[glyf] Support and test cubic spline

Lib/fontTools/ttLib/tables/_g_l_y_f.py

@@ -1427,6 +1427,7 @@ class Glyph(object):
                 cubic = all(cuFlags)
                 if cubic:
                     count = len(contour)
+                    assert count % 2 == 0, "Odd number of cubic off-curves undefined"
                     for i in range(0, count, 2):
                         p1 = contour[i]
                         p2 = contour[i + 1]
@@ -1462,11 +1463,23 @@ class Glyph(object):
                         if cubic:
                             assert all(
                                 cubicFlags
-                            ), "Mixed segments not currently supported"
+                            ), "Mixed cubic and quadratic segment undefined"
+
+                            count = nextOnCurve
                             assert (
-                                len(cubicFlags) == 2
+                                count >= 3
-                            ), "Cubic multi-segments not currently supported"
+                            ), "At least two cubic off-curve points required"
-                            pen.curveTo(*contour[:nextOnCurve])
+                            assert (
+                                count - 1
+                            ) % 2 == 0, "Odd number of cubic off-curves undefined"
+                            for i in range(0, count - 3, 2):
+                                p1 = contour[i]
+                                p2 = contour[i + 1]
+                                p4 = contour[i + 2]
+                                p3 = ((p2[0] + p4[0]) * 0.5, (p2[1] + p4[1]) * 0.5)
+                                lastOnCurve = p3
+                                pen.curveTo(p1, p2, p3)
+                            pen.curveTo(*contour[count - 3 : count])
                         else:
                             pen.qCurveTo(*contour[:nextOnCurve])
                     contour = contour[nextOnCurve:]

Tests/ttLib/tables/_g_l_y_f_test.py

@@ -8,6 +8,7 @@ from fontTools.ttLib.tables._g_l_y_f import (
     Glyph,
     GlyphCoordinates,
     GlyphComponent,
+    flagOnCurve,
     flagCubic,
     ARGS_ARE_XY_VALUES,
     SCALED_COMPONENT_OFFSET,
@@ -771,6 +772,32 @@ class GlyphCubicTest:
                 ("closePath", ()),
             ]
 
+    def test_spline(self):
+        glyph = Glyph()
+        glyph.numberOfContours = 1
+        glyph.coordinates = GlyphCoordinates(
+            [(0, 0), (1, 0), (1, 0), (1, 1), (1, 1), (0, 1), (0, 1)]
+        )
+        glyph.flags = array.array("B", [flagOnCurve] + [flagCubic] * 6)
+        glyph.endPtsOfContours = [6]
+        glyph.program = ttProgram.Program()
+
+        for i in range(2):
+
+            if i == 1:
+                glyph.compile(None)
+
+            pen = RecordingPen()
+            glyph.draw(pen, None)
+
+            assert pen.value == [
+                ("moveTo", ((0, 0),)),
+                ("curveTo", ((1, 0), (1, 0), (1.0, 0.5))),
+                ("curveTo", ((1, 1), (1, 1), (0.5, 1.0))),
+                ("curveTo", ((0, 1), (0, 1), (0, 0))),
+                ("closePath", ()),
+            ]
+
 
 if __name__ == "__main__":
     import sys