I have added a new procedure to the pdb in CVS head by making some
modifications to plug-ins/common/compose.c. 

It is called "Recompose", and is accessible from Filters->Colors and
Image->Mode.  It can be run on any of the grayscale images produced by
Decompose, and runs without asking any questions or showing any dialog
-- its effect is to compose the layers in the inverse of the way that
they were decomposed, and substitute the result for the layer that was
originally decomposed to produce them.  The Recompose operation can be
undone by undoing on the image that was originally decomposed.

The net result is that this should make it much more convenient to
do some operations on the individual color channels of a layer:  you
simply decompose it, manipulate the grayscale layers however you
please, then recompose.

This was made possible by modifying Decompose to add a parasite to the
images that it creates, containing the necessary information about
which layer was decomposed and how it was done.  The parasite is
simple ascii text, and non-persistent, so it won't be saved in
XCF files.  This change should be invisible to users of Decompose.

In the testing I have done, it seems pretty robust and straightforward
to use, but of course I am open to any suggestions for improvements.

Note: while working on this, I noticed that Decompose did not add
alpha channels to the layers it created.  I have fixed this both
in cvs head and in the 2.2 branch.

  -- Bill

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