-The alpha channel of each pixel in I<src> is used to control how much
-the existing colour in I<im> is replaced, if it is 255 then the colour
-is completely replaced, if it is 0 then the original colour is left
+The alpha channel of each pixel in C<src> is used to control how much
+the existing color in C<im> is replaced, if it is 255 then the color
+is completely replaced, if it is 0 then the original color is left