Do arithmetic on 2D images, pixel-by-pixel.
Usage: imcalc options
where the options are:
  -datum type = Coerce the output data to be stored as the given type,
                  which may be byte, short, or float.
                  [default = datum of first input image on command line]
  -a dname    = Read image 'dname' and call the voxel values 'a'
                  in the expression.  'a' may be any letter from 'a' to 'z'.
               ** If some letter name is used in the expression, but not
                  present in one of the image options here, then that
                  variable is set to 0.
  -expr "expression"
                Apply the expression within quotes to the input images,
                  one voxel at a time, to produce the output image.
                  ("sqrt(a*b)" to compute the geometric mean, for example)
  -output name = Use 'name' for the output image filename.

See the output of '3dcalc -help' for details on what kinds of expressions
are possible.  Note that complex-valued images cannot be processed (byte,