HOWTO: Postprocessing Photographed Books
Piotr Sobolewski
This HOWTO is intended for people who already know how to use Gimp.
Sometimes, if you take a photo of some book in the library the result looks like this:
The light is never equal on all picture - some parts are brighter, some parts are darker. This is bad if you plan to OCR it or convert to djvu. Here I will describe how to postprocess such photo in Gimp, so it is equally light in all parts.
I open this photo in Gimp. Then I do a second copy of a layer. Now I have two layers with two copies of the same image.
Then I blur the upper layer with Gaussian Blur, with a large radius, e.g. 100 pixels.
The effect looks like this:
As you can see, the blured image shows us what parts of the image are dark and what parts are bright.
Now I change the mode of this upper (blured) layer to divide.
This causes the bottom layer to be divided by the upper one. The result looks like this:
Now I flatten the image.
I correct levels a little bit and have something like this:
