Re: [Gimp-user] threshold function parameters that vary by position? (going to B&W from a book page photo)

Here is the way I compensate for such exposure  when you know the
background should all be one colour:

1) Duplicate your source layer.
2) Select the duplicate layer and run Filters->Generic-> Dilate.
3) Repeat the dilate (ctrl-f) until the (black) print is gone. (For your
sample I did it three times)
4) Apply a larger radius Gaussian blur (10 times the number of dilates you
did seems to work for me, so in this case, 30px)
5) This layer is kind of a low-pass filter version of your page with the
text removed.  Set this layer to grain extract to kind of remove it from
the original.
6) Do Layer->New from Visible to get the composite.
7) Now go and play with levels/curves/thresholding.

Note this works best with low noise images (i.e. tiff or png is better than
jpeg) and higher resolution scans...

Here is what I ended up with:

-Rob A>

Thanks, this method gives virtually perfect results.  Also, thanks to
everyone for writing your suggestions.

I like how the generality of the "grain extract" and its siblings
would seem to make it a powerful tool in a number of areas, and I'm
now fascinated. :-)  I wonder if there exists a sort of compendium of
gimp techniques from the signal processing perspective....


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