THRESHOLDING
Macro: PROTHR
Function: Threshold am image of type byte.
Parameters:
1. Input output image planes.
2. Window containing image displayed in lin grayscale.
3. Window to perform threshold operation containing slider(S), text(T), and button(B) items.
-- (S): click with left mouse button to fix threshold, image display changes corresponding to threshold.
-- Threshold[ ](T): current threshold.
-- Convert(B): execute the conversion.
-- Quit(B): cancel the operation.
