demo08 of Im2mesh package

demo08 - Function plotMeshes
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 Overview Import image Generate mesh Select colormap Set display mode Other settings Highlight low quality elements

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 = 300; height = 300;
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'))

Overview

We will demostrate function plotMeshes in this demo.
There are 3 typical usage of function plotMeshes
% usage 1:
% plotMeshes( vert, ele, tnum );
%
% usage 2:
% plotMeshes( vert, ele, tnum, color_code );
%
% usage 3:
% plotMeshes( vert,ele,tnum, color_code, opt )
The meanings of the input arguments are as follows:
color_code: Color code for selecting colormap. Interger. Value: 0-10. Default value: 0
0: grayscale, 1: lines, 2: parula, 3: turbo, 4: jet, 5: hot, 6: cool, 7: summer, 8: winter, 9: bone, 10: pink.
opt: a structure array. It is the extra options for plotMeshes. It stores extra parameter settings for plotMeshes.
opt.mode: display mode. Value: 1 or 2.
opt.wid: line width of the plotted edges. Default value: 0.5
opt.alpha: edge line transparency. It's a scalar value in range [0,1]. Default value: 0.5
opt.beta: brightness adjustment of colormap. [-1, 1]. Default value: 0
opt.tf_gs: Boolean value. Whether to use graphics smoothing when plotting. Default value: 1
opt.q_thresh: threshold for mesh quality. Those elements with mesh quality lower than opt.q_thresh will be highlighted. Default value: 0

Import image

Let's start demo. Import image.
im = imread('logo.tif');
if size(im,3) == 3; im = rgb2gray( im ); end
imshow( im,'InitialMagnification','fit' );

Generate mesh

opt = []; % reset
opt.tolerance = 1;
opt.grad_limit = 0.2;
[ vert, tria, tnum, vert2, tria2 ] = im2mesh( im, opt );
Refine triangulation... ------------------------------------------------------- |ITER.| |CDT1(X)| |CDT2(X)| ------------------------------------------------------- 9 125 108 10 125 218 17 158 433 Refine triangulation... ------------------------------------------------------- |ITER.| |CDT1(X)| |CDT2(X)| ------------------------------------------------------- 10 271 108 18 297 108 20 302 830 30 310 2114 33 311 2126 Smooth triangulation... ------------------------------------------------------- |ITER.| |MOVE(X)| |DTRI(X)| ------------------------------------------------------- 10 84 2108
Plot mesh
plotMeshes( vert, tria, tnum )

Select colormap

color_code = 2;
plotMeshes( vert,tria,tnum, color_code )

Set display mode

When opt.mode=1, plot faces and edges. Slower.
When opt.mode=2, plot edges only. Faster.
Default value: 1
color_code = 2;
opt = []; % reset
opt.mode = 2;
plotMeshes( vert,tria,tnum, color_code, opt )

Other settings

When the mesh data is huge, I usually set 'opt.alpha' to 0.2-0.3.
color_code = 2;
opt = []; % reset
opt.mode = 2;
opt.wid = 0.5;
opt.alpha = 0.3;
opt.beta = 0.5;
plotMeshes( vert,tria,tnum, color_code, opt )

Highlight low quality elements

Those elements with mesh quality lower than 'opt.q_thresh' will be highlighted.
You can set color_code to 0 to faciliate the visualization.
color_code = 0;
opt = []; % reset
opt.mode = 1;
opt.wid = 0.5;
opt.alpha = 0.2;
opt.beta = 0;
opt.q_thresh = 0.6;
plotMeshes( vert,tria,tnum, color_code, opt )
% reset image size
set(groot, 'DefaultFigurePosition', 'factory')
% end of demo