Ken --- close. but no cigar... 
the steps: 1) Colors -> Desaturate; 2) Colors -> Invert leaves me with a 
greyscale image... all color is gone :-( I'm looking for a filter that will 
invert only black, white and grey pixels... leaving color saturated pixels 
alone.

Owen --- I will investigate the python solution. thanks for the advice.

All --- Maybe the best question to ask is this... where online should I go to 
learn about how GIMP filters are developed? I suppose once I learn how to make 
on filter.. I suspect I will quickly conjure up other filter ideas as well...

For completeness, my original question is this: I have an RGB image that is 
basically a black background with many
colorful objects and white text.  --- I'd like to invert the image so
that it is a white background with black text but a simple invert
inverts the colors in all the colored objects as well. Is there a
technique or tool to invert any part of the image that is only a shade of grey?
i.e. invert pixeils with a saturation of ~0 (if that is how you say
it). I could imagine a filter that does a simple RGB invert for any
pixel whose calculated RGB saturation value is below a set threshold.
Does this type of tool/script exist?


continued thanks,
-rich


----- Original Message ----
From: Ken Warner <kwarner...@verizon.net>
To: Rich Evans <reva...@yahoo.com>
Cc: Gimp-user@lists.XCF.Berkeley.EDU
Sent: Tue, February 2, 2010 8:12:09 PM
Subject: Re: [Gimp-user] how to invert greyscale values only...

Colors -> Desaturate; Colors -> Invert

No?

Rich Evans wrote:
> 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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