STSUB

       Subroutine of STSUBIM to read a subset of a SDAS image file.

       User can specify a subimage range or a step size    Called by STSUBIM

       Result =  stsub( unit, x1, x2, y1, y2, step)

       UNIT      =  Unit number of file, must be from 1 to 9.
                    Unit must have been opened with SXOPEN.
       x1        =  lower x value
       x2        =  upper x value
       y1        =  lower y value
       y2        =  upper y value
       step      =  used to extract every nth pixel.  If step = 1, a full res.
                    subimage is extracted; step = 2, every other pixel is
                    extracted, etc.  Defaults to 1.  The minimum value is 1.

       Result of function = array constructed from designated record.

       Uses idl common stcommn to access parameters (see SXOPEN)

       Written, M. Greason, STX, July 1990.
       Remove initialization of array for increased efficiency  January, 1991
       Removed call to STSUBC.EXE, do it all in IDL
                                      - K. Venkatakrishna, STX, April 1992
       Converted to IDL V5.0   W. Landsman   September 1997

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