demo08 of Im2mesh package

demo08 - Select phases for meshing
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
Note Initialize Import image & generate mesh Select phases for meshing

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. 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')
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'))

Import image & generate mesh

im = imread('Phases.tif');
if size(im,3) == 3; im = rgb2gray( im ); end
imshow( im,'InitialMagnification','fit' );
Show the grayscale levels in the image.
intensity = unique( im );
intensity
intensity = 7×1 uint8 column vector
0 40 80 120 160 200 240
There're 7 phases.
Let's generate mesh using the default setting.
[ vert, tria, tnum ] = im2mesh( im );
Refine triangulation... ------------------------------------------------------- |ITER.| |CDT1(X)| |CDT2(X)| ------------------------------------------------------- 3 71 100 8 90 179 Refine triangulation... ------------------------------------------------------- |ITER.| |CDT1(X)| |CDT2(X)| ------------------------------------------------------- 5 131 100 10 142 586 12 142 600 Smooth triangulation... ------------------------------------------------------- |ITER.| |MOVE(X)| |DTRI(X)| -------------------------------------------------------
plotMeshes( vert, tria, tnum )

Select phases for meshing

However, we don't need some of the phases in the image. Those phases may be air voids or background. We don't want them to show up in the finite element meshes.
Function im2mesh support phase selection for meshing. We need to assign a index vector to opt.select_phase. I'll show you how to do that.
We knew that the image has the following grayscales.
intensity
intensity = 7×1 uint8 column vector
0 40 80 120 160 200 240
For example, we are interested in the grayscale: 40, 80, 120, 200, 240. We don't want grayscle of 0 and 160. Let's use the following operation to obtain the index vector for opt.select_phase.
grayscale_we_like = [ 40, 80, 120, 200, 240 ]';
ind_vec = find( ismember(intensity, grayscale_we_like) );
% ind_vec is the index vector we need
ind_vec
ind_vec = 5×1
2 3 4 6 7
Let's generate mesh.
opt = []; % reset opt
opt.select_phase = ind_vec; % assign index vector to opt.select_phase
[ vert, tria, tnum ] = im2mesh( im, opt );
Refine triangulation... ------------------------------------------------------- |ITER.| |CDT1(X)| |CDT2(X)| ------------------------------------------------------- 3 67 76 8 83 140 Refine triangulation... ------------------------------------------------------- |ITER.| |CDT1(X)| |CDT2(X)| ------------------------------------------------------- 5 119 76 10 130 462 12 130 474 Smooth triangulation... ------------------------------------------------------- |ITER.| |MOVE(X)| |DTRI(X)| ------------------------------------------------------- 10 0 470
plotMeshes( vert, tria, tnum )
% reset image size
set(groot, 'DefaultFigurePosition', 'factory')
% end of demo