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

[Nick LaForge <nicklaforge@xxxxxxxxx>]

> 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:
> http://www.majhost.com/gallery/ffaat/gimp/more2/results.jpg
>
> -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....

Nick
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