Quickly Find The Differences in Two Photos Using Photoshop

Recently, ScanCafe ran a promotion offering up to 35% off their service. But, there was a catch ... "We made some changes to the photograph of Lange. Can you find all five? For each one you find, we'll take 5% off." The photos they provided were the following:<p><div class='p_embed p_image_embed'>


You *could* just look at the photos, maybe blow them up a bit, and figure out manually what changes they made. I spotted three immediately. But, I prefer to know rather than guess. So, I decided to quickly drop the images into Adobe Photoshop to confirm my guesses. The steps are as follows:

1) Open the images in Photoshop CS4.

2) Separate them into individual layers. (Select the 2nd image, right-click, and choose "Layer via Cut").

3) Align the two layers so that they are on top of one another. (I did this by making the top layer 50% opaque, and sliding it over until there was no ghosting. This is similar to onion-skinning. Then, I changed the opacity back to 100%).


4) Now change the layer blending method to "Difference". You'll get a mostly black image with white or grey highlights where differences exist. As a caution, if images contain a lot of JPEG compression noise, or if a large amount of sharpening was used, those will show up as differences.


5) Identify the differences. Here, there are four that stand out right away. First is the alteration to woman, now holding a camera. Second is the deletion of the windshield wiper that's present in the first photo. Third, is the (subtle) change to the license plate's digits. Fourth is the modification of the tree line on the ridge, extending its length in the second frame. That's one that I hadn't spotted without Photoshop's help. In looking for #5, you might be misled by the back door of the wagon. Yes, there's been an alteration there, but that probably comes as a result of sharpening, given that the pixel differences are only on the edges of the back door and wheel. It's also possible that they were shifted a few pixels or so, but I don't think that's what the contest is looking for. My guess for number five is the scratch in the right center of the first frame ahead of the vehicle's bumper. That doesn't stand out strongly in the difference analysis, but it does appear more noticeably if you switch the layer order and re-apply the difference blending. That makes my count Five, plus sharpening artifacts.

So, that's it! Two minutes in photoshop and I feel much more confident about my answers !

[Disclaimer-ish Note: In case you were wondering, ScanCafe seems to have taken the original image from http://en.wikipedia.org/wiki/File:Lange_car.jpg or from a government source, therefore reproduction would be permissible as a public domain work created by a US Government employee during the course of the person's official duties. The modified image by ScanCafe (and my changes in this post) are derivative works, insufficiently different enough from the original to be regarded as a new work. I'm certainly not going to be trying to register my chicken-scratch red circles a new work ...]