NAG Library Function Document

nag_mesh2d_inc (d06aac)


    1  Purpose
    7  Accuracy


nag_mesh2d_inc (d06aac) generates a triangular mesh of a closed polygonal region in 2, given a mesh of its boundary. It uses a simple incremental method.


#include <nag.h>
#include <nagd06.h>
void  nag_mesh2d_inc (Integer nvb, Integer nvmax, Integer nedge, const Integer edge[], Integer *nv, Integer *nelt, double coor[], Integer conn[], const double bspace[], Nag_Boolean smooth, double coef, double power, Integer itrace, const char *outfile, NagError *fail)


nag_mesh2d_inc (d06aac) generates the set of interior vertices using a process based on a simple incremental method. A smoothing of the mesh is optionally available. 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).


George P L and Borouchaki H (1998) Delaunay Triangulation and Meshing: Application to Finite Elements Editions HERMES, Paris


1:     nvb IntegerInput
On entry: the number of vertices in the input boundary mesh.
Constraint: 3nvbnvmax.
2:     nvmax IntegerInput
On entry: the maximum number of vertices in the mesh to be generated.
3:     nedge IntegerInput
On entry: the number of boundary edges in the input mesh.
Constraint: nedge1.
4:     edge[3×nedge] const IntegerInput
Note: the i,jth element of the matrix is stored in edge[j-1×3+i-1].
On entry: the specification of the boundary edges. edge[j-1×3] and edge[j-1×3+1] contain the vertex numbers of the two end points of the jth boundary edge. edge[j-1×3+2] is a user-supplied tag for the jth boundary edge and is not used by nag_mesh2d_inc (d06aac). Note that the edge vertices are numbered from 1 to nvb.
Constraint: 1edge[j-1×3+i-1]nvb and edge[j-1×3]edge[j-1×3+1], for i=1,2 and j=1,2,,nedge.
5:     nv Integer *Output
On exit: the total number of vertices in the output mesh (including both boundary and interior vertices). If nvb=nvmax, no interior vertices will be generated and nv=nvb.
6:     nelt Integer *Output
On exit: the number of triangular elements in the mesh.
7:     coor[2×nvmax] doubleInput/Output
Note: the i,jth element of the matrix is stored in coor[j-1×2+i-1].
On entry: coor[i-1×2] contains the x coordinate of the ith input boundary mesh vertex; while coor[i-1×2+1] contains the corresponding y coordinate, for i=1,2,,nvb.
On exit: coor[i-1×2] will contain the x coordinate of the i-nvbth generated interior mesh vertex; while coor[i-1×2+1] will contain the corresponding y coordinate, for i=nvb+1,,nv. The remaining elements are unchanged.
8:     conn[3×2×nvmax-1] IntegerOutput
Note: the i,jth element of the matrix is stored in conn[j-1×3+i-1].
On exit: the connectivity of the mesh between triangles and vertices. For each triangle j, conn[j-1×3+i-1] gives the indices of its three vertices (in anticlockwise order), for i=1,2,3 and j=1,2,,nelt. Note that the mesh vertices are numbered from 1 to nv.
9:     bspace[nvb] const doubleInput
On entry: the desired mesh spacing (triangle diameter, which is the length of the longer edge of the triangle) near the boundary vertices.
Constraint: bspace[i-1]>0.0, for i=1,2,,nvb.
10:   smooth Nag_BooleanInput
On entry: indicates whether or not mesh smoothing should be performed.
If smooth=Nag_TRUE, the smoothing is performed; otherwise no smoothing is performed.
11:   coef doubleInput
On entry: the coefficient in the stopping criteria for the generation of interior vertices. This argument controls the triangle density and the number of triangles generated is in Ocoef2. The mesh will be finer if coef is greater than 0.7165 and 0.75 is a good value.
Suggested value: 0.75.
12:   power doubleInput
On entry: controls the rate of change of the mesh size during the generation of interior vertices. The smaller the value of power, the faster the decrease in element size away from the boundary.
Suggested value: 0.25.
Constraint: 0.1power10.0.
13:   itrace IntegerInput
On entry: the level of trace information required from nag_mesh2d_inc (d06aac).
No output is generated.
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 itrace=0, unless you are experienced with finite element mesh generation.
14:   outfile const 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.
15:   fail NagError *Input/Output
The NAG error argument (see Section 3.7 in How to Use the NAG Library and its Documentation).

Error Indicators and Warnings

Dynamic memory allocation failed.
See Section in How to Use the NAG Library and its Documentation for further information.
On entry, argument value had an illegal value.
On entry, nedge=value.
Constraint: nedge1.
On entry, nvb=value and nvmax=value.
Constraint: 3nvbnvmax.
On entry, the end points of the edge J have the same index I: J=value and I=value.
On entry, edgeI,J=value, I=value, J=value and nvb=value.
Constraint: edgeI,J1 and edgeI,Jnvb, where edgeI,J denotes edge[J-1×3+I-1].
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.
See Section 2.7.6 in How to Use the NAG Library and its Documentation for further information.
An error has occurred during the generation of the interior mesh. Check the inputs of the boundary.
Your licence key may have expired or may not have been installed correctly.
See Section 2.7.5 in How to Use the NAG Library and its Documentation for further information.
Cannot close file value.
Cannot open file value for writing.
On entry, power=value.
Constraint: power10.0.
On entry, power=value.
Constraint: power0.1.
On entry, bspace[I-1]=value and I=value.
Constraint: bspace[I-1]>0.0.


Not applicable.

Parallelism and Performance

nag_mesh2d_inc (d06aac) is not threaded in any implementation.

Further Comments

The position of the internal vertices is a function of the positions of the vertices on the given boundary. A fine mesh on the boundary results in a fine mesh in the interior. The algorithm allows you to obtain a denser interior mesh by varying nvmax, bspace, coef and power. But you are advised to manipulate the last two arguments 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.


In this example, a geometry with two holes (two interior circles inside an exterior one) is meshed using the simple incremental method (see the d06 Chapter Introduction). The exterior circle is centred at the origin with a radius 1.0, the first interior circle is centred at the point -0.5,0.0 with a radius 0.49, and the second one is centred at the point -0.5,0.65 with a radius 0.15. Note that the points -1.0,0.0 and -0.5,0.5) are points of ‘near tangency’ between the exterior circle and the first and second circles.
The boundary mesh has 100 vertices and 100 edges (see Figure 1 in Section 10.3). Note that the particular mesh generated could be sensitive to the machine precision and therefore may differ from one implementation to another. Figure 2 in Section 10.3 contains the output mesh.

Program Text

Program Text (d06aace.c)

Program Data

Program Data (d06aace.d)

Program Results

Program Results (d06aace.r)

GnuplotProduced by GNUPLOT 4.6 patchlevel 3 Example Program Figure 1: The Geometry of Circular Region With Two Holes gnuplot_plot_1
GnuplotProduced by GNUPLOT 4.6 patchlevel 3 Figure 2: Mesh Generated on the Geometry With Two Holes gnuplot_plot_1
© The Numerical Algorithms Group Ltd, Oxford, UK. 2017