -168 -54 -92 rlineto
           22 -26 rmoveto
           152 270 rlineto
           -536 vlineto
+          endchar
         </CharString>
         <CharString name="A">
           50 hmoveto
           32 hlineto
           140 396 28 80 24 68 24 82 rlinecurve
           4 hlineto
-          24 -82 24 -68 28 -80 138 -396 rcurveline
-          34 hlineto
-          -236 660 rlineto
+          24 -82 24 -68 28 -80 rrcurveto
+          138 -396 34 0 -236 660 rlineto
           -28 hlineto
           -134 -424 rmoveto
           293 28 -293 hlineto
+          endchar
         </CharString>
         <CharString name="T">
           284 hmoveto
           32 632 234 28 -500 -28 234 hlineto
+          endchar
         </CharString>
         <CharString name="dollar">
           311 34 rmoveto
@@ -74,14 +72,16 @@
           -99 -78 -54 -88 hvcurveto
           -166 338 28 -156 vvcurveto
           -70 -56 -50 -103 -85 -66 38 34 -40 vhcurveto
-          -18 -22 45 -38 73 -40 91 0 rlinecurve
+          -18 -22 rlineto
+          -38 45 73 -40 91 hhcurveto
           -18 566 rmoveto
           30 hlineto
           50 0 50 50 vvcurveto
           -30 hlineto
           -50 0 -50 -50 vvcurveto
           -562 vmoveto
           -148 30 148 vlineto
+          endchar
         </CharString>
         <CharString name="glyph00003">
           311 34 rmoveto
@@ -97,6 +97,7 @@
           -38 45 73 -40 91 hhcurveto
           -70 -146 rmoveto
           158 860 -30 4 -158 -860 rlineto
+          endchar
         </CharString>
       </CharStrings>
     </CFFFont>