The Threshold Alpha command converts
semi-transparent areas of the active layer into completely transparent 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.
If the Keep transparency option is checked in the
Layers dialog, the command displays an error message.
7.37.1. Activating the Command
You can access this command from the image menubar through
Layer → Transparency → Threshold Alpha.
7.37.2. Description of the Dialog Window
The only one option of the “Threshold Alpha”
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
completely transparent. The transition is abrupt.