% ICOSAHEDRON Generates vertices and graph of icosahedron
%
% Usage: [xyz, A, F] = icosahedron(radius)
%
% Argument: radius - Optional radius of icosahedron. Defaults to 1.
% Returns: xyz - 12x3 matrix of vertex coordinates.
% A - Adjacency matrix defining connectivity of vertices.
% F - 20x3 matrix specifying the 3 nodes that define each face.
%
% See also: GEODOME, GPLOT3D, DRAWFACES
% Copyright (c) 2009 Peter Kovesi
% www.peterkovesi.com
%
% Permission is hereby granted, free of charge, to any person obtaining a copy
% of this software and associated documentation files (the "Software"), to deal
% in the Software without restriction, subject to the following conditions:
%
% The above copyright notice and this permission notice shall be included in
% all copies or substantial portions of the Software.
%
% The Software is provided "as is", without warranty of any kind.
% May 2009
function [xyz, A, F] = icosahedron(radius)
if ~exist('radius','var')
radius = 1;
end
% Compute the 12 vertices
phi = (1+sqrt(5))/2; % Golden ratio
xyz = [0 1 phi
0 -1 phi
0 1 -phi
0 -1 -phi
1 phi 0
-1 phi 0
1 -phi 0
-1 -phi 0
phi 0 1
-phi 0 1
phi 0 -1
-phi 0 -1];
% Scale to required radius
xyz = xyz*radius/(sqrt(1+phi^2));
% Define the adjacency matrix
% 1 2 3 4 5 6 7 8 9 10 11 12
A = [0 1 0 0 1 1 0 0 1 1 0 0
1 0 0 0 0 0 1 1 1 1 0 0
0 0 0 1 1 1 0 0 0 0 1 1
0 0 1 0 0 0 1 1 0 0 1 1
1 0 1 0 0 1 0 0 1 0 1 0
1 0 1 0 1 0 0 0 0 1 0 1
0 1 0 1 0 0 0 1 1 0 1 0
0 1 0 1 0 0 1 0 0 1 0 1
1 1 0 0 1 0 1 0 0 0 1 0
1 1 0 0 0 1 0 1 0 0 0 1
0 0 1 1 1 0 1 0 1 0 0 0
0 0 1 1 0 1 0 1 0 1 0 0];
% Define nodes that make up each face
F = [1 2 9
1 9 5
1 5 6
1 6 10
1 10 2
2 7 9
9 7 11
9 11 5
5 11 3
5 3 6
6 3 12
6 12 10
10 12 8
10 8 2
2 8 7
4 7 8
4 8 12
4 12 3
4 3 11
4 11 7];
% The icosahedron defined above is oriented so that an edge is at the
% top. The following code transforms the vertex coordinates so that a
% vertex is at the top to give a more conventional view.
%
% Define coordinate frame where z passes through one of the vertices and
% transform the vertices so that one of the vertices is at the 'top'.
Z = xyz(1,:)'; % Z passes through vertex 1
X = xyz(2,:)'; % Choose adjacent vertex as an approximate X
Y = cross(Z,X); % Y is perpendicular to Z and this approx X
X = cross(Y,Z); % Final X is perpendicular to Y and Z
X = X/norm(X); Y = Y/norm(Y); Z = Z/norm(Z); % Ensure unit vectors
xyz = ([X Y Z]'*xyz')'; % Transform points;