d06 Chapter Contents
d06 Chapter Introduction
NAG C Library Manual

# NAG Library Function Documentnag_mesh2d_delaunay (d06abc)

## 1  Purpose

nag_mesh2d_delaunay (d06abc) generates a triangular mesh of a closed polygonal region in ${ℝ}^{2}$, given a mesh of its boundary. It uses a Delaunay–Voronoi process, based on an incremental method.

## 2  Specification

 #include #include
 void nag_mesh2d_delaunay (Integer nvb, Integer nvint, Integer nvmax, Integer nedge, const Integer edge[], Integer *nv, Integer *nelt, double coor[], Integer conn[], const double weight[], Integer npropa, Integer itrace, const char *outfile, NagError *fail)

## 3  Description

nag_mesh2d_delaunay (d06abc) generates the set of interior vertices using a Delaunay–Voronoi 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 function 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:     nvbIntegerInput
On entry: the number of vertices in the input boundary mesh.
Constraint: ${\mathbf{nvb}}\ge 3$.
2:     nvintIntegerInput
On entry: the number of fixed interior mesh vertices to which a weight will be applied.
Constraint: ${\mathbf{nvint}}\ge 0$.
3:     nvmaxIntegerInput
On entry: the maximum number of vertices in the mesh to be generated.
Constraint: ${\mathbf{nvmax}}\ge {\mathbf{nvb}}+{\mathbf{nvint}}$.
4:     nedgeIntegerInput
On entry: the number of boundary edges in the input mesh.
Constraint: ${\mathbf{nedge}}\ge 1$.
5:     edge[$3×{\mathbf{nedge}}$]const IntegerInput
On entry: the specification of the boundary edges. ${\mathbf{edge}}\left[\left(j-1\right)×3+0\right]$ and ${\mathbf{edge}}\left[\left(j-1\right)×3+1\right]$ contain the vertex numbers of the two end points of the $j$th boundary edge. ${\mathbf{edge}}\left[\left(j-1\right)×3+2\right]$ is a user-supplied tag for the $j$th boundary edge and is not used by nag_mesh2d_delaunay (d06abc). Note that the edge vertices are numbered from $1$ to nvb.
Constraint: $1\le {\mathbf{edge}}\left[\left(\mathit{j}-1\right)×3+\mathit{i}-1\right]\le {\mathbf{nvb}}$ and ${\mathbf{edge}}\left[\left(\mathit{j}-1\right)×3+0\right]\ne {\mathbf{edge}}\left[\left(\mathit{j}-1\right)×3+1\right]$, for $\mathit{i}=1,2$ and $\mathit{j}=1,2,\dots ,{\mathbf{nedge}}$.
6:     nvInteger *Output
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:     neltInteger *Output
On exit: the number of triangular elements in the mesh.
8:     coor[$2×{\mathbf{nvmax}}$]doubleInput/Output
On entry: ${\mathbf{coor}}\left[\left(\mathit{i}-1\right)×2+0\right]$ contains the $x$ coordinate of the $\mathit{i}$th input boundary mesh vertex, for $\mathit{i}=1,2,\dots ,{\mathbf{nvb}}$. ${\mathbf{coor}}\left[\left(\mathit{i}-1\right)×2+0\right]$ 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, ${\mathbf{coor}}\left[\left(\mathit{i}-1\right)×2+1\right]$ contains the corresponding $y$ coordinate, for $\mathit{i}=1,2,\dots ,{\mathbf{nvb}}+{\mathbf{nvint}}$.
On exit: ${\mathbf{coor}}\left[\left(\mathit{i}-1\right)×2+0\right]$ 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 ${\mathbf{coor}}\left[\left(i-1\right)×2+1\right]$ will contain the corresponding $y$ coordinate. The remaining elements are unchanged.
9:     conn[$3×\left(2×{\mathbf{nvmax}}+5\right)$]IntegerOutput
On exit: the connectivity of the mesh between triangles and vertices. For each triangle $\mathit{j}$, ${\mathbf{conn}}\left[\left(\mathit{j}-1\right)×3+\mathit{i}-1\right]$ gives the indices of its three vertices (in anticlockwise order), for $\mathit{i}=1,2,3$ and $\mathit{j}=1,2,\dots ,{\mathbf{nelt}}$. Note that the mesh vertices are numbered from $1$ to nv.
10:   weight[$\mathit{dim}$]const doubleInput
Note: the dimension, dim, 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 ${\mathbf{nvint}}>0$, ${\mathbf{weight}}\left[\mathit{i}-1\right]>0.0$, for $\mathit{i}=1,2,\dots ,{\mathbf{nvint}}$.
11:   npropaIntegerInput
On entry: the propagation type and coefficient, the argument npropa is used when the internal points are created. They are distributed in a geometric manner if npropa is positive and in an arithmetic manner if it is negative. For more details see Section 8.
Constraint: ${\mathbf{npropa}}\ne 0$.
12:   itraceIntegerInput
On entry: the level of trace information required from nag_mesh2d_delaunay (d06abc).
${\mathbf{itrace}}\le 0$
No output is generated.
${\mathbf{itrace}}\ge 1$
Output from the meshing solver is printed. 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.
13:   outfileconst char *Input
On entry: the name of a file to which diagnostic output will be directed. If outfile is NULL the diagnostic output will be directed to standard output.
14:   failNagError *Input/Output
The NAG error argument (see Section 3.6 in the Essential Introduction).

