PIXWT

       Circle-rectangle overlap area computation.

       Compute the fraction of a unit pixel that is interior to a circle.
       The circle has a radius r and is centered at (xc, yc).  The center of
       the unit pixel (length of sides = 1) is at (x, y).

       CCD data processing

       area = Pixwt( xc, yc, r, x, y )

       xc, yc : Center of the circle, numeric scalars
       r      : Radius of the circle, numeric scalars
       x, y   : Center of the unit pixel, numeric scalar or vector

       None.

       None.

       Function value: Computed overlap area.

       What is the area of overlap of a circle with radius 3.44 units centered
       on the point 3.23, 4.22 with the pixel centered at [5,7]
       IDL> print,pixwt(3.23,4.22,3.44,5,7)  ==>  0.6502

    None.

       Divides the circle and rectangle into a series of sectors and
       triangles.  Determines which of nine possible cases for the
       overlap applies and sums the areas of the corresponding sectors
       and triangles.    Called by aper.pro

      If improved speed is needed then a C version of this routines, with
      notes on how to linkimage it to IDL is available at
       ftp://ftp.lowell.edu/pub/buie/idl/custom/

     Ported by Doug Loucks, Lowell Observatory, 1992 Sep, from the
    routine pixwt.c, by Marc Buie.

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