On Thu 30-Sep-2010 at 18:59 +0200, Milan Vancura wrote:
> I tried to find tips how to "clean" a photo of the drawing but 
> with no success.  I have a photo of a paper with drawing (usually 
> a combination of heavy lines by marker and thin pencil ones) and 
> want to improve the image for printing. So I want to get white 
> background of 100% of the image (no shadows) and still have all 
> details of drawing kept. Even if they are sometimes lighter than 
> shadows in different part of the image. For example a dark shadow 
> in the cormer and light pencil line in the middle.

This is what I do:

Duplicate the layer and remove all the dark lines/text with the 
Dilate filter, repeat until they are all gone.

Gaussian Blur this duplicated layer with a large radius.

Effectively you want to subtract this duplicated layer from the 
original, do this by setting the layer Mode to Grain Extract, then 

The result will have an even grey background, adjust the Levels to 
make it white again.

This works very well, I use it for cleaning up photos of sketches, 
whiteboards etc...  There is an alternative method using a 
Convolution Matrix, but it doesn't get such good results for me.

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