gdk-pixbuf:pixbuf-add-alpha (pixbuf substitute red green blue)

pixbuf -- a gdk-pixbuf:pixbuf object
substitute -- a boolean whether to set a color to zero opacity, if this is false, then the red, green, blue arguments will be ignored
red -- an unsigned char with the red value to substitute
green -- an unsigned char with the green value to substitute
blue -- an unsigned char with the blue value to substitute

The newly created pixbuf with a reference count of 1.

Takes an existing pixbuf and adds an alpha channel to it. If the existing pixbuf already had an alpha channel, the channel values are copied from the original; otherwise, the alpha channel is initialized to 255 (full opacity).

If the substitute argument is true, then the color specified by red, green, blue will be assigned zero opacity. That is, if you pass (255, 255, 255) for the substitute color, all white pixels will become fully transparent.