plucky (3) ppditr.3NCARG.gz

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.

SEE ALSO

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

       Hardcopy: None.

COPYRIGHT

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