demo10 of Im2mesh package

demo10 - 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

Jiexian Ma, mjx0799@gmail.com Project website List of demo

Table of Contents

pixelMesh Initialize Generate mesh Outputs of pixelMesh Export mesh Scale node coordinates Export mesh as bdf file (Nastran bulk data) Export mesh as inp file (Abaqus) Linear element Quadratic element Phase selection before meshing

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. No need to install any MATLAB toolboxes when running this demo.

Initialize

Before we start, please set folder "Im2mesh_Matlab" as your current folder of MATLAB.

clearvars

Set default image size (optional).

x = 250; y = 250; width = 250; height = 250;
set(groot, 'DefaultFigurePosition', [x,y,width,height])
% To reset:
% set(groot, 'DefaultFigurePosition', 'factory')

Generate mesh

First, we'll use function pixelMesh to generate mesh. In the next step, we'll export the mesh.

Import image.

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])

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;

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 mesh as bdf file (Nastran bulk data)

precision = 8;
file_name = 'test_2d.bdf';
printBdf2d( vert, ele, tnum, [], precision, file_name );
printBdf2d Done! Check the bdf file!

Export mesh as inp file (Abaqus)

Please refer to the following link to learn more about exporting mesh: https://github.com/mjx888/writeMesh/blob/main/README.md

Linear element

ele_type = 'CPS4';
precision = 8;
file_name = 'test_linear.inp';
printInp2d( vert, ele, tnum, ele_type, precision, file_name );
printInp2d Done! Check the inp file!

Quadratic element

ele_type = 'CPS8';
precision = 8;
file_name = 'test_quadratic.inp';
printInp2d( vert2, ele2, tnum, ele_type, precision, file_name );
printInp2d Done! Check the inp file!

Phase selection before meshing

We can specify phase selection before meshing. See demo08 for example.

opt = []; % reset

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])

% reset image size
set(groot, 'DefaultFigurePosition', 'factory')
% end of demo