Visual diff. The operation of finding differences between two files by eyeball search. The term optical diff has also been reported, and is sometimes more specifically used for the act of superimposing two nearly identical printouts on one another and holding them up to a light to spot differences. Though this method is poor for detecting omissions in the ‘rear’ file, it can also be used with printouts of graphics, a claim few if any diff programs can make. See diff.
An interesting variant of the vdiff technique usable by anyone who has sufficient control over the parallax of their eyeballs (e.g. those who can easily view random-dot stereograms), is to hold up two paper printouts and go cross-eyed to superimpose them. This invokes deep, fast, built-in image comparison wetware (the same machinery responsible for depth perception) and differences stand out almost immediately. This technique is good for finding edits in graphical images, or for comparing an image with a compressed version to spot artifacts.