demo01 of Im2mesh package
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')
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'))
im2mesh
Let's start demo01. We'll demostate the usage of function im2mesh.
Import image kumamon.tif.
im = imread('kumamon.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
Let's use function im2mesh to generate meshes based on the kumamon image. We will use default settings. Note that function im2mesh has incorporated the workflow that you saw in im2mesh_GUI: extract polygonal boundaries from image, search & label control points, smooth boundary, simplify boundary, select phase, and generate mesh.
[ vert, tria, tnum ] = im2mesh( im );
Function im2mesh has 3 ouptut variables: vert, tria, tnum. They defines linear elements of generated triangular mesh.
vert - Node data. N-by-2 array.
vert(i,1:2) = [x_coordinate, y_coordinate] of the i-th node
tria - Node numbering for each triangle. M-by-3 array.
tria(j,1:3) = [node_numbering_of_3_nodes] of the j-th element
tnum - Label of material phase. P-by-1 array.
tnum(j,1) = k; means the j-th element is belong to the k-th phase
We can plot the mesh using function plotMeshes.
plotMeshes( vert, tria, tnum )
Mesh quality
We can use function tricost to check the mesh quality. Function tricost is written by by Darren Engwirda.
tricost( vert, tria, tnum );
We can get the mean value of Q.
mean(triscr2( vert, tria ))
ans = 0.9550
Wonderful! Let's check the total number of triangles in the mesh.
size( tria, 1 )
ans = 1783
That's a lot of triangles.
Change parameter
We can change the parameters of function im2mesh to see whether it help to reduce the number of triangles.
opt = []; % initialize. opt is a structure array.
opt.tolerance = 1; % default value for opt.tolerance is 0.3
[ vert, tria, tnum ] = im2mesh( im, opt );
plotMeshes( vert, tria, tnum )
size( tria, 1 )
ans = 708
Now, the number of triangles decreases a lot. That's great.
Parameters of function im2mesh
As I showed before, there're two ways to call function im2mesh.
[ vert, tria, tnum ] = im2mesh( im ); % this use default opt setting
[ vert, tria, tnum ] = im2mesh( im, opt );
Variable opt means options. It's a structure array used to stored input parameters for function im2mesh. We can take a look at the default value of opt.
% opt is a structure array for parameters
opt.tf_avoid_sharp_corner = false;
opt.lambda = 0.5;
opt.mu = -0.5;
opt.iters = 100;
opt.threshold_num_turning = 0;
opt.threshold_num_vert_Smo = 0;
opt.tolerance = 0.3;
opt.threshold_num_vert_Sim = 0;
opt.select_phase = [];
opt.grad_limit = 0.25;
opt.hmax = 500;
opt.mesh_kind = 'delaunay';
opt.tf_mesh = true;
You don't need to memorize the names of these parameters. The meaning of these parameters are listed the file "Im2mesh functions and parameters.pdf". Please refer to "Im2mesh_GUI Tutorial.pdf" about how to choose the value of these parameters. You may change the default values by editting function setOption in "im2mesh.m".
Quadratic element
Function im2mesh can also generate quadratic element via the following syntax.
[ vert, tria, tnum, vert2, tria2 ] = im2mesh( im ); % this use default opt setting
[ vert, tria, tnum, vert2, tria2 ] = im2mesh( im, opt );
Ouput variables - vert, tria, tnum define linear elements.
Ouput variables - vert2, tria2, tnum define quadratic elements.
Their meanings are as follows.
vert - Node data. N-by-2 array.
vert(i,1:2) = [x_coordinate, y_coordinate] of the i-th node
tria - Node numbering for each triangle. M-by-3 array.
tria(j,1:3) = [node_numbering_of_3_nodes] of the j-th element
tnum - Label of material phase. P-by-1 array.
tnum(j,1) = k; means the j-th element is belong to the k-th phase
vert2 - Node data (2nd order element). P-by-2 array. Due to new vertices, the length of vert2 is much longer than vert.
vert2(i,1:2) = [x_coordinate, y_coordinate] of the i-th node
tria2 - Node numbering for each triangle (2nd order element). M-by-6 array.
tria2(j,1:6) = [node_numbering_of_6_nodes] of the j-th element
You can also plot mesh using function plotMeshes, or eveluate mesh quality using function tricost.
plotMeshes( vert2, tria2, tnum );
tricost( vert2, tria2, tnum );
Example 1
Let's show some examples with different opt.
% example 1
opt = []; % reset opt
opt.tolerance = 1e-10; % no polyline simplification
[ vert, tria, tnum ] = im2mesh( im, opt );
plotMeshes( vert, tria, tnum );
Example 2
% example 2
opt = []; % reset opt
opt.tolerance = 1;
opt.select_phase = [1 2 4]; % select phase for meshing
[ vert, tria, tnum ] = im2mesh( im, opt );
plotMeshes( vert, tria, tnum );
Example 3
What if we don't need to generate mesh and we want to check the simplified polygonal boundary?
In this special case, we can set "opt.tf_mesh = false;". It means we do not generate mesh. We will use this method in demo13, 15 and 16.
% example 3
opt = []; % reset opt
opt.tolerance = 1;
opt.select_phase = [1 2 4]; % select phase for meshing
opt.tf_mesh = false; % do not generate mesh
bounds = im2mesh( im, opt );
bounds is the simplified polygonal boundaries. We use function plotBounds to plot it.
plotBounds(bounds);
We can plot the polygons with a face color. We will show more about this in demo15. Please refer to the following link for how to set face color: https://www.mathworks.com/help/matlab/ref/polyshape.plot.html
% convert to a cell array of polyshapes
psCell = bound2polyshape(bounds);
figure
hold on; axis equal;
for i = 1: length(psCell)
plot( psCell{i} );
end
hold off
% reset image size
set(groot, 'DefaultFigurePosition', 'factory')
% end of demo