demo12 of Im2mesh package
demo12 - Demonstrate function pixelMesh (pixel-based quadrilateral mesh)
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')
pixelMesh
pixelMesh is a function used to generate pixel-based quadrilateral mesh. It’s written by me many years ago. It’s for some special applications.
Let's start demo. First, we'll use function pixelMesh 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.
[ vert, ele, tnum, vert2, ele2 ] = pixelMesh( im );
plotMeshes( vert, ele, tnum )
Zoom in.
plotMeshes( vert, ele, tnum )
xlim([9.9 28.5])
ylim([15.4 33.6])
Phase selection before meshing
The syntax of function pixelMesh is the same as function im2mesh. We can specify phase selection before meshing
opt.select_phase = [2 3 4];
[ vert, ele, tnum, vert2, ele2 ] = pixelMesh( im, opt );
plotMeshes( vert, ele, tnum )
Zoom in.
plotMeshes( vert, ele, tnum )
xlim([9.9 28.5])
ylim([15.4 33.6])
Outputs of pixelMesh
vert is mesh nodes (for linear element)
ele is mesh elements (for linear element)
tnum is label of phase
vert2 is mesh nodes (for quadratic element)
ele2 is mesh elements (for quadratic element)
Export mesh
We have demonstrated how to do this in demo03
Scale node coordinates
Setup scale.
% physical dimensions of a pixel in the image
dx = 1; dy = 1;
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;
Export mesh as bdf file (Nastran bulk data)
precision_nodecoor = 8;
file_name = 'test_2d.bdf';
printBdf2d( vert, ele, tnum, [], precision_nodecoor, file_name );
Export mesh as inp file (Abaqus)
Linear element
ele_type = 'CPS4';
precision_nodecoor = 8;
file_name = 'test_linear.inp';
printInp2d( vert, ele, tnum, ele_type, precision_nodecoor, file_name );
Quadratic element
ele_type = 'CPS8';
precision_nodecoor = 8;
file_name = 'test_quadratic.inp';
printInp2d( vert2, ele2, tnum, ele_type, precision_nodecoor, file_name );
Comments
You will find that when exporting mesh, the functions for triangular and quadrilateral mesh is the same. That's very convenient.
% reset image size
set(groot, 'DefaultFigurePosition', 'factory')
% end of demo