add ComposeMethod.Compose

compose.go

@@ -1,5 +1,7 @@
 package pixel
 
+import "errors"
+
 // ComposeTarget is a BasicTarget capable of Porter-Duff composition.
 type ComposeTarget interface {
 	BasicTarget
@@ -26,3 +28,38 @@ const (
 	ComposePlus
 	ComposeCopy
 )
+
+// Compose composes two colors together according to the ComposeMethod. A is the foreground, B is
+// the background.
+func (cm ComposeMethod) Compose(a, b RGBA) RGBA {
+	var fa, fb float64
+
+	switch cm {
+	case ComposeOver:
+		fa, fb = 1, 1-a.A
+	case ComposeIn:
+		fa, fb = b.A, 0
+	case ComposeOut:
+		fa, fb = 1-b.A, 0
+	case ComposeAtop:
+		fa, fb = b.A, 1-a.A
+	case ComposeRover:
+		fa, fb = 1-b.A, 1
+	case ComposeRin:
+		fa, fb = 0, a.A
+	case ComposeRout:
+		fa, fb = 0, 1-a.A
+	case ComposeRatop:
+		fa, fb = 1-b.A, a.A
+	case ComposeXor:
+		fa, fb = 1-b.A, 1-a.A
+	case ComposePlus:
+		fa, fb = 1, 1
+	case ComposeCopy:
+		fa, fb = 1, 0
+	default:
+		panic(errors.New("Compose: invalid ComposeMethod"))
+	}
+
+	return a.Mul(Alpha(fa)).Add(b.Mul(Alpha(fb)))
+}