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.

UNIX                                                July 1997                                     TDOTRI(3NCARG)