catflap.org Online Dictionary Query

  vdiff
       
          /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]
       
       

  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.