Add methods to reflect mesh polygons along a plane

Sources/Mesh+CSG.swift

@@ -511,6 +511,11 @@ public extension Mesh {
         }
         return edges
     }
+
+    /// Reflects each polygon of the mesh along a plane.
+    /// - Parameter plane: The ``Plane`` against which the vertices are to be reflected.
+    /// - Returns: A ``Mesh`` representing the reflected mesh.
+    func reflect(along plane: Plane) -> Self { Self(polygons.map { $0.reflect(along: plane) }) }
 }
 
 private func boundsTest(

Sources/Polygon+CSG.swift

@@ -82,6 +82,27 @@ public extension Polygon {
         intersect(with: plane, edges: &edges)
         return edges
     }
+
+    /// Reflects each vertex of the polygon along a plane.
+    /// - Parameter plane: The ``Plane`` against which the vertices are to be reflected.
+    /// - Returns: A ``Polygon`` representing the reflected vertices.
+    func reflect(along plane: Plane) -> Self {
+        mapVertices {
+            let p = $0.position.project(onto: plane)
+            let d = $0.position - p
+
+            //https://math.stackexchange.com/questions/13261/how-to-get-a-reflection-vector
+            //𝑟=𝑑−2(𝑑⋅𝑛)𝑛
+            let n = plane.normal - 2.0 * plane.normal.dot($0.normal) * $0.normal
+
+            return Vertex(
+                p - d,
+                n,
+                $0.texcoord,
+                $0.color
+            )
+        }.inverted()
+    }
 }
 
 extension Polygon {