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.