This command converts partial transparency into all-or-none transparency, by applying a user-specified cutoff threshold to the layer's alpha channel. It only works on alpha-channel-possessing layers from RGB images: if the image is Grayscale or Indexed, or the layer lacks an alpha channel, the menu entry will be insensitive. If Keep transparency has been toggled on in the Layers dialog, the command will show an error message and refuse to operate.

10.40.2.  Dialog window description

Figure 10.79.  The “Threshold Alpha” dialog

Threshold

With this option you can set the transparency value which will be used as a threshold. You can do that by using the slider or by entering value in the input box (0-255). All transparency values below this threshold will turn opaque. All transparency values above this threshold will turn to full transparency.