By now, photoshopping — and, yes, Adobe lawyers, I’m afraid I just violated every single one of your language rules by writing “photoshopping” in lower case — is so popular a pasttime that the Internet is flooded with photos that are cunning pastiches. And as the John-Kerry-Jane-Fonda flap of several months ago proves, the venerable Stalinist tradition of tampering with photos for political gain is alive and well amongst Bush’s supporters. But are there any ways to tell if a picture has been digitally tampered with?

Yes there are — and my friend Noah Shachtman has an excellent piece in today’s New York Times on how it’s done. Professor Hany Farid of Dartmouth College has developed a cool way to analyze the pixels in a photo:

Take a picture that is 10 pixels by 10 pixels, for a total of 100. Stretch it to 10 by 20 pixels, and image-editing software like Adobe Photoshop will assign the picture’s original pixels to every other slot in the new picture. That leaves 100 pixels “blank,” or without values. Image-editing software fills in the gaps by examining what their neighbors look like, and then applying an average. To oversimplify, if pixel A is blue, and pixel C is red, the blank pixel B will become purple.

This kind of averaging becomes “pretty obvious” after some analysis of the image, Professor Farid said.

The only problem with the technique is that it doesn’t work on highly compressed digital images, such as JPEGs.