## 6  Error Indicators and Warnings

NE_ALLOC_FAIL
Dynamic memory allocation failed.
On entry, argument $〈\mathit{\text{value}}〉$ had an illegal value.
NE_INT
On entry, ${\mathbf{nedge}}=〈\mathit{\text{value}}〉$.
Constraint: ${\mathbf{nedge}}\ge 1$.
On entry, ${\mathbf{npropa}}=0$.
On entry, ${\mathbf{nvb}}=〈\mathit{\text{value}}〉$.
Constraint: ${\mathbf{nvb}}\ge 3$.
On entry, ${\mathbf{nvint}}=〈\mathit{\text{value}}〉$.
Constraint: ${\mathbf{nvint}}\ge 0$.
NE_INT_2
On entry, the endpoints of the edge $j$ have the same index $i$: $j=〈\mathit{\text{value}}〉$ and $i=〈\mathit{\text{value}}〉$.
NE_INT_3
On entry, ${\mathbf{nvb}}=〈\mathit{\text{value}}〉$, ${\mathbf{nvint}}=〈\mathit{\text{value}}〉$ and ${\mathbf{nvmax}}=〈\mathit{\text{value}}〉$.
Constraint: ${\mathbf{nvmax}}\ge {\mathbf{nvb}}+{\mathbf{nvint}}$.
NE_INT_4
On entry, ${\mathbf{EDGE}}\left(i,j\right)=〈\mathit{\text{value}}〉$, $i=〈\mathit{\text{value}}〉$, $j=〈\mathit{\text{value}}〉$ and ${\mathbf{nvb}}=〈\mathit{\text{value}}〉$.
Constraint: ${\mathbf{EDGE}}\left(i,j\right)\ge 1$ and ${\mathbf{EDGE}}\left(i,j\right)\le {\mathbf{nvb}}$, where ${\mathbf{EDGE}}\left(i,j\right)$ denotes ${\mathbf{edge}}\left[\left(j-1\right)×3+i-1\right]$.
NE_INTERNAL_ERROR
An internal error has occurred in this function. Check the function call and any array sizes. If the call is correct then please contact NAG for assistance.
NE_MESH_ERROR
An error has occurred during the generation of the boundary mesh. It appears that nvmax is not large enough: ${\mathbf{nvmax}}=〈\mathit{\text{value}}〉$.
An error has occurred during the generation of the interior mesh. Check the inputs of the boundary.
NE_NOT_CLOSE_FILE
Cannot close file $〈\mathit{\text{value}}〉$.
NE_NOT_WRITE_FILE
Cannot open file $〈\mathit{\text{value}}〉$ for writing.
NE_REAL_ARRAY_INPUT
On entry, ${\mathbf{weight}}\left[i-1\right]=〈\mathit{\text{value}}〉$ and $i=〈\mathit{\text{value}}〉$.
Constraint: ${\mathbf{weight}}\left[i-1\right]>0.0$.

## 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. To dilute the influence of the data on the interior of the domain, the value of npropa can be changed. The propagation coefficient is calculated as: $\omega =1+\frac{a-1.0}{20.0}$, where $a$ is the absolute value of npropa. 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)). This generation uses the coefficient $\omega$, and it is geometric if ${\mathbf{npropa}}>0$, and arithmetic otherwise. But increasing the value of $a$ may lead to failure of the process, due to precision, especially in geometries with holes. So you are advised to manipulate the argument npropa with care.
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 $\left(1.0,0.0\right)$ with a radius $3$, the first RAE wing begins at the origin and it is normalized, and the last wing is a result from the first one after a translation, a scale reduction and a rotation. 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 $296$ vertices and $296$ 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. The interior meshes for different values of npropa are given in Figure 1.

### 9.1  Program Text

Program Text (d06abce.c)

### 9.2  Program Data

Program Data (d06abce.d)

### 9.3  Program Results

Program Results (d06abce.r)

Figure 1: The boundary mesh (top), the interior mesh with ${\mathbf{npropa}}=-5$ (middle left), $-1$ (middle right),
$1$ (bottom left) and $5$ (bottom right) of a double RAE wings inside a circle geometry