vdiff

vdiff
 /vee'dif/, 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.
    

vdiff
optical diff

   /vee'dif/ 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}.

   [{Jargon File}]