tdotri(3NCARG)
NAME
TDOTRI - Order the triangles defined by a triangle list.
SYNOPSIS
CALL TDOTRI (RTRI, MTRI, NTRI, RTWK, ITWK, IORD)
C-BINDING SYNOPSIS
#include <ncarg/ncargC.h> void c_tdotri(float *rtri, int mtri, int *ntri, float *rtwk, int *itwk, int iord)
DESCRIPTION
This routine, given a list of NTRI triangles in the array RTRI and a
real scratch array RTWK of length at least MTRI x 2 , determines the
order in which the triangles are to be rendered and returns a
permutation of the integers from 1 to NTRI in the array ITWK, defining
that permutation.
The caller may select any of three ways in which the triangles are to
be ordered, the first two of which are essentially identical: When the
argument IORD is given the value 0, the distances of the midpoints of
the triangles from the viewpoint are computed and the triangles are
sorted by decreasing order of these distances. When IORD is given the
value -1, the result is the same, except that the distances of the
farthest points of the triangles from the viewpoint are computed and
the triangles are put in decreasing order of those distances. Both of
these possibilities are appropriate for situations in which the
triangles represent smooth surfaces that do not intersect each other or
themselves; the occasional small errors in the resulting rendering
order should be acceptable.
If any of the triangles in the list intersect each other or if the
surfaces being depicted are too rough, then the third option should be
used: When IORD is given the value +1, TDOTRI executes an algorithm
taken from the reference "Computer Graphics Principles and Practice",
by Foley and Van Dam. It starts by ordering the triangles as if IORD
had the value -1 (using distances of the far points of the triangles
from the viewpoint), but then it checks for situations in which this
ordering is in error and fixes the errors. Executing this algorithm can
be time-consuming, so it should not be done unless it is really
necessary; one possible way to proceed might be to use IORD = -1 while
checking out a code and then use IORD = +1 only when doing final plots.
Sometimes, when IORD = +1, triangles must be broken into smaller
triangles, thereby increasing the total number of triangles in RTRI.
If, as a result of this, NTRI becomes equal to MTRI, no error exit is
taken; instead, TDOTRI just returns control to the caller. Therefore,
it's a good idea, after calling TDOTRI, to check the value of NTRI
against the dimension MTRI; if they're equal, it probably means that
the triangle list filled up and that using the permutation returned in
ITWK will result in an incorrect rendering of the triangles.
The arguments of TDOTRI are as follows:
RTRI (an input/output array, of type REAL, dimensioned 10 x MTRI) -
a list of triangles, probably created by means of calls to
TDSTRI, TDITRI, and/or TDMTRI. As described above, the number
of triangles in the list may increase as a result of calling
TDOTRI.
MTRI (an input expression of type INTEGER) - the second dimension of
RTRI and thus the maximum number of triangles the triangle list
will hold.
NTRI (an input/output variable of type INTEGER) - specifies the
number of triangles currently in the list. It is the user's
responsibility to zero this initially; its value is increased
by each call to a triangle-generating routine like TDSTRI or
TDITRI and may be increased by a call to TDOTRI.
RTWK (a scratch array of type REAL, dimensioned at least MTRI x 2).
ITWK (an output array, of type INTEGER, dimensioned at least MTRI) -
returned containing a permutation of the integers from 1 to
NTRI, specifying the order in which the triangles ought to be
rendered.
IORD (an input expression of type INTEGER) - says how the triangles
are to be ordered. The value 0 implies ordering by decreasing
distance of the triangle midpoints from the eye, -1 implies
ordering by decreasing distance of the triangle farpoints from
the eye, and +1 implies ordering by decreasing distance of the
triangle farpoints from the eye, with adjustments made by
running an algorithm from the reference "Computer Graphics
Principles and Practice", by Foley and Van Dam.
C-BINDING DESCRIPTION
The C-binding argument descriptions are the same as the FORTRAN argument descriptions.
ACCESS
To use TDOTRI or c_tdotri, load the NCAR Graphics libraries ncarg, ncarg_gks, and ncarg_c, preferably in that order.
SEE ALSO
Online: tdclrs, tdctri, tddtri, tdgeti, tdgetr, tdgrds, tdgrid, tdgtrs, tdinit, tditri, tdlbla, tdlbls, tdline, tdlnpa, tdmtri, tdpack, tdpack_params, tdpara, tdplch, tdprpa, tdprpi, tdprpt, tdseti, tdsetr, tdsort, tdstri, tdstrs
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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