InvertColorsEffect does not work correct for alpha values != 255

reason is that cairo images use premultiplied alpha values
krvax · Dec 2, 2015 · b4b013b · b4b013b
1 parent 481f6fa
commit b4b013b
Showing 1 changed file with 4 additions and 1 deletion.
diff --git a/Pinta.Core/Effects/UnaryPixelOps.cs b/Pinta.Core/Effects/UnaryPixelOps.cs
@@ -250,7 +250,10 @@ public class Invert
         {
             public override ColorBgra Apply(ColorBgra color)
             {
-                return ColorBgra.FromBgra((byte)(255 - color.B), (byte)(255 - color.G), (byte)(255 - color.R), color.A);
+                //Note: Cairo images use premultiplied alpha values
+                //The formula for changing B would be: (255 - B * 255 / A) * A / 255
+                //This can be simplified to: A - B
+                return ColorBgra.FromBgra((byte)(color.A - color.B), (byte)(color.A - color.G), (byte)(color.A - color.R), color.A);
             }
         }