[transform] Add DecomposedTransform.fromTransform()

Lib/fontTools/misc/transform.py

@@ -351,41 +351,7 @@ class Transform(NamedTuple):
 
     def toDecomposed(self) -> "DecomposedTransform":
         """Decompose into a DecomposedTransform."""
-        # Adapted from an answer on
+        return DecomposedTransform.fromTransform(self)
-        # https://math.stackexchange.com/questions/13150/extracting-rotation-scale-values-from-2d-transformation-matrix
-        a, b, c, d, x, y = self
-        delta = a * d - b * c
-
-        rotation = 0
-        scaleX = scaleY = 0
-        skewX = skewY = 0
-
-        # Apply the QR-like decomposition.
-        if a != 0 or b != 0:
-            r = math.sqrt(a * a + b * b)
-            rotation = math.acos(a / r) if b > 0 else -math.acos(a / r)
-            scaleX, scaleY = (r, delta / r)
-            skewX, skewY = (math.atan((a * c + b * d) / (r * r)), 0)
-        elif c != 0 or d != 0:
-            s = math.sqrt(c * c + d * d)
-            rotation = math.pi / 2 - (math.acos(-c / s) if d > 0 else -math.acos(c / s))
-            scaleX, scaleY = (delta / s, s)
-            skewX, skewY = (0, math.atan((a * c + b * d) / (s * s)))
-        else:
-            # a = b = c = d = 0
-            pass
-
-        return DecomposedTransform(
-            x,
-            y,
-            math.degrees(rotation),
-            scaleX,
-            scaleY,
-            -math.degrees(skewX),
-            math.degrees(skewY),
-            0,
-            0,
-        )
 
     def __bool__(self):
         """Returns True if transform is not identity, False otherwise.
@@ -457,6 +423,44 @@ class DecomposedTransform:
     tCenterX: float = 0
     tCenterY: float = 0
 
+    @classmethod
+    def fromTransform(self, transform):
+        # Adapted from an answer on
+        # https://math.stackexchange.com/questions/13150/extracting-rotation-scale-values-from-2d-transformation-matrix
+        a, b, c, d, x, y = transform
+        delta = a * d - b * c
+
+        rotation = 0
+        scaleX = scaleY = 0
+        skewX = skewY = 0
+
+        # Apply the QR-like decomposition.
+        if a != 0 or b != 0:
+            r = math.sqrt(a * a + b * b)
+            rotation = math.acos(a / r) if b > 0 else -math.acos(a / r)
+            scaleX, scaleY = (r, delta / r)
+            skewX, skewY = (math.atan((a * c + b * d) / (r * r)), 0)
+        elif c != 0 or d != 0:
+            s = math.sqrt(c * c + d * d)
+            rotation = math.pi / 2 - (math.acos(-c / s) if d > 0 else -math.acos(c / s))
+            scaleX, scaleY = (delta / s, s)
+            skewX, skewY = (0, math.atan((a * c + b * d) / (s * s)))
+        else:
+            # a = b = c = d = 0
+            pass
+
+        return DecomposedTransform(
+            x,
+            y,
+            math.degrees(rotation),
+            scaleX,
+            scaleY,
+            -math.degrees(skewX),
+            math.degrees(skewY),
+            0,
+            0,
+        )
+
     def toTransform(self):
         """Return the Transform() equivalent of this transformation.