[2] People with sufficient control over the parallax of their eyeballs (e.g. those who can easily view random-dot stereograms) can 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.
[3] Visual comparison with a standard chart or reference is often used as a means of measuring complex phenomena such as the weather, sea states or the roughness of a river.
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 diff programs can make.