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      Box-average a 1 or 2 dimensional array.


      This procedure differs from the intrinsic REBIN function in the follow
      2 ways:
      (1) the box size parameter is specified rather than the output
              array size
      (2) for INTEGER arrays, BOXAVE computes intermediate steps using REAL*4
              (or REAL*8 for 64bit integers) arithmetic. This is
              considerably slower than REBIN but avoids integer truncation

Calling Sequence

      result = BOXAVE( Array, Xsize,[ Ysize ] )


      ARRAY - Two dimensional input Array to be box-averaged. Array may be
              one or 2 dimensions and of any type except character.

Optional Inputs

      XSIZE - Size of box in the X direction, over which the array is to
              be averaged. If omitted, program will prompt for this
      YSIZE - For 2 dimensional arrays, the box size in the Y direction.
              If omitted, then the box size in the X and Y directions are
              assumed to be equal


      RESULT - Output array after box averaging. If the input array has
              dimensions XDIM by YDIM, then RESULT has dimensions
              XDIM/NBOX by YDIM/NBOX. The type of RESULT is the same as
              the input array. However, the averaging is always computed
              using REAL arithmetic, so that the calculation should be exact.
              If the box size did not exactly divide the input array, then
              then not all of the input array will be boxaveraged.


      BOXAVE boxaverages all points simultaneously using vector subscripting


      If im_int is a 512 x 512 integer (16 bit) array, then the two statements
              IDL> im = fix(round(rebin(float(im_int), 128, 128)))
              IDL> im = boxave( im_int,4)
      give equivalent results. The use of REBIN is faster, but BOXAVE is
      is less demanding on virtual memory, since one does not need to make
      a floating point copy of the entire array.

Revision History

      Written, W. Landsman, October 1986
      Call REBIN for REAL*4 and REAL*8 input arrays, W. Landsman Jan, 1992
      Removed /NOZERO in output array definition W. Landsman 1995
      Fixed occasional integer overflow problem W. Landsman Sep. 1995
      Allow unsigned data types W. Landsman Jan. 2000
      Assume since V5.4, Allow 64bit integers W. Landsman Apr 2006

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