Add Transform.inverted() method #292

Author: viblo

CHANGELOG.rst

@@ -2,6 +2,9 @@
 Changelog 
 =========
 
+.. NEXT
+  - Added Tranform.inverted() to get the inverse transform.
+
 Pymunk 7.1.0 (2025-06-29)
 -------------------------
 

TODO.txt

@@ -40,7 +40,6 @@ v6.x
 - Update benchmark
 - Make benchmark that compares different pymunk and python versions easily
 - Update get_vertices doc to use body.local_to_world shorthand to conv.
-- Package pymunk for Pyodide
 - Add Canvas util module (after pyodide) 
 - Use https://diataxis.fr/ method of organizing docs (tutorials, how-to, explanation, reference)
 - Make sure cffi extensions included in wheel and zip!

pymunk/tests/test_transform.py

@@ -5,34 +5,6 @@
 
 
 class UnitTestTransform(unittest.TestCase):
-    def testInit(self) -> None:
-
-        t = p.Transform(1, 2, 3, 4, 5, 6)
-        self.assertEqual(t.a, 1)
-        self.assertEqual(t.b, 2)
-        self.assertEqual(t.c, 3)
-        self.assertEqual(t.d, 4)
-        self.assertEqual(t.tx, 5)
-        self.assertEqual(t.ty, 6)
-        self.assertEqual(str(t), "Transform(a=1, b=2, c=3, d=4, tx=5, ty=6)")
-
-        t = p.Transform(b=4, ty=2)
-        self.assertEqual(t.a, 1)
-        self.assertEqual(t.b, 4)
-        self.assertEqual(t.c, 0)
-        self.assertEqual(t.d, 1)
-        self.assertEqual(t.tx, 0)
-        self.assertEqual(t.ty, 2)
-
-    def testIdentity(self) -> None:
-        t = p.Transform.identity()
-        self.assertEqual(t.a, 1)
-        self.assertEqual(t.b, 0)
-        self.assertEqual(t.c, 0)
-        self.assertEqual(t.d, 1)
-        self.assertEqual(t.tx, 0)
-        self.assertEqual(t.ty, 0)
-
     def testPickle(self) -> None:
         x = p.Transform.identity()
         s = pickle.dumps(x, 2)

pymunk/transform.py

@@ -164,6 +164,36 @@ def rotated(self, t: float) -> "Transform":
         """
         return self @ Transform.rotation(t)
 
+    def inverted(self):
+        """Invert this Transform and return the result
+
+        >>> t = Transform.translation(3,4).scaled(2)
+        >>> v1 = Vec2d(5, 6)
+        >>> v2 = (t @ v1)
+        >>> t.inverted() @ v2 == v1
+        True
+
+        >>> t = Transform(1,2,3,4,5,6)
+        >>> t @ t.inverted()
+        Transform(a=1.0, b=0.0, c=0.0, d=1.0, tx=0.0, ty=0.0)
+
+        >>> i = Transform.identity()
+        >>> i == i.inverted()
+        True
+        """
+        det = self.a * self.d - self.b * self.c
+        if det == 0:
+            raise ValueError("Singular transform cannot be inverted")
+
+        inv_a = self.d / det
+        inv_b = -self.b / det
+        inv_c = -self.c / det
+        inv_d = self.a / det
+        inv_tx = (self.c * self.ty - self.d * self.tx) / det
+        inv_ty = (self.b * self.tx - self.a * self.ty) / det
+
+        return Transform(inv_a, inv_b, inv_c, inv_d, inv_tx, inv_ty)
+
     @staticmethod
     def translation(x: float, y: float) -> "Transform":
         """A translation transform