There is an option menu which allows the user to control values that are outside the specified range of grey levels. If the option menu is set to black, then all values outside the specified range are set to black. Similarly if the option menu is set to white, then the outside ranges are set to white. The Ignore option will not alter existing values that are outside the range of the PCT.
The ``Compress'', ``Map'', or ``Stretch'' push button is used to actually display the selected colour scale in the image window. Until this button is pressed, you will not actually see the effects of changing the colour bar or histogram.