The Keep transparency option is checked in the
Layers dialog, the command displays an error message.
semi-transparent areas of the active layer into completely transparent or
or completely opaque areas, based on a threshold you set, between 0 and
255. It only works on layers of RGB images which have an alpha
channel. If the image is Grayscale or Indexed, or if the layer does not
have an alpha channel, the menu entry is insensitive and grayed out.
Activating the Command
Description of the Dialog Window
The “Threshold Alpha” dialog
You can set the transparency value to be used as a threshold by
using the slider or by entering a value between 0 and 255 in
the input box. All transparency values below this threshold will
become opaque and all transparency values above it will become