add Transform.transformVector method

Adds a transformVector and transformVectors methods to the Transform class. Useful for things like https://github.com/googlefonts/ufo2ft/pull/515

Lib/fontTools/misc/transform.py

@@ -171,6 +171,31 @@ class Transform(NamedTuple):
 		xx, xy, yx, yy, dx, dy = self
 		return [(xx*x + yx*y + dx, xy*x + yy*y + dy) for x, y in points]
 
+	def transformVector(self, v):
+		"""Transform an (dx, dy) vector, treating translation as zero.
+
+		Example:
+			>>> t = Transform(2, 0, 0, 2, 10, 20)
+			>>> t.transformVector((3, -4))
+			(6, -8)
+			>>>
+		"""
+		(dx, dy) = v
+		xx, xy, yx, yy = self[:4]
+		return (xx*dx + yx*dy, xy*dx + yy*dy)
+
+	def transformVectors(self, vectors):
+		"""Transform a list of (dx, dy) vector, treating translation as zero.
+
+		Example:
+			>>> t = Transform(2, 0, 0, 2, 10, 20)
+			>>> t.transformVectors([(3, -4), (5, -6)])
+			[(6, -8), (10, -12)]
+			>>>
+		"""
+		xx, xy, yx, yy = self[:4]
+		return [(xx*dx + yx*dy, xy*dx + yy*dy) for dx, dy in vectors]
+
 	def translate(self, x=0, y=0):
 		"""Return a new transformation, translated (offset) by x, y.
 

Tests/misc/transform_test.py

@@ -23,6 +23,14 @@ class TransformTest(object):
             [(0, 0), (0, 100), (100, 100), (100, 0)]
         ) == [(0, 0), (0, 300), (200, 300), (200, 0)]
 
+    def test_transformVector(self):
+        t = Transform(2, 0, 0, 3, -10, 30)
+        assert t.transformVector((-4, 5)) == (-8, 15)
+
+    def test_transformVectors(self):
+        t = Transform(2, 0, 0, 3, -10, 30)
+        assert t.transformVectors([(-4, 5), (-6, 7)]) == [(-8, 15), (-12, 21)]
+
     def test_translate(self):
         t = Transform()
         assert t.translate(20, 30) == Transform(1, 0, 0, 1, 20, 30)