Geom.Mesh.RotateFace is a new function that will rotate a Face by a given amount, based on an optional center of rotation.

src/geom/mesh/RotateFace.js

@@ -0,0 +1,59 @@
+/**
+ * @author       Richard Davey <rich@photonstorm.com>
+ * @copyright    2020 Photon Storm Ltd.
+ * @license      {@link https://opensource.org/licenses/MIT|MIT License}
+ */
+
+/**
+ * Rotates the vertices of a Face to the given angle.
+ *
+ * The actual vertex positions are adjusted, not their transformed positions.
+ *
+ * Therefore, this updates the vertex data directly.
+ *
+ * @function Phaser.Geom.Mesh.RotateFace
+ * @since 3.50.0
+ *
+ * @param {Phaser.Geom.Mesh.Face} face - The Face to rotate.
+ * @param {number} angle - The angle to rotate to, in radians.
+ * @param {number} [cx] - An optional center of rotation. If not given, the Face in-center is used.
+ * @param {number} [cy] - An optional center of rotation. If not given, the Face in-center is used.
+ */
+var RotateFace = function (face, angle, cx, cy)
+{
+    var x;
+    var y;
+
+    //  No point of rotation? Use the inCenter instead, then.
+    if (cx === undefined && cy === undefined)
+    {
+        var inCenter = face.getInCenter();
+
+        x = inCenter.x;
+        y = inCenter.y;
+    }
+
+    var c = Math.cos(angle);
+    var s = Math.sin(angle);
+
+    var v1 = face.vertex1;
+    var v2 = face.vertex2;
+    var v3 = face.vertex3;
+
+    var tx = v1.x - x;
+    var ty = v1.y - y;
+
+    v1.set(tx * c - ty * s + x, tx * s + ty * c + y);
+
+    tx = v2.x - x;
+    ty = v2.y - y;
+
+    v2.set(tx * c - ty * s + x, tx * s + ty * c + y);
+
+    tx = v3.x - x;
+    ty = v3.y - y;
+
+    v3.set(tx * c - ty * s + x, tx * s + ty * c + y);
+};
+
+module.exports = RotateFace;

src/geom/mesh/index.js

@@ -16,6 +16,7 @@ var Mesh = {
     GenerateVerts: require('./GenerateVerts'),
     ParseObj: require('./ParseObj'),
     ParseObjMaterial: require('./ParseObjMaterial'),
+    RotateFace: require('./RotateFace'),
     Vertex: require('./Vertex')
 
 };