Add InvertMask and an example

context.go

@@ -496,6 +496,18 @@ func (dc *Context) AsMask() *image.Alpha {
 	return mask
 }
 
+// InvertMask inverts the alpha values in the current clipping mask such that
+// a fully transparent region becomes fully opaque and vice versa.
+func (dc *Context) InvertMask() {
+	if dc.mask == nil {
+		dc.mask = image.NewAlpha(dc.im.Bounds())
+	} else {
+		for i, a := range dc.mask.Pix {
+			dc.mask.Pix[i] = 255 - a
+		}
+	}
+}
+
 // Clip updates the clipping region by intersecting the current
 // clipping region with the current path as it would be filled by dc.Fill().
 // The path is cleared after this operation.

examples/invert-mask.go

@@ -0,0 +1,14 @@
+package main
+
+import "github.com/fogleman/gg"
+
+func main() {
+	dc := gg.NewContext(1024, 1024)
+	dc.DrawCircle(512, 512, 384)
+	dc.Clip()
+	dc.InvertMask()
+	dc.DrawRectangle(0, 0, 1024, 1024)
+	dc.SetRGB(0, 0, 0)
+	dc.Fill()
+	dc.SavePNG("out.png")
+}