demo03 of Im2mesh package

demo03 - Export: save mesh as inp, bdf, and msh file; save geometry as dxf file, geo file or PSLG data.
Cite as: Ma, J., & Li, Y. (2025). Im2mesh: A MATLAB/Octave package for generating finite element mesh based on 2D multi-phase image (2.1.5). Zenodo. https://doi.org/10.5281/zenodo.14847059
Jiexian Ma, mjx0799@gmail.com. Project website.
Table of Contents
Note Setup Overview Part 1. Export mesh Generate mesh Prepare for mesh export Export as inp file (Abaqus) Linear element Quadratic element Export as bdf file (Nastran bulk data) Export as msh file (Gmsh mesh file) Other file formats Comments Part 2. Export geometry Obtain polygonal boundary Export as dxf file (CAD) Export as geo file (for Gmsh) Export as PSLG

Note

I suggest familiarizing yourself with Im2mesh_GUI before learning Im2mesh package. With graphical user interface, Im2mesh_GUI will help you better understand the workflow and parameters of Im2mesh package.
Im2mesh_GUI: https://www.mathworks.com/matlabcentral/fileexchange/179684-im2mesh_gui-2d-image-to-finite-element-mesh
If you are using Im2mesh package in MATLAB, you need to install MATLAB Image Processing Toolbox and Mapping Toolbox because Im2mesh package use a few functions in these toolboxes.

Setup

Before we start, please set folder "Im2mesh_Matlab" as your current folder of MATLAB.
clearvars
Set default image size.
x = 250; y = 250; width = 250; height = 250;
set(groot, 'DefaultFigurePosition', [x,y,width,height])
% To reset:
% set(groot, 'DefaultFigurePosition', 'factory')
Function im2mesh use a mesh generator called MESH2D (developed by Darren Engwirda). We can use the following command to add the folder 'mesh2d-master' to the path of MATLAB.
addpath(genpath('mesh2d-master'))

Overview

This demo has 2 parts.
We'll use function printInp2d, printBdf2d, and printMsh to export mesh. Check the link below to learn about the input arguments for these functions.
https://github.com/mjx888/writeMesh/blob/main/README.md

Part 1. Export mesh

Generate mesh

First, we'll use function im2mesh to generate mesh. In the next step, we'll export the mesh.
Import image Circle.tif.
im = imread('logo.tif');
if size(im,3) == 3; im = rgb2gray( im ); end
imshow( im,'InitialMagnification','fit' );
Generate mesh.
opt = []; % reset opt
opt.tolerance = 1;
[ vert, tria, tnum, vert2, tria2 ] = im2mesh( im, opt );
Refine triangulation... ------------------------------------------------------- |ITER.| |CDT1(X)| |CDT2(X)| ------------------------------------------------------- 9 125 108 10 125 218 17 158 433 Refine triangulation... ------------------------------------------------------- |ITER.| |CDT1(X)| |CDT2(X)| ------------------------------------------------------- 10 255 108 15 272 108 20 284 1509 29 299 1821 Smooth triangulation... ------------------------------------------------------- |ITER.| |MOVE(X)| |DTRI(X)| ------------------------------------------------------- 10 39 1809
plotMeshes( vert, tria, tnum )

Prepare for mesh export

Setup scale.
% physical dimensions of a pixel in the image
dx = 0.2;
Scale node coordinates according to dx.
% scale node coordinates of linear elements
vert = vert * dx;
% scale node coordinates of quadratic elements
vert2 = vert2 * dx;

Export as inp file (Abaqus)

We use function printInp2d to write 2d mesh to inp file.
The exported inp file would have a model with one part, which contains multiple sections. Each section corresponds to one phase in the mesh.

Linear element

Before exporting, we specify element type and precision.
precision is the number of digits to the right of the decimal point.
ele_type = 'CPS3';
precision = 8;
Write mesh to inp file.
file_name = 'test_linear.inp';
printInp2d( vert, tria, tnum, ele_type, precision, file_name );
printInp2d Done! Check the inp file!

Quadratic element

We can do the same operation to quadratic elements.
ele_type = 'CPS6';
precision = 8;
file_name = 'test_quadratic.inp';
printInp2d( vert2, tria2, tnum, ele_type, precision, file_name );
printInp2d Done! Check the inp file!

Export as bdf file (Nastran bulk data)

We use function printBdf2d to write 2d mesh to bdf file.
precision = 8;
file_name = 'test_2d.bdf';
printBdf2d( vert, tria, tnum, [], precision, file_name );
printBdf2d Done! Check the bdf file!

Export as msh file (Gmsh mesh file)

We use function printMsh to write 2d mesh to msh file.
precision = 8;
file_name = 'test_2d.msh';
printMsh( vert, tria, tnum, [], precision, file_name );
printMsh Done! Check the msh file!

Other file formats

For other formats, we can open the msh file in Gmsh and then export as other file formats.
Gmsh supports a lot of mesh file formats, such as STL, KEY, and VTK.

Comments

Im2mesh can also export mesh as .node/.ele file using function printTria.
Function printTria was written by me many years ago but I didn't test it.
% .node/.ele file
precision = 8;
printTria( vert, tria, tnum, precision );
clearvars

Part 2. Export geometry

We can save polygonal boundaries (obtained from an image) as dxf file, geo file, or PSLG data.

Obtain polygonal boundary

Import image Circle.tif.
im = imread('logo.tif');
if size(im,3) == 3; im = rgb2gray( im ); end
imshow( im,'InitialMagnification','fit' );
We can get boundries from image. We have demostrate how to do this in example3 of demo01.mlx
opt = []; % reset opt
opt.tolerance = 1;
opt.tf_mesh = false;
bounds = im2mesh( im, opt );
plotBounds( bounds )

Export as dxf file (CAD)

We use function printDxf to save boundries as dxf file.
path_to_dxf = 'test.dxf';
printDxf( bounds, path_to_dxf );
printDxf Done! Check the dxf file!
Multiple dxf files will be created depending on the number of phases in the 'bounds'.
For example, the length of 'bounds' is 3, meaning total 3 phases in 'bounds'. After running function printDxf, 4 dxf files will be created.
'test.dxf' is for all the phases, with different phases storing in different layers.
The other dxf files ('test_Layer_1.dxf', ...) are for each phase in 'bounds'.

Export as geo file (for Gmsh)

geo file is a text file that contains Gmsh’s own scripting language. It’s used to define geometry and mesh generation instructions for Gmsh.
% setup parameters
opt = []; % reset
opt.tf_includeMesh = false;
path_to_geo = 'test.geo';
bounds2geo( bounds, path_to_geo, opt );
bounds2geo Done! Check the geo file!

Export as PSLG

We can save polygonal boundaries as PSLG data.
PSLG is planar straight-line graphs. It's the input for mesh generators.
PSLG is represent by these 3 variables: node, edge, part
We can use function getPolyNodeEdge and regroup to obtain these 3 variables (node, edge, part).
[ poly_node, poly_edge ] = getPolyNodeEdge( bounds );
[ node, edge, part ] = regroup( poly_node, poly_edge );
We may save them them for future usage.
path_file_name = 'PSLG.mat';
save( path_file_name, 'node', 'edge', 'part' );
% reset image size
set(groot, 'DefaultFigurePosition', 'factory')
% end of demo