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

[Rob Antonishen <rob antonishen gmail com>]

Regarding the suggestions: I think I've played with levels enough
without results.  On the other hand, I don't know about the layer mask
approach.  From my limited abilities, I can only see that layer masks
modulate the opacity of the resulting image after having applied some
filter to a duplicate layer, whereas I want to modulate the input
parameters of a "filter" (threshold) to the original layer.

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>