Ubuntu Manpage: PPDITR - generates and returns a set of trapezoids representing the interior of the "difference" polygon,

NAME

       PPDITR - generates and returns a set of trapezoids representing the interior of the "difference" polygon,
       which consists of all points that are inside the subject polygon but not inside the clip polygon.

SYNOPSIS

       CALL PPDITR (XCCP,YCCP,NCCP,XCSP,YCSP,NCSP,RWRK,IWRK,NWRK,URPT,IERR)

C-BINDING SYNOPSIS

       #include <ncarg/ncargC.h>

       void c_ppditr(
            float *xccp,
            float *yccp,
            int nccp,
            float *xcsp,
            float *ycsp,
            int ncsp,
            float *rwrk,
            int *iwrk,
            int nwrk,
            int (*urpt_)(float *xcbl,
                         float *xcbr,
                         float *ycob,
                         float *dxle,
                         float *dxre,
                         float *ycot),
            int *ierr)

DESCRIPTION

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). The routines
that produce trapezoids use a lot less workspace than the ones that produce polygons. The
amount of space used at a particular time during the execution of one of these routines is
roughly ten or eleven times the number of intersections of a horizontal "scan line" with the
edge segments making up the input polygons, plus about three times the number of points above
the current scan line that are "local minima". (A "local minimum" is a point on one of the
polygons that is connected to two other points on the polygon having larger Y coordinates
than its own). I have not yet developed a rule of thumb for setting the value of NWRK.
Ultimately, I would like to at least 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.

URPT is the name of a user-provided routine to process the trapezoids. This name must appear in
an EXTERNAL statement in the routine that calls PPDITR and the routine itself must have the
following form:

SUBROUTINE URPP (XCBL,XCBR,YCOB,DXLE,DXRE,YCOT)
...(code to process a trapezoid)...
RETURN
END

The bottom and top of the trapezoid are horizontal (parallel to the X axis); the arguments
XCBL and XCBR define the X coordinates of its bottom left and bottom right corners, YCOB is
the Y coordinate of its bottom edge, DXLE and DXRE are the inverses (dx/dy) of the slopes of
its left and right edges, and YCOT is the Y coordinate of its top edge. The corners of the
trapezoid are therefore as follows: (XCBL,YCOB), (XCBR,YCOB), (XCBL+DXLE*(YCOT-YCOB),YCOT),
and (XCBR+DXRE*(YCOT-YCOB),YCOT).

IERR (an output variable of type INTEGER) is returned with the value zero if no errors occurred in
the execution of PPDITR 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 probably indicate some problem with
the algorithm. If IERR is returned non-zero, one can be sure that there have been no calls
to URPT only if IERR = 1 or 2; otherwise, some trapezoids probably have been generated and
delivered to URPT.

C-BINDING DESCRIPTION

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

USAGE

       The FORTRAN statement

       CALL PPDITR (XCCP,YCCP,NCCP,XCSP,YCSP,NCSP,RWRK,IWRK,NWRK,URPT,IERR)

       causes  the  formation  of  a  difference  polygon  (the  subject polygon minus the clip polygon) and the
       delivery of trapezoids representing the interior of that polygon, one at a time,  to  the  user-specified
       trapezoid-processing routine URPT.

EXAMPLES

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

ACCESS

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

COPYRIGHT

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

UNIX                                               March 1995                                     PPDITR(3NCARG)