D06 Chapter Contents
D06 Chapter Introduction
NAG Library Manual

# NAG Library Routine DocumentD06ACF

Note:  before using this routine, please read the Users' Note for your implementation to check the interpretation of bold italicised terms and other implementation-dependent details.

## 1  Purpose

D06ACF generates a triangular mesh of a closed polygonal region in ${ℝ}^{2}$, 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  Parameters

1:     NVB – INTEGERInput
On entry: the number of vertices in the input boundary mesh.
Constraint: ${\mathbf{NVB}}\ge 3$.
2:     NVINT – INTEGERInput
On entry: the number of fixed interior mesh vertices to which a weight will be applied.
Constraint: ${\mathbf{NVINT}}\ge 0$.
3:     NVMAX – INTEGERInput
On entry: the maximum number of vertices in the mesh to be generated.
Constraint: ${\mathbf{NVMAX}}\ge {\mathbf{NVB}}+{\mathbf{NVINT}}$.
4:     NEDGE – INTEGERInput
On entry: the number of boundary edges in the input mesh.
Constraint: ${\mathbf{NEDGE}}\ge 1$.
5:     EDGE($3$,NEDGE) – 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 user-supplied 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:     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:     NELT – INTEGEROutput
On exit: the number of triangular elements in the mesh.
8:     COOR($2$,NVMAX) – 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:     CONN($3$,$2×{\mathbf{NVMAX}}+5$) – 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:   WEIGHT($*$) – 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:   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:   RWORK(LRWORK) – REAL (KIND=nag_wp) arrayWorkspace
13:   LRWORK – INTEGERInput
On entry: the dimension of the array RWORK as declared in the (sub)program from which D06ACF is called.
Constraint: ${\mathbf{LRWORK}}\ge 12×{\mathbf{NVMAX}}+30015$.
14:   IWORK(LIWORK) – INTEGER arrayWorkspace
15:   LIWORK – INTEGERInput
On entry: the dimension of the array IWORK as declared in the (sub)program from which D06ACF is called.
Constraint: ${\mathbf{LIWORK}}\ge 8×{\mathbf{NEDGE}}+53×{\mathbf{NVMAX}}+2×{\mathbf{NVB}}+10078$.
16:   IFAIL – INTEGERInput/Output
On entry: IFAIL must be set to $0$, $-1\text{​ or ​}1$. If you are unfamiliar with this parameter you should refer to Section 3.3 in the Essential Introduction 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 parameter, 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 ${\mathbf{IFAIL}}={\mathbf{0}}$ or $-{\mathbf{1}}$, 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 ${\mathbf{LRWORK}}<12×{\mathbf{NVMAX}}+30015$, or ${\mathbf{LIWORK}}<8×{\mathbf{NEDGE}}+53×{\mathbf{NVMAX}}+2×{\mathbf{NVB}}+10078$.
${\mathbf{IFAIL}}=2$
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.

## 7  Accuracy

Not applicable.

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.

## 9  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.

### 9.1  Program Text

Program Text (d06acfe.f90)

### 9.2  Program Data

Program Data (d06acfe.d)

### 9.3  Program Results

Program Results (d06acfe.r)

Figure 1: The boundary mesh (top), the interior mesh (bottom) of a
double wing inside a circle geometry