diff --git a/draw2d/rgba_interpolation.go b/draw2d/rgba_interpolation.go
index 29056b1..f746e91 100644
--- a/draw2d/rgba_interpolation.go
+++ b/draw2d/rgba_interpolation.go
@@ -1,3 +1,4 @@
+// see http://pippin.gimp.org/image_processing/chap_resampling.html
 package draw2d
 
 import (