add reflect method for matrix

geometry.go

@@ -3,6 +3,8 @@ package pixel
 import (
 	"fmt"
 	"math"
+
+	"github.com/scottwinkler/pixel"
 )
 
 // Clamp returns x clamped to the interval [min, max].
@@ -371,6 +373,15 @@ func (m Matrix) Rotated(around Vec, angle float64) Matrix {
 	return m
 }
 
+//Reflect reflects everything around a given point by the given angle in radians
+func (m Matrix) Reflect(around pixel.Vec, angle float64) pixel.Matrix {
+	sin2t, cos2t := math.Sincos(2 * angle)
+	m[4], m[5] = m[4]-around.X, m[5]-around.Y
+	m = m.Chained(pixel.Matrix{cos2t, sin2t, sin2t, -cos2t, 0, 0})
+	m[4], m[5] = m[4]+around.X, m[5]+around.Y
+	return m
+}
+
 // Chained adds another Matrix to this one. All tranformations by the next Matrix will be applied
 // after the transformations of this Matrix.
 func (m Matrix) Chained(next Matrix) Matrix {