Thanks to all who replied. I finally got it to work.
The missing piece of information in everything that I had been trying
is that, unlike any other GUI-based program I have ever used, it
seems that there are Gimp functions that one cannot find by wandering
through the menus! The trick was to discover that I had to do an alt-
click to be able to edit the layer mask, which I eventually
discoevered in the Gimp manual, when I did a PDF search on "layer
mask". I could find no menu item that allowed me to do this, just the
alt-click trick.
Thanks for everyone's help.
Doc Evans
