add ComposeTarget interface

interface.go

@@ -38,6 +38,30 @@ type BasicTarget interface {
  SetColorMask(color.Color)
 }
 
+// ComposeTarget is a BasicTarget capable of Porter-Duff composition.
+type ComposeTarget interface {
+ BasicTarget
+
+ // SetComposeMethod sets a Porter-Duff composition method to be used.
+ SetComposeMethod(ComposeMethod)
+}
+
+// ComposeMethod is a Porter-Duff composition method.
+type ComposeMethod int
+
+// Here's the list of all available Porter-Duff composition methods. User ComposeOver for the basic
+// alpha blending.
+const (
+ ComposeOver ComposeMethod = iota
+ ComposeIn
+ ComposeOut
+ ComposeAtop
+ ComposeDstIn
+ ComposeDstOut
+ ComposeDstAtop
+ ComposeXor
+)
+
 // Triangles represents a list of vertices, where each three vertices form a triangle. (First,
 // second and third is the first triangle, fourth, fifth and sixth is the second triangle, etc.)
 type Triangles interface {