MATLAB Examples

# Divide a cube into six tetrahedra

There exist only 13 manners to split a cube in tetrahedras built exclusively with the 8 corners of the cube without counting simple rotations und reflections of the splittings. There are 12 splittings in 6 tetrahedra and one in 5 tetrahedra.

## Initial data

Set the random number generator.

rng(1);

Set the dimension of the problem.

d=3;

Define the 3-dimensional hypercube (cube).

vert=[0 0 0;1 0 0;1 1 0;0 1 0;0 0 1;1 0 1;1 1 1;0 1 1];

## Processing

Perform random perturbations to the vertices of the hypercube to avoid roundoff errors in convhull_nd.

vert=vert+0.001*rand(size(vert));

Find the Delaunay triangulation of the hypercube.

chull=delaunay_nd(vert);

Delete the simplices with very small volume. Such simplices are created due to the small random perturbations performed in the initial hypercube vertices.

V=zeros(size(chull,1),1);
for i=1:size(chull,1)
A=[vert(chull(i,:),:), ones(d+1,1)];
V(i)=abs(det(A));
end
V=V/prod(1:d);
chull=chull(V>0.01,:);
V=V(V>0.01,:);

## Verification

Verify that the volume of the unit cube is unity.

sum(V)
ans =

1.0002

## Plots

Plot the simplex components of the square.

Find the first and the second point identity of each edge of the hypercube.

node1=[1;2;3;4;5;6;7;8;1;2;3;4];
node2=[2;3;4;1;6;7;8;5;5;6;7;8];

Find the x and y coordinates of the first and second point of each edge of the hypercube.

x1=vert(node1,1);
x2=vert(node2,1);
y1=vert(node1,2);
y2=vert(node2,2);
z1=vert(node1,3);
z2=vert(node2,3);

Arrange the coordinate data.

X1=[x1,x2]';
Y1=[y1,y2]';
Z1=[z1,z2]';

Plot the simplices.

FigHandle=figure('Name','Simplices','NumberTitle','off');
set(FigHandle,'Position',[50, 50, 1800, 500]);
for i=1:size(chull,1)
subplot(1,6,i)
line(X1,Y1,Z1,'marker','.','markersize',20,'markeredgecolor',[1 0.5 0],...
'linestyle','-', 'linewidth',2,'color','blue');
hold on
tetramesh(chull(i,:),vert,'FaceColor',[1 0.5 0],...
'EdgeColor',[0 0 1],'FaceAlpha',0.3);
view(3)
axis equal
end

## Contact author

(c) 2014 by George Papazafeiropoulos
First Lieutenant, Infrastructure Engineer, Hellenic Air Force
Civil Engineer, M.Sc., Ph.D. candidate, NTUA

Website: http://users.ntua.gr/gpapazaf/