NAG Library Routine Document
D06ACF
1 Purpose
D06ACF generates a triangular mesh of a closed polygonal region in ${\mathbb{R}}^{2}$, given a mesh of its boundary. It uses an Advancing Front process, based on an incremental method.
2 Specification
SUBROUTINE D06ACF ( 
NVB, NVINT, NVMAX, NEDGE, EDGE, NV, NELT, COOR, CONN, WEIGHT, ITRACE, RWORK, LRWORK, IWORK, LIWORK, IFAIL) 
INTEGER 
NVB, NVINT, NVMAX, NEDGE, EDGE(3,NEDGE), NV, NELT, CONN(3,2*NVMAX+5), ITRACE, LRWORK, IWORK(LIWORK), LIWORK, IFAIL 
REAL (KIND=nag_wp) 
COOR(2,NVMAX), WEIGHT(*), RWORK(LRWORK) 

3 Description
D06ACF generates the set of interior vertices using an Advancing Front process, based on an incremental method. It allows you to specify a number of fixed interior mesh vertices together with weights which allow concentration of the mesh in their neighbourhood. For more details about the triangulation method, consult the
D06 Chapter Introduction as well as
George and Borouchaki (1998).
This routine is derived from material in the MODULEF package from INRIA (Institut National de Recherche en Informatique et Automatique).
4 References
George P L and Borouchaki H (1998) Delaunay Triangulation and Meshing: Application to Finite Elements Editions HERMES, Paris
5 Arguments
 1: $\mathrm{NVB}$ – INTEGERInput

On entry: the number of vertices in the input boundary mesh.
Constraint:
${\mathbf{NVB}}\ge 3$.
 2: $\mathrm{NVINT}$ – INTEGERInput

On entry: the number of fixed interior mesh vertices to which a weight will be applied.
Constraint:
${\mathbf{NVINT}}\ge 0$.
 3: $\mathrm{NVMAX}$ – INTEGERInput

On entry: the maximum number of vertices in the mesh to be generated.
Constraint:
${\mathbf{NVMAX}}\ge {\mathbf{NVB}}+{\mathbf{NVINT}}$.
 4: $\mathrm{NEDGE}$ – INTEGERInput

On entry: the number of boundary edges in the input mesh.
Constraint:
${\mathbf{NEDGE}}\ge 1$.
 5: $\mathrm{EDGE}\left(3,{\mathbf{NEDGE}}\right)$ – INTEGER arrayInput

On entry: the specification of the boundary edges. ${\mathbf{EDGE}}\left(1,j\right)$ and ${\mathbf{EDGE}}\left(2,j\right)$ contain the vertex numbers of the two end points of the $j$th boundary edge. ${\mathbf{EDGE}}\left(3,j\right)$ is a usersupplied tag for the $j$th boundary edge and is not used by D06ACF.
Constraint:
$1\le {\mathbf{EDGE}}\left(\mathit{i},\mathit{j}\right)\le {\mathbf{NVB}}$ and ${\mathbf{EDGE}}\left(1,\mathit{j}\right)\ne {\mathbf{EDGE}}\left(2,\mathit{j}\right)$, for $\mathit{i}=1,2$ and $\mathit{j}=1,2,\dots ,{\mathbf{NEDGE}}$.
 6: $\mathrm{NV}$ – INTEGEROutput

On exit: the total number of vertices in the output mesh (including both boundary and interior vertices). If ${\mathbf{NVB}}+{\mathbf{NVINT}}={\mathbf{NVMAX}}$, no interior vertices will be generated and ${\mathbf{NV}}={\mathbf{NVMAX}}$.
 7: $\mathrm{NELT}$ – INTEGEROutput

On exit: the number of triangular elements in the mesh.
 8: $\mathrm{COOR}\left(2,{\mathbf{NVMAX}}\right)$ – REAL (KIND=nag_wp) arrayInput/Output

