NAME:
    pixwt
 PURPOSE: (one line)
    Circle-rectangle overlap area computation.
 DESCRIPTION:
    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).
 CATEGORY:
    CCD data processing
 CALLING SEQUENCE:
    Pixwt( xc, yc, r, x, y )
 INPUTS:
    xc, yc : Center of the circle.
    r      : Radius of the circle.
    x, y   : Center of the unit pixel.
 OPTIONAL INPUT PARAMETERS:
    None.
 KEYWORD PARAMETERS:
    None.
 OUTPUTS:
    Function value: Computed overlap area.
 COMMON BLOCKS:
    None.
 SIDE EFFECTS:
 RESTRICTIONS:
    None.
 PROCEDURE:
    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.
 MODIFICATION HISTORY:
    Ported by Doug Loucks, Lowell Observatory, 1992 Sep, from the
    routine pixwt.c, by Marc Buie.