PPDIPO - generates and returns the boundary of the "difference" polygon, which consists of all points

       To  use PPDIPO or c_ppdipo, load the NCAR Graphics libraries ncarg, ncarg_gks, and ncarg_c, preferably in
       that order.

       The C-binding argument descriptions are the same as the FORTRAN argument descriptions.

       #include <ncarg/ncargC.h>

       void c_ppdipo(
            float *xccp,
            float *yccp,
            int nccp,
            float *xcsp,
            float *ycsp,
            int ncsp,
            float *rwrk,
            int *iwrk,
            int nwrk,
            int (*urpp_)( float *xcra, float *ycra, int *ncra),
            int *ierr)

       Copyright (C) 1987-2009
       University Corporation for Atmospheric Research
       The use of this Software is governed by a License Agreement.

UNIX                                               March 1995                                     PPDIPO(3NCARG)

       XCCP        (an input array of type REAL) is the X coordinate array for the clip polygon.

       YCCP        (an input array of type REAL) is the Y coordinate array for the clip polygon.

       NCCP        (an input expression of type INTEGER) is the number of points defining the clip polygon.

       XCSP        (an input array of type REAL) is the X coordinate array for the subject polygon.

       YCSP        (an input array of type REAL) is the Y coordinate array for the subject polygon.

       NCSP        (an input expression of type INTEGER) is the number of points defining the subject polygon.

       RWRK        (a  scratch array, dimensioned NWRK, of type REAL) is a real workspace array.  Because of the
                   way in which they are used, RWRK and IWRK may be  EQUIVALENCEd  (and,  to  save  space,  they
                   should be).

       IWRK        (a  scratch array, dimensioned NWRK, of type INTEGER) is an integer workspace array.  Because
                   of the way in which they are used, RWRK and IWRK may be EQUIVALENCEd  (and,  to  save  space,
                   they should be).

       NWRK        (an  input  expression of type INTEGER) is the length of the workspace array(s).  It is a bit
                   difficult to describe how much space might be required. At  the  moment,  I  would  recommend
                   using  NWRK  equal to about ten times the total of the number of points in the input polygons
                   and the number of intersection points. This situation will change  with  time;  at  the  very
                   least,  I  would  like  to put in an internal parameter that will tell one how much space was
                   actually used on a given call, but I have not yet done so.

       URPP        is the name of a user-provided routine to process the  polygon-boundary  pieces.   This  name
                   must  appear in an EXTERNAL statement in the routine that calls PPDIPO and the routine itself
                   must have the following form:

                        SUBROUTINE URPP (XCRA,YCRA,NCRA)
                          DIMENSION XCRA(NCRA),YCRA(NCRA)
                          ...(code to process a polygon boundary piece)...
                          RETURN
                        END

                   Each of the arguments XCRA and YCRA is a real array, dimensioned NCRA; the former holds the X
                   coordinates, and the latter the Y coordinates, of a piece of the polygon boundary. It will be
                   the case that XCRA(NCRA)=XCRA(1) and YCRA(NCRA)=YCRA(1).

       IERR        (an output variable of type INTEGER) is returned with the value zero if no errors occurred in
                   the execution of PPDIPO or with a small positive value if an error did occur.   The  value  1
                   indicates  that a degenerate clip polygon was detected, the value 2 that a degenerate subject
                   polygon was detected, and the value 3 that the  workspace  provided  was  too  small;  values
                   greater  than  3  should  be  reported  to the author, as they indicate some problem with the
                   algorithm. Currently, if IERR is returned non-zero, one can be sure that  no  calls  to  URPP
                   were executed; in the future, this could change, but, in that case, there will be an internal
                   parameter allowing one to request the current behavior.

       Use the ncargex command to see the following relevant examples: ppex01, tppack, c_ppex01.

       PPDIPO - generates and returns the boundary of the "difference" polygon, which consists of all points
       that are inside the subject polygon but not inside the clip polygon.

       Online: polypack, ppditr, ppinpo, ppintr, ppplcl, ppppap, ppunpo, ppuntr, ncarg_cbind.

       Hardcopy: None.

       CALL PPDIPO (XCCP,YCCP,NCCP,XCSP,YCSP,NCSP,RWRK,IWRK,NWRK,URPP,IERR)

       The FORTRAN statement

       CALL PPDIPO (XCCP,YCCP,NCCP,XCSP,YCSP,NCSP,RWRK,IWRK,NWRK,URPP,IERR)

       causes  the  formation  of  a  difference  polygon  (the  subject polygon minus the clip polygon) and the
       delivery of that polygon's boundary, piece by piece, to  the  user-specified  polygon-processing  routine
       URPP.