Zone System Separator

This script-fu does some annoying work for a photographer who wants to analyze a black and white-photo according to the zone system of Ansel Adams.

What it does

ZS-Separator creates 11 masked layers as copies from the active layer, each representing one of the 11 tonal zones of the zone system. The mask of each layer makes all pixels transparent which do not belong to the gray values of the corresponding zone. The stack of all layers gives the complete photo. Additionally, four differently colored layers (red, cyan, black, white) are inserted below the stack of the zone layers which can be used to highlight pixels belonging to selected zones.

How to use it

Activate the layer you want to analyze and run the script. You will
find the menu entry in Filters/Generic/Zone System Separator. Turn the inserted colored layers below the zone layer stack invisible except one. Play around with the visibility or opacity of the zone layers.

Please note: This script does nothing fancy with your image. It just provides a means for evaluating black and white photos according to the zone system. If you need more information on the zone system in black and white photography have a look at http://en.wikipedia.org/wiki/Zone_system.

Known shortcomings
- works only with RGB-images.
- does not check whether it has already been run. Hence, if it is
run again all layers will be created again
- suits only for analyzing a given state of a photo. There's no dynamic adaption if
you make changes to your original layer or to the zone layers.
- excessively commented. Since the author is not a programmer he would
have appreciated to find more comments in other scripts while learning
script-fu

The value ranges for each zone are derived from an article (in german) written by
Thomas Brotzler on focussiert.com:
http://fokussiert.com/2013/03/20/digitale-schwarzweisfotografie-das-zone...

Any hints to make this script better are highly appreciated.