ppditr(3NCARG)
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.
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