Fix issue with sparse masters when .notdef contain cubic curves

Lib/ufo2ft/__init__.py

@@ -408,7 +408,7 @@ def compileInterpolatableTTFsFromDS(designSpaceDoc, **kwargs):
     kwargs["skipExportGlyphs"] = designSpaceDoc.lib.get("public.skipExportGlyphs", [])
 
     if kwargs["notdefGlyph"] is None:
-        kwargs["notdefGlyph"] = _getDefaultNotdefGlyph(designSpaceDoc)
+        kwargs["notdefGlyph"] = _getDefaultNotdefGlyph(designSpaceDoc, empty=True)
 
     kwargs["extraSubstitutions"] = defaultdict(set)
     for rule in designSpaceDoc.rules:

Lib/ufo2ft/util.py

@@ -475,7 +475,7 @@ def getDefaultMasterFont(designSpaceDoc):
     return defaultSource.font
 
 
-def _getDefaultNotdefGlyph(designSpaceDoc):
+def _getDefaultNotdefGlyph(designSpaceDoc, empty=False):
     from ufo2ft.errors import InvalidDesignSpaceData
 
     try:
@@ -488,6 +488,18 @@ def _getDefaultNotdefGlyph(designSpaceDoc):
             notdefGlyph = baseUfo[".notdef"]
         except KeyError:
             notdefGlyph = None
+        else:
+            if empty:
+                # create a new empty .notdef glyph with the same width/height
+                # as the default master's .notdef glyph to be use for sparse layer
+                # master TTFs, so that it won't participate in gvar interpolation
+                # TODO(anthrotype): Use sentinel values for width/height to
+                # mark the glyph as non-participating for HVAR if/when fonttools
+                # supports that, https://github.com/googlefonts/ufo2ft/issues/501
+                emptyGlyph = _getNewGlyphFactory(notdefGlyph)(".notdef")
+                emptyGlyph.width = notdefGlyph.width
+                emptyGlyph.height = notdefGlyph.height
+                notdefGlyph = emptyGlyph
     return notdefGlyph
 
 

tests/integration_test.py

@@ -7,7 +7,11 @@
 
 import pytest
 from fontTools.pens.boundsPen import BoundsPen
-from fontTools.ttLib.tables._g_l_y_f import OVERLAP_COMPOUND, flagOverlapSimple
+from fontTools.ttLib.tables._g_l_y_f import (
+    OVERLAP_COMPOUND,
+    flagCubic,
+    flagOverlapSimple,
+)
 
 from ufo2ft import (
     compileInterpolatableTTFs,
@@ -450,17 +454,21 @@ def test_compileTTF_glyf1_not_allQuadratic(self, testufo):
 
         assert ttf["head"].glyphDataFormat == 1
 
+    @staticmethod
+    def drawCurvedContour(glyph):
+        pen = glyph.getPen()
+        pen.moveTo((500, 0))
+        pen.curveTo((500, 277.614), (388.072, 500), (250, 500))
+        pen.curveTo((111.928, 500), (0, 277.614), (0, 0))
+        pen.closePath()
+
     def test_compileVariableTTF_glyf1_not_allQuadratic(self, designspace):
         base_master = designspace.findDefault()
         assert base_master is not None
         # add a glyph with some curveTo to exercise the cu2qu codepath
         glyph = base_master.font.newGlyph("curved")
         glyph.width = 1000
-        pen = glyph.getPen()
-        pen.moveTo((500, 0))
-        pen.curveTo((500, 277.614), (388.072, 500), (250, 500))
-        pen.curveTo((111.928, 500), (0, 277.614), (0, 0))
-        pen.closePath()
+        self.drawCurvedContour(glyph)
 
         vf = compileVariableTTF(designspace, allQuadratic=False)
         expectTTX(vf, "TestVariableFont-TTF-not-allQuadratic.ttx", tables=["glyf"])
@@ -480,6 +488,26 @@ def test_compileTTF_overlap_simple_flag(self, testufo):
         assert not ttf["glyf"]["g"].components[0].flags & OVERLAP_COMPOUND
         assert ttf["glyf"]["h"].components[0].flags & OVERLAP_COMPOUND
 
+    def test_compileVariableTTF_notdefGlyph_with_curves(self, designspace):
+        # The test DS contains two full masters (Regular and Bold) and one intermediate
+        # 'sparse' (Medium) master, which does not contain a .notdef glyph and as such
+        # is supposed to inherit one from the default master. If the notdef contains
+        # any curves, an error occured because these are weren't been converted to
+        # quadratic: https://github.com/googlefonts/ufo2ft/issues/501
+
+        # First we draw an additional contour containing cubic curves in the Regular
+        # and Bold's .notdef glyphs
+        for src_idx in (0, 2):
+            notdef = designspace.sources[src_idx].font[".notdef"]
+            self.drawCurvedContour(notdef)
+        assert ".notdef" not in designspace.sources[1].font.layers["Medium"]
+
+        # this must NOT fail!
+        vf = compileVariableTTF(designspace, convertCubics=True, allQuadratic=True)
+
+        # and because allQuadratic=True, we expect .notdef contains no cubic curves
+        assert not any(f & flagCubic for f in vf["glyf"][".notdef"].flags)
+
 
 if __name__ == "__main__":
     sys.exit(pytest.main(sys.argv))