1PPPPAP(3NCARG)                   NCAR GRAPHICS                  PPPPAP(3NCARG)
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NAME

6       PPPPAP - can be called to preprocess a polygon in such a way as to
7       remove certain peculiarities that can cause minor cosmetic errors in
8       the output from the routines that return trapezoids.
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SYNOPSIS

11       CALL PPPPAP (XCIP,YCIP,NCIP,NBTS)
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C-BINDING SYNOPSIS

14       #include <ncarg/ncargC.h>
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16       void c_ppppap(float *xcip, float *ycip, int ncip, int nbts)
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DESCRIPTION

19       XCIP        (an input/output array of type REAL) is the X coordinate
20                   array for a polygon to be used as input to one of the
21                   routines PPDITR, PPINTR, or PPUNTR.  PPPPAP will alter the
22                   contents of this array.
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24       YCIP        (an input/output array of type REAL) is the Y coordinate
25                   arrays for a polygon to be used as input to one of the
26                   routines PPDITR, PPINTR, or PPUNTR.  PPPPAP will alter the
27                   contents of this array.
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29       NCIP        (an input/output variable of type INTEGER) is the number of
30                   points defining the input polygon. PPPPAP may reduce the
31                   value of NCIP.
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33       NBTS        (an input expression of type INTEGER) is the number of
34                   significant bits to be left unzeroed in the fractional part
35                   of each coordinate. Generally, one would probably not want
36                   to use a value less than 10 or 12. On a 32-bit machine on
37                   which reals have 24-bit fractions, 18 may be a good choice;
38                   on a 64-bit machine with 48-bit fractions, larger values
39                   may be desirable.
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C-BINDING DESCRIPTION

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

46       The FORTRAN statement
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48         CALL PPPPAP (XCIP,YCIP,NCIP,NBTS)
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50       causes preprocessing of the X and Y coordinates of the points defining
51       a polygon which is to be used as input to one of the principal POLYPACK
52       routines that produce trapezoids. The object is to cure an annoying
53       (but basically cosmetic) problem that sometimes occurs.
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55       The nature of the problem is as follows: Sometimes, when adjacent
56       points have Y coordinates that differ only very slightly, there will
57       be, among the output trapezoids, degenerates, of essentially zero
58       height, that stick out to the left or right from the body of the
59       polygon of which each trapezoid is a part. This happens because, in the
60       calls to the user-defined trapezoid-processing routine URPT, the values
61       of DXLE and DXRE become very large and the difference between the
62       values of YCOT and YCOB becomes very small; in URPT, then, to get the X
63       coordinates at the ends of the top of the trapezoid, the very large
64       values and the very small values are multiplied together and the result
65       is highly inaccurate.
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67       What PPPPAP does is this: Each of the input coordinates is first
68       modified by zeroing out all but the first NBTS bits of its fractional
69       part; then, any point with the same coordinates as the preceding point
70       is culled. This ensures that there are no adjacent points with X or Y
71       coordinates that are so nearly identical as to cause the observed
72       problem, but has little real effect on the values of the coordinates.
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74       There are several reasons why this was not done automatically: 1)
75       Because it's a little time-consuming (zeroing the low-order bits in a
76       way that doesn't violate the FORTRAN-77 standard is a bit difficult),
77       it shouldn't be done if it doesn't have to be done, and the user may
78       have reason to know that the problem doesn't arise.  2) The problem may
79       arise in one of the polygons, but not the other (most likely, in the
80       subject polygon, but not in the clip polygon).  3) The user knows best
81       what precision needs to be maintained in the data.
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EXAMPLES

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

88       To use PPPPAP or c_ppppap, load the NCAR Graphics libraries ncarg,
89       ncarg_gks, and ncarg_c, preferably in that order.
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SEE ALSO

92       Online: polypack, ppdipo, ppditr, ppinpo, ppintr, ppplcl, ppunpo,
93       ppuntr, ncarg_cbind.
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95       Hardcopy: None.
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98       Copyright (C) 1987-2009
99       University Corporation for Atmospheric Research
100       The use of this Software is governed by a License Agreement.
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104UNIX                              March 1995                    PPPPAP(3NCARG)
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