demo03 of Im2mesh package
demo03 - How to export mesh as inp and bdf file
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
Table of Contents
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'))
Circle
Let's start demo. First, we'll use function im2mesh to generate mesh. In the next step, we'll export the mesh.
Import image Circle.tif.
im = imread('Circle.tif');
if size(im,3) == 3; im = rgb2gray( im ); end
imshow( im,'InitialMagnification','fit' );
Generate mesh.
opt = []; % reset opt
opt.tf_avoid_sharp_corner = true;
opt.threshold_num_turning = 10;
opt.threshold_num_vert_Smo = 20;
opt.threshold_num_vert_Sim = 20;
[ vert, tria, tnum, vert2, tria2 ] = im2mesh( im, opt );
plotMeshes( vert, tria, tnum )
Prepare for mesh export
Setup parameters.
% parameters for mesh export
dx = 1; dy = 1; % scale of your imgage
% dx - column direction, dy - row direction
% e.g. scale of your imgage is 0.11 mm/pixel, try
% dx = 0.11; and dy = 0.11;
precision_nodecoor = 8; % precision of node coordinates, for printInp
% e.g. precision_nodecoor=4, dx=1 -> 0.5000
% precision_nodecoor=3, dx=0.111, -> 0.055
Scale node coordinates according to dx and dy.
% scale node coordinates of linear elements
vert( :, 1 ) = vert( :, 1 ) * dx;
vert( :, 2 ) = vert( :, 2 ) * dy;
% scale node coordinates of quadratic elements
vert2( :, 1 ) = vert2( :, 1 ) * dx;
vert2( :, 2 ) = vert2( :, 2 ) * dy;
Reorgnize nodes and elements
We use function getNodeEle to reorganize node coordinates and elements from the mesh, and add node numbering and element numbering.
% for linear element
[ nodecoor_list, nodecoor_cell, ele_cell ] = getNodeEle( vert, tria, tnum );
[ nodecoor_list, nodecoor_cell, ele_cell ] is an alternative way to represent the triangular mesh. The format is convenient for importing the mesh into commercial finite element software. The meaning of these three variable is listed as follows. We will write these three variables nodecoor_list, nodecoor_cell, ele_cell to inp file and bdf file later.
nodecoor_list is node coordinates, N-by-3 array.
nodecoor_list(i,1:3) = [node_numbering, x_coordinate, y_coordinate];
nodecoor_cell is a 1-by-P cell array. nodecoor_cell{i} represents the node numbering and node coordinates in the i-th phase.
nodecoor_cell{i}(j,1:3) = [node_numbering, x_coordinate, y_coordinate];
ele_cell is a 1-by-P cell array. ele_cell{i} represent elements in the i-th phase.
% When the elements are linear
ele_cell{i}(j,1:4) = [ element_numbering, node_numbering_of_node_1, ...
node_numbering_of_node_2, ...
node_numbering_of_node_3
];
We can do the same operation to quadratic elements using the same function.
% for quadratic element
[ nodecoor_list_Q, nodecoor_cell_Q, ele_cell_Q ] = getNodeEle( vert2, tria2, tnum );
Export mesh as bdf file (Nastran bulk data)
We use function printBdf to do that. The bdf file 'test.bdf' will be saved to current folder.
printBdf( nodecoor_list, ele_cell, precision_nodecoor );
We can explictly specify the file name.
file_name = 'test_2025.bdf';
printBdf( nodecoor_list, ele_cell, precision_nodecoor, file_name );
Export mesh as inp file (Abaqus)
Here, we use function printInp_multiSect to save multiple phases in the mesh as multiple sections.
The exported inp file would have a model with one part, which contains multiple sections. Each section corresponds to one phase in your image.
Linear element
Before exporting mesh, we specify element type.
ele_type = 'CPS3'; % element type, for printInp
We use function printInp_multiSect to save multiple phases in the mesh as multiple sections. The inp file will be saved to current folder.
% print as multiple sections
file_name = 'test_linear.inp';
printInp_multiSect( nodecoor_list, ele_cell, ele_type, precision_nodecoor, file_name );
Quadratic element
We can do the same operation to quadratic elements.
ele_type_Q = 'CPS6'; % element type, for printInp
To avoid confusion, you can explictly specify the file name.
% print as multiple sections
file_name = 'test_quadratic.inp';
printInp_multiSect( nodecoor_list_Q, ele_cell_Q, ele_type_Q, precision_nodecoor, file_name );
Other comments
Multiple phases in the mesh can also be saved as multiple parts. We use function printInp_multiPart.
The exported inp file would has a model with multiple parts, where each part corresponds to one phase in the image.
Caution: function printInp_multiPart (written by me in 2018) maybe outdated. It may has bug when doing simulation in Abaqus.
% print as multiple parts
% Linear element
printInp_multiPart( nodecoor_cell, ele_cell, ele_type, precision_nodecoor );
% Quadratic element
file_name = 'test_multi_parts_Q.inp';
printInp_multiPart( nodecoor_cell_Q, ele_cell_Q, ele_type_Q, precision_nodecoor, file_name );
Im2mesh can also export mesh as .node/.ele file, but I didn't test the following function.
% .node/.ele file
% haven't been tested
printTria( vert, tria, tnum, precision_nodecoor );
% reset image size
set(groot, 'DefaultFigurePosition', 'factory')
% end of demo