Code covered by the BSD License

# geom3d

### David Legland (view profile)

19 Jun 2009 (Updated )

Library to handle 3D geometric primitives: create, intersect, display, and make basic computations

### Editor's Notes:

This file was selected as MATLAB Central Pick of the Week

minConvexHull(nodes, varargin)
```function faces = minConvexHull(nodes, varargin)
%MINCONVEXHULL Return the unique minimal convex hull of a set of 3D points
%
%   FACES = minConvexHull(NODES)
%   NODES is a set of 3D points  (as a Nx3 array). The function computes
%   the convex hull, and merge contiguous coplanar faces. The result is a
%   set of polygonal faces, such that there are no coplanar faces.
%   FACES is a cell array, each cell containing the vector of indices of
%   nodes given in NODES for the corresponding face.
%
%   FACES = minConvexHull(NODES, PRECISION)
%   Adjust the threshold for deciding if two faces are coplanar or
%   parallel. Default value is 1e-14.
%
%   Example
%   [n e f] = createCube;
%   f2 = minConvexHull(n);
%   drawMesh(n, f);
%
%   meshes3d, drawMesh, convhull, convhulln
%
%
% ------
% Author: David Legland
% e-mail: david.legland@jouy.inra.fr
% Created: 2006-07-05
% Copyright 2006 INRA - CEPIA Nantes - MIAJ (Jouy-en-Josas).

% HISTORY
%   20/07/2006 add tolerance for coplanarity test
%   21/08/2006 fix small bug due to difference of methods to test
%       coplanarity, sometimes resulting in 3 points of a face being not
%       coplanar ! Also add control on precision
%   18/09/2007 ensure faces are given as horizontal vectors

% set up precision
acc = 1e-14;
if ~isempty(varargin)
acc = varargin{1};
end

% triangulated convex hull. It is not uniquely defined.
hull = convhulln(nodes);

% number of base triangular faces
N = size(hull, 1);

% compute normals of given faces
normals = planeNormal(createPlane(...
nodes(hull(:,1),:), nodes(hull(:,2),:), nodes(hull(:,3),:)));

% initialize empty faces
faces = {};

% Processing flag for each triangle
% 1 : triangle to process, 0 : already processed
% in the beginning, every triangle face need to be processed
flag = ones(N, 1);

% iterate on each triangle face
for i = 1:N

% check if face was already performed
if ~flag(i)
continue;
end

% indices of faces with same normal
ind = find(abs(vectorNorm3d(cross(repmat(normals(i, :), [N 1]), normals)))<acc);
ind = ind(ind~=i);

% keep only coplanar faces (test coplanarity of points in both face)
ind2 = i;
for j = 1:length(ind)
if isCoplanar(nodes([hull(i,:) hull(ind(j),:)], :), acc)
ind2 = [ind2 ind(j)]; %#ok<AGROW>
end
end

% compute order of the vertices in current face
vertices = unique(hull(ind2, :));
[tmp, I]  = angleSort3d(nodes(vertices, :)); %#ok<ASGLU>

% add a new face to the list
face = vertices(I);
faces = [faces {face(:)'}]; %#ok<AGROW>

% mark processed faces
flag(ind2) = 0;
end

```