sfddiff [-merge outfile] [-ignorehints] [-ignorenames] [-ignoregpos]
  [-ignoregsub] [-ignorebitmaps] [-exact] [-warn] [-version] [-help] fontfile1
  fontfile2
  
Sfddiff compares (or merges) two font files (in any format fontforge can read) and checks for differences.
It will notice:
  If the -merge flag is specified the following argument should
  be the name of an output file into which sfddiff will store a merged version
  of the two fonts. This will contain everything from fontfile1 as well as
  any characters present in fontfile2 but not in fontfile1. For any characters
  with different outlines or references, the background of the character
  will contain the splines from fontfile2 (sadly references can not be placed
  in the background).
  
  If the -ignorehints flag is specified, then no hint differences
  will be reported.
  
  -help will provide a mini description and will list the available
  options.
  
  -usage will list the available options.
  
  -version will display the current version of sfddiff.