This week's column by Ctein
Here's a wonderful little trick I learned last year from one of my readers, Carl Kracht.
One of the difficulties that pops up with scanning prints is that if there are any creases, folds, ripples, tears or cracks in the paper, they catch the directional light in the scanner like you wouldn't believe. Figure 1 is a great example of it; notice how below the crack is darker than above it. It's not hard to use spot healing or cloning tools to get rid of the actual cracks, but what you do about the patches of different brightness? My old answer to that would be lots of tedious dodging and burning in in an adjustment layer. Carl gave me a much better one.
Rotate the print 180° and make a second scan, as shown in figure 2. Reversing the direction the scanning light comes from reverses the pattern of shadowing and highlighting in the paper.
Then average those two scans together (figure 3). It eliminates that problem! It can even make some fine cracks and damage disappear.
In Photoshop CS6 Extended, this algorithm is buried under "File/Scripts/Statistics... ." Choose Stack Mode: Mean and point Photoshop at the two files you want Photoshop to combine. Make sure that the "Attempt to Automatically Align Source Images" box is checked. Click OK, and Photoshop generates an averaged blend of the two files.
Any image processing program that has a similar averaging function can do this. (Don't ask me which ones they are. Betcha our readers know, though.)
In theory you can get even better results by averaging scans in four directions at 90° to each other. In practice many scanners have a very slightly different pixel pitch across the sensor array than in the direction of carriage travel. Normally, the fraction of a percent distortion doesn't matter, but it may make a sharp blending of scans at right angles to each other impossible.
I make it easier for Photoshop to produce a sharp result by modifying my scans in Photoshop to bring them into very close alignment before blending them. I open both files, make a copy of "scan 2," and paste it into "scan 1," where it appears as a new layer. I set that layer's opacity to 50% to make that easier to see what's going on and rotate and translate that layer until I get a good match. I copy out the modified second layer, paste it into a new file, save it, and use that file to blend with the scan 1 file.
It takes longer to do a scan this way, but I save a whole lot more time by making a whole lot of crap disappear so that I don't have to clean it up later.
©2013 by Ctein, all rights reserved
Ctein's weekly columns, which scan best from top to bottom, appear on Wednesdays on TOP. He is the author of Digital Restoration from Start to Finish: How to repair old and damaged photographs from Focal Press, now in its Second Edition.
Original contents copyright 2013 by Michael C. Johnston and/or the bylined author. All Rights Reserved. Links in this post may be to our affiliates; sales through affiliate links may benefit this site.
(To see all the comments, click on the "Comments" link below.)
Featured Comments from:
Matthew Miller: "This also works really well for scans of prints on textured paper."
Stan Greenberg: "As per Matthew's comment, this trick very often works for scanning textured photos. Always worth a try, because everything else essentially boils down to making the image selectively fuzzy. Yes, this trick can be done in Paint Shop Pro, using layers. One other additional trick: when aligning the two scans, use 'Difference' as the blending mode. When the blend is pure black (or as close as you can get to pure black) the two layers are aligned.
"Another maybe superfluous comment: you have to perform two physically different scans, you cannot simply tell the scanning software to rescan the second time with a 90 degree rotation. (May sound obvious, but I had to learn this the hard way.) This is a wonderful technique—I think I discovered it on a Retouch Pro web site forum. Stan Greenberg, Kibbutz Kabri, Western Galilee."
Ctein replies: Yeah, works well with textured papers. Doesn't always work with the embossed "honeycomb" portrait papers, because sometimes there's an actual difference in the density. But always worth a try. The difference trick for alignment sounds Fine. I knew that...but forgot it! Thanks. Yes, the print needs to be physically rotated on the scanner platen, so it's facing the opposite way during the second scan. Thanks for emphasizing that.