On entry: ${\mathbf{COOR}}\left(1,\mathit{i}\right)$ contains the $x$ coordinate of the $\mathit{i}$th input boundary mesh vertex, for $\mathit{i}=1,2,\dots ,{\mathbf{NVB}}$.
${\mathbf{COOR}}\left(1,\mathit{i}\right)$ contains the $x$ coordinate of the $\left(\mathit{i}{\mathbf{NVB}}\right)$th fixed interior vertex, for $\mathit{i}={\mathbf{NVB}}+1,\dots ,{\mathbf{NVB}}+{\mathbf{NVINT}}$. For boundary and interior vertices,
${\mathbf{COOR}}\left(2,\mathit{i}\right)$ contains the corresponding $y$ coordinate, for $\mathit{i}=1,2,\dots ,{\mathbf{NVB}}+{\mathbf{NVINT}}$.
On exit: ${\mathbf{COOR}}\left(1,\mathit{i}\right)$ will contain the $x$ coordinate of the $\left(\mathit{i}{\mathbf{NVB}}{\mathbf{NVINT}}\right)$th generated interior mesh vertex, for $\mathit{i}={\mathbf{NVB}}+{\mathbf{NVINT}}+1,\dots ,{\mathbf{NV}}$; while ${\mathbf{COOR}}\left(2,i\right)$ will contain the corresponding $y$ coordinate. The remaining elements are unchanged.
 9: $\mathrm{CONN}\left(3,2\times {\mathbf{NVMAX}}+5\right)$ – INTEGER arrayOutput

On exit: the connectivity of the mesh between triangles and vertices. For each triangle
$\mathit{j}$, ${\mathbf{CONN}}\left(\mathit{i},\mathit{j}\right)$ gives the indices of its three vertices (in anticlockwise order), for $\mathit{i}=1,2,3$ and $\mathit{j}=1,2,\dots ,{\mathbf{NELT}}$.
 10: $\mathrm{WEIGHT}\left(*\right)$ – REAL (KIND=nag_wp) arrayInput

Note: the dimension of the array
WEIGHT
must be at least
$\mathrm{max}\phantom{\rule{0.125em}{0ex}}\left(1,{\mathbf{NVINT}}\right)$.
On entry: the weight of fixed interior vertices. It is the diameter of triangles (length of the longer edge) created around each of the given interior vertices.
Constraint:
if ${\mathbf{NVINT}}>0$, ${\mathbf{WEIGHT}}\left(\mathit{i}\right)>0.0$, for $\mathit{i}=1,2,\dots ,{\mathbf{NVINT}}$.
 11: $\mathrm{ITRACE}$ – INTEGERInput

On entry: the level of trace information required from D06ACF.
 ${\mathbf{ITRACE}}\le 0$
 No output is generated.
 ${\mathbf{ITRACE}}\ge 1$
 Output from the meshing solver is printed on the current advisory message unit (see X04ABF). This output contains details of the vertices and triangles generated by the process.
You are advised to set ${\mathbf{ITRACE}}=0$, unless you are experienced with finite element mesh generation.
 12: $\mathrm{RWORK}\left({\mathbf{LRWORK}}\right)$ – REAL (KIND=nag_wp) arrayWorkspace
 13: $\mathrm{LRWORK}$ – INTEGERInput

On entry: the dimension of the array
RWORK as declared in the (sub)program from which D06ACF is called.
Constraint:
${\mathbf{LRWORK}}\ge 12\times {\mathbf{NVMAX}}+30015$.
 14: $\mathrm{IWORK}\left({\mathbf{LIWORK}}\right)$ – INTEGER arrayWorkspace
 15: $\mathrm{LIWORK}$ – INTEGERInput

On entry: the dimension of the array
IWORK as declared in the (sub)program from which D06ACF is called.
Constraint:
${\mathbf{LIWORK}}\ge 8\times {\mathbf{NEDGE}}+53\times {\mathbf{NVMAX}}+2\times {\mathbf{NVB}}+10078$.
 16: $\mathrm{IFAIL}$ – INTEGERInput/Output

On entry:
IFAIL must be set to
$0$,
$1\text{ or}1$. If you are unfamiliar with this argument you should refer to
Section 3.4 in How to Use the NAG Library and its Documentation for details.
For environments where it might be inappropriate to halt program execution when an error is detected, the value
$1\text{ or}1$ is recommended. If the output of error messages is undesirable, then the value
$1$ is recommended. Otherwise, if you are not familiar with this argument, the recommended value is
$0$.
When the value $\mathbf{1}\text{ or}\mathbf{1}$ is used it is essential to test the value of IFAIL on exit.
On exit:
${\mathbf{IFAIL}}={\mathbf{0}}$ unless the routine detects an error or a warning has been flagged (see
Section 6).
6 Error Indicators and Warnings
If on entry
${\mathbf{IFAIL}}=0$ or
$1$, explanatory error messages are output on the current error message unit (as defined by
X04AAF).
Errors or warnings detected by the routine:
 ${\mathbf{IFAIL}}=1$

