demo09 of Im2mesh package

demo09 - How to 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.
Table of Contents
Note Setup Phases 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.
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'))

Phases

Let's start demo. Import image Phases.tif.
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'
ans = 1×7 uint8 row vector
0 40 80 120 160 200 240
There're 7phases. Let's generate mesh using the default setting.
[ vert, tria, tnum ] = im2mesh( im );
Refine triangulation... ------------------------------------------------------- |ITER.| |CDT1(X)| |CDT2(X)| ------------------------------------------------------- 3 71 100 8 91 178 Refine triangulation... ------------------------------------------------------- |ITER.| |CDT1(X)| |CDT2(X)| ------------------------------------------------------- 5 128 100 10 140 550 11 140 556 Smooth triangulation... ------------------------------------------------------- |ITER.| |MOVE(X)| |DTRI(X)| ------------------------------------------------------- 4 113 554 8 18 554
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)| ------------------------------------------------------- 5 118 76 10 129 431 12 129 437 Smooth triangulation... ------------------------------------------------------- |ITER.| |MOVE(X)| |DTRI(X)| ------------------------------------------------------- 4 93 437 8 12 437
plotMeshes( vert, tria, tnum )
% reset image size
set(groot, 'DefaultFigurePosition', 'factory')
% end of demo