[pens] Add qu2cuPen.py

Lib/fontTools/pens/qu2cuPen.py

@@ -0,0 +1,62 @@
+# Copyright 2016 Google Inc. All Rights Reserved.
+# Copyright 2023 Behdad Esfahbod. All Rights Reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from fontTools.qu2cu import quadratic_to_curve
+from fontTools.pens.filterPen import FilterPen
+from fontTools.pens.reverseContourPen import ReverseContourPen
+
+
+class Qu2CuPen(FilterPen):
+    """A filter pen to convert quadratic bezier splines to cubic curves
+    using the FontTools SegmentPen protocol.
+
+    Args:
+
+        other_pen: another SegmentPen used to draw the transformed outline.
+        max_err: maximum approximation error in font units. For optimal results,
+            if you know the UPEM of the font, we recommend setting this to a
+            value equal, or close to UPEM / 1000.
+        reverse_direction: flip the contours' direction but keep starting point.
+        stats: a dictionary counting the point numbers of cubic segments.
+    """
+
+    def __init__(
+        self,
+        other_pen,
+        max_err,
+        reverse_direction=False,
+        stats=None,
+    ):
+        if reverse_direction:
+            other_pen = ReverseContourPen(other_pen)
+        super().__init__(other_pen)
+        self.max_err = max_err
+        self.stats = stats
+
+    def _quadratic_to_curve(self, points):
+        quadratics = (self.current_pt,) + points
+        curves = quadratic_to_curves(quadratics, self.max_err)
+        if self.stats is not None:
+            n = str(len(curves))
+            self.stats[n] = self.stats.get(n, 0) + 1
+        for curve in curves:
+            self.curveTo(*curve[1:])
+
+    def qCurveTo(self, *points):
+        n = len(points)
+        if n < 2:
+            self.lineTo(*points)
+        else:
+            self._quadratic_to_curve(points)