unrule removes lineature from an image.
The name is coming from the opposite of rule (like hrule or vrule in TeX, which mean horizontal rule and vertical rule) and is meant to un-rule, to remove the rule from the paper. But it is also a wink on against the rules.
The tool uses a tripartite sliding window on the scan lines (horizontal line of pixels) of an image to decide, if some pixels in the inner window should be changed to the average of the left and right outer windows.
The inner window is ins (for inside) pixels wide. The left and right outer windows are each stretch pixels wide.
At the moment, the output is a black-/white image.
numpy, matplotlib, PIL (Python Pillow)
The lineature must be aligned with the image, so a rotated scan will very likely not give useful results.
Also, the paper that was used to develop unrule, so far was white paper. If the color is different, results may be unsatisfactory. In that case, please let me know via Github-issues.
Call unrule on the command line with the filenames as arguments. There are also two parameters: ins and stretch.
The ins parameter is the size in pixels of the inner (inside) window, as described above. The stretch parameter is the size in pixels of the outer area left and right of the inside window.
The default settings are:
stretch = 3
ins = 3
This gives a complete window size of stretch + ins + stretch = 3 + 3 + 3 = 9. So the window is 9 pixels, and the inside window-part is 3 pixels wide.
To get useful results, the number of pixels of the lineature (horizontal and/or vertical rule) is the value that must be set as ins value.
If you change the ins value much, the stretch value very likely must be adapted.