[Tex2002ans]

Originally Posted by BetterRed

Have a look for imaging forensic tools.

Hmmm... that may be another angle to research.

I know a lot of times they "average" pixel colors of entire rows/columns to get semi-unique fingerprints. Perhaps something like that could be used to detect lines too.

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Tonight I was dabbling a bit more with Hough Lines.

I had quite a bit of success locating the line through the text.

ImageMagick Hough Lines

Original Image:



Step 1: Inverse the scan using ImageMagick's canny (see fmwconcepts link in Post #1):

convert test.png -canny 0x1+10%+40% test_inverse.png

Step 2: Then calculate Hough Lines:

From testing, on this specific book, I found a threshold between 500-700 worked:

Spoiler:

convert -hough-lines 5x5+500 -fill red -transparent white test_inverse.png test_lines_500.png
convert -hough-lines 5x5+550 -fill red -transparent white test_inverse.png test_lines_550.png
convert -hough-lines 5x5+600 -fill red -transparent white test_inverse.png test_lines_600.png
convert -hough-lines 5x5+650 -fill red -transparent white test_inverse.png test_lines_650.png
convert -hough-lines 5x5+700 -fill red -transparent white test_inverse.png test_lines_700.png

The higher the threshold, the more "false positives" disappeared.

Step 3: Overlay Hough Lines with image:

Spoiler:

convert test.png ( test_lines_500.png ) -compose over -composite test4_composite_500.png
convert test.png ( test_lines_550.png ) -compose over -composite test4_composite_550.png
convert test.png ( test_lines_600.png ) -compose over -composite test4_composite_600.png
convert test.png ( test_lines_650.png ) -compose over -composite test4_composite_650.png
convert test.png ( test_lines_700.png ) -compose over -composite test4_composite_700.png

Here's the same steps with another page:



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Side Note: To see what a Hough Line calculation is actually doing, I found this part of the video did a decent job explaining it visually:

https://youtu.be/4zHbI-fFIlI?t=219

It goes row-by-row detecting each white pixel, then spins a line in a 360. Plotting this leads to points of various strength (which tells you probable locations + angles of lines).