1PPDITR(3NCARG) NCAR GRAPHICS PPDITR(3NCARG)
2
6 PPDITR - generates and returns a set of trapezoids representing the
7 interior of the "difference" polygon, which consists of all points that
8 are inside the subject polygon but not inside the clip polygon.
9
11 CALL PPDITR (XCCP,YCCP,NCCP,XCSP,YCSP,NCSP,RWRK,IWRK,NWRK,URPT,IERR)
12
14 #include <ncarg/ncargC.h>
15 void c_ppditr(
17 float *xccp,
18 float *yccp,
19 int nccp,
20 float *xcsp,
21 float *ycsp,
22 int ncsp,
23 float *rwrk,
24 int *iwrk,
25 int nwrk,
26 int (*urpt_)(float *xcbl,
27 float *xcbr,
28 float *ycob,
29 float *dxle,
30 float *dxre,
31 float *ycot),
32 int *ierr)
33
35 XCCP (an input array of type REAL) is the X coordinate array for
36 the clip polygon.
37 YCCP (an input array of type REAL) is the Y coordinate array for
39 the clip polygon.
40 NCCP (an input expression of type INTEGER) is the number of
42 points defining the clip polygon.
43 XCSP (an input array of type REAL) is the X coordinate array for
45 the subject polygon.
46 YCSP (an input array of type REAL) is the Y coordinate array for
48 the subject polygon.
49 NCSP (an input expression of type INTEGER) is the number of
51 points defining the subject polygon.
52 RWRK (a scratch array, dimensioned NWRK, of type REAL) is a real
54 workspace array. Because of the way in which they are
55 used, RWRK and IWRK may be EQUIVALENCEd (and, to save
56 space, they should be).
57 IWRK (a scratch array, dimensioned NWRK, of type INTEGER) is an
59 integer workspace array. Because of the way in which they
60 are used, RWRK and IWRK may be EQUIVALENCEd (and, to save
61 space, they should be).
62 NWRK (an input expression of type INTEGER) is the length of the
64 workspace array(s). The routines that produce trapezoids
65 use a lot less workspace than the ones that produce
66 polygons. The amount of space used at a particular time
67 during the execution of one of these routines is roughly
68 ten or eleven times the number of intersections of a
69 horizontal "scan line" with the edge segments making up the
70 input polygons, plus about three times the number of points
71 above the current scan line that are "local minima". (A
72 "local minimum" is a point on one of the polygons that is
73 connected to two other points on the polygon having larger
74 Y coordinates than its own). I have not yet developed a
75 rule of thumb for setting the value of NWRK. Ultimately, I
76 would like to at least put in an internal parameter that
77 will tell one how much space was actually used on a given
78 call, but I have not yet done so.
79 URPT is the name of a user-provided routine to process the
81 trapezoids. This name must appear in an EXTERNAL statement
82 in the routine that calls PPDITR and the routine itself
83 must have the following form:
84 SUBROUTINE URPP (XCBL,XCBR,YCOB,DXLE,DXRE,YCOT)
86 ...(code to process a trapezoid)...
87 RETURN
88 END
89 The bottom and top of the trapezoid are horizontal
91 (parallel to the X axis); the arguments XCBL and XCBR
92 define the X coordinates of its bottom left and bottom
93 right corners, YCOB is the Y coordinate of its bottom edge,
94 DXLE and DXRE are the inverses (dx/dy) of the slopes of its
95 left and right edges, and YCOT is the Y coordinate of its
96 top edge. The corners of the trapezoid are therefore as
97 follows: (XCBL,YCOB), (XCBR,YCOB), (XCBL+DXLE*(YCOT-
98 YCOB),YCOT), and (XCBR+DXRE*(YCOT-YCOB),YCOT).
99 IERR (an output variable of type INTEGER) is returned with the
101 value zero if no errors occurred in the execution of PPDITR
102 or with a small positive value if an error did occur. The
103 value 1 indicates that a degenerate clip polygon was
104 detected, the value 2 that a degenerate subject polygon was
105 detected, and the value 3 that the workspace provided was
106 too small; values greater than 3 should be reported to the
107 author, as they probably indicate some problem with the
108 algorithm. If IERR is returned non-zero, one can be sure
109 that there have been no calls to URPT only if IERR = 1 or
110 2; otherwise, some trapezoids probably have been generated
111 and delivered to URPT.
112
114 The C-binding argument descriptions are the same as the FORTRAN
115 argument descriptions.
116
118 The FORTRAN statement
119 CALL PPDITR (XCCP,YCCP,NCCP,XCSP,YCSP,NCSP,RWRK,IWRK,NWRK,URPT,IERR)
121 causes the formation of a difference polygon (the subject polygon minus
123 the clip polygon) and the delivery of trapezoids representing the
124 interior of that polygon, one at a time, to the user-specified
125 trapezoid-processing routine URPT.
126
128 Use the ncargex command to see the following relevant examples: ppex01,
129 tppack, c_ppex01.
130
132 To use PPDITR or c_ppditr, load the NCAR Graphics libraries ncarg,
133 ncarg_gks, and ncarg_c, preferably in that order.
134
136 Online: polypack, ppdipo, ppinpo, ppintr, ppplcl, ppppap, ppunpo,
137 ppuntr, ncarg_cbind.
138 Hardcopy: None.
140
142 Copyright (C) 1987-2009
143 University Corporation for Atmospheric Research
144 The use of this Software is governed by a License Agreement.
145UNIX March 1995 PPDITR(3NCARG)