vdiff

/veedif/ , v.,n. 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.

