Legacy ImageMagick Discussions Archive

One of the lightbulbs on our overhead scanner was dim, causing the left side of each scan to be darker than the right. Hundreds of books were scanned this way before the problem was identified. (I'm not involved in the scanning.) I was hoping there might be some way to batch convert the images, so that they're all the same uniform brightness.

Example left: http://dimenovels.lib.niu.edu/islandora ... J/download
Example right: http://dimenovels.lib.niu.edu/islandora ... J/download

Side-by-side view: http://dimenovels.lib.niu.edu/islandora ... 2/mode/2up

So far I've tried equalize and Fred's histomatch script, but without success. The images look more similar, but only by changing them a bright shade of orange. I'm almost completely unfamiliar with image processing, so I was hoping someone here might be able to point me in the right direction.

Version: 6.9.1
Platform: OS X Yosemite

Thanks!

Matthew Short
Metadata Librarian
Northern Illinois University

I'm not sure what you are trying to do. Perhaps you want to lighten the left side so the paper is the same lightness as the right (but a different colour). If so, just multiply the channels by an appropriate factor.

At a rough guess, the left image is 10% darker. So we can make it 10% lighter: A better estimate of the factor could be found by cropping a blank area from each page, converting to grayscale, and finding the mean. Divide one by the other, and that is the factor to use.

Thanks for your help. When viewed side-by-side, as in an open book, both pages should be the same color. Basically, I want to compensate for the fact that the left side was scanned with less light. I was focusing on brightness, but maybe I should be changing the color instead? Apologies for not articulating the problem very well. Your code helps some, but there's still a noticeable difference.

You might want to change all the pages to the same shade of gray. To get a gray, the three RGB colour channels need to have the same value.

Taking a sample from a blank part of each page, without any text:
ObjDatastream2 (the right page) is rgb(87%, 75%, 60%)
ObjDatastream is (the left page) rgb(75%, 71%, 57%)

We could make all the values the same as the right page Red channel, 87% gray.

To do that, we need to multiply each channel:
ObjDatastream is (the left page) rgb(1.16, 1.225, 1.526)
ObjDatastream2 (the right page) is rgb(1.0, 1.16, 1.45)
To increase legibility, we can gently increase contrast: The left page, being mostly gray, more clearly shows colour, in this case a pink at the sides. This could also be removed.

try my script textcleaner and convert them to gray via the -g option. See the link below to get to my web site