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 substitute_color is TRUE, then the color specified by (r, g, b) will be
assigned zero opacity. That is, if you pass (255, 255, 255) for the
substitute color, all white pixels will become fully transparent.
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 substitute_color is TRUE, then the color specified by (r, g, b) will be assigned zero opacity. That is, if you pass (255, 255, 255) for the substitute color, all white pixels will become fully transparent.