On entry,  ${\mathbf{NVB}}<3$, 
or  ${\mathbf{NVINT}}<0$, 
or  ${\mathbf{NVB}}+{\mathbf{NVINT}}>{\mathbf{NVMAX}}$, 
or  ${\mathbf{NEDGE}}<1$, 
or  ${\mathbf{EDGE}}\left(i,j\right)<1$ or ${\mathbf{EDGE}}\left(i,j\right)>{\mathbf{NVB}}$, for some $i=1,2$ and $j=1,2,\dots ,{\mathbf{NEDGE}}$, 
or  ${\mathbf{EDGE}}\left(1,j\right)={\mathbf{EDGE}}\left(2,j\right)$, for some $j=1,2,\dots ,{\mathbf{NEDGE}}$, 
or  if ${\mathbf{NVINT}}>0$, ${\mathbf{WEIGHT}}\left(i\right)\le 0.0$, for some $i=1,2,\dots ,{\mathbf{NVINT}}$; 
or  ${\mathbf{LRWORK}}<12\times {\mathbf{NVMAX}}+30015$, 
or  ${\mathbf{LIWORK}}<8\times {\mathbf{NEDGE}}+53\times {\mathbf{NVMAX}}+2\times {\mathbf{NVB}}+10078$. 
 ${\mathbf{IFAIL}}=2$

An error has occurred during the generation of the interior mesh. Check the definition of the boundary (arguments
COOR and
EDGE) as well as the orientation of the boundary (especially in the case of a multiple connected component boundary). Setting
${\mathbf{ITRACE}}>0$ may provide more details.
 ${\mathbf{IFAIL}}=99$
An unexpected error has been triggered by this routine. Please
contact
NAG.
See
Section 3.9 in How to Use the NAG Library and its Documentation for further information.
 ${\mathbf{IFAIL}}=399$
Your licence key may have expired or may not have been installed correctly.
See
Section 3.8 in How to Use the NAG Library and its Documentation for further information.
 ${\mathbf{IFAIL}}=999$
Dynamic memory allocation failed.
See
Section 3.7 in How to Use the NAG Library and its Documentation for further information.
7 Accuracy
Not applicable.
8 Parallelism and Performance
D06ACF makes calls to BLAS and/or LAPACK routines, which may be threaded within the vendor library used by this implementation. Consult the documentation for the vendor library for further information.
Please consult the
X06 Chapter Introduction for information on how to control and interrogate the OpenMP environment used within this routine. Please also consult the
Users' Note for your implementation for any additional implementationspecific information.
The position of the internal vertices is a function position of the vertices on the given boundary. A fine mesh on the boundary results in a fine mesh in the interior. During the process vertices are generated on edges of the mesh
${\mathcal{T}}_{i}$ to obtain the mesh
${\mathcal{T}}_{i+1}$ in the general incremental method (consult the
D06 Chapter Introduction or
George and Borouchaki (1998)).
You are advised to take care to set the boundary inputs properly, especially for a boundary with multiply connected components. The orientation of the interior boundaries should be in clockwise order and opposite to that of the exterior boundary. If the boundary has only one connected component, its orientation should be anticlockwise.
10 Example
In this example, a geometry with two holes (two wings inside an exterior circle) is meshed using a Delaunay–Voronoi method. The exterior circle is centred at the point $\left(1.5,0.0\right)$ with a radius $4.5$, the first wing begins at the origin and it is normalized, finally the last wing is also normalized and begins at the point $\left(0.8,0.3\right)$. To be able to carry out some realistic computation on that geometry, some interior points have been introduced to have a finer mesh in the wake of those airfoils.
The boundary mesh has
$120$ vertices and
$120$ edges (see
Figure 1 top). Note that the particular mesh generated could be sensitive to the
machine precision and therefore may differ from one implementation to another.
10.1 Program Text
Program Text (d06acfe.f90)
10.2 Program Data
Program Data (d06acfe.d)
10.3 Program Results
Program Results (d06acfe.r)
Figure 1: The boundary mesh (top), the interior mesh (bottom) of a
double wing inside a circle geometry