I'm grateful for the replies I have received, but I was thinking that if there 
isn't a well-known tool or method already, that maybe someone could point me to 
a site that describes how custom filters can be made from scratch. I'm not 
afraid to learn what I need to to make the filter, ...I've just never done it 
before. *grin* I just need someone to point me to a decent tutorial (I can do c 
programming if that's what it is, but my working assumption is that, all things 
being equal,  it should be easier to make a filter for GIMP than it would be to 
write a c program from scratch).Would anyone consider this... easy?

continue thanks,
-Rich



----- Original Message ----
From: Cédric Gémy <radar.ma...@free.fr>
To: Gimp-user@lists.XCF.Berkeley.EDU
Sent: Tue, February 2, 2010 5:58:51 PM
Subject: Re: [Gimp-user] how to invert greyscale values only...

not completely satisfied with sven's method, working on some pictures
but not any, depending on the shades.
I used to do it in CMYK a long time ago with photoshop by just pushing
the image to CMYK and inverting the K curve.
I guess something might be done with decompose color filter or so. But
the quicker/best average i've found is just adding a mask based on the
layer (L from Lab or N form CMYK could be used too), and play with the
curve to adapt to the shades+refine with brush on mask. It takes few
seconds if the picture is not too complex. Then put a white layer below
and it's quite done. 
But i'm not completely satisfied with it.

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