implementation of subclass from triangulation

Hi everyone,

I am trying to implement a derived class from the 'triangulation' class available in Matlab 2014. The aim is to introduce additional properties and methods. I tried to mimic the example available in the Matlab documentation, but I failed even in creating a simple constructor. The minimal example is the following

classdef mymesh < triangulation
  properties
      Obj = [];
  end

    methods
        % constructor
        function primal = mymesh(P,T,M)   % M is the object code
            primal = primal@triangulation(T,P);
            primal.Obj = M;
        end
    end
  end

But trying to call it with the code

P = [0 0 0; 0 1 0; 1 1 0; 1 1 1];
T = [1 2 3 4];
M = ones(size(T,1),1);
c = mymesh(P,T,M);

makes Matlab crash. Can anyone point my were I am wrong?

Thank you in advance

Fabio

[Fabio Freschi]

The crash happens in Matlab 2014b. With version 2015b the error message is:

 Class 'mymesh' is not an allowed subclass of class 'triangulation'.

Answer 1

[Tom Clark]

Fabio,

To follow up on Rebecca's (correct, IMO) answer, you can overload properties and methods of the triangulation class, to make your new class look exactly like a triangulation.

 classdef MyTri

   properties(Hidden = true)
      TR
   end

   properties(Dependent)
      Points
      ConnectivityList
   end

   methods
      function obj = MyTri(varargin)
         % your constructor here, which creates a triangulation and stores it in obj.TR
      end
      function p = get.Points(obj)
         % Property getter method, so that MyTri.Points behaves like triangulation.Points
         p = obj.TR.Points;
      end
      function c = get.ConnectivityList(obj)
         % Another property getter
         c = obj.TR.ConnectivityList;
      end
   end
end

If you're familiar with python, using the dependent property with a getter is similar to using python's @property decorator on a class method (I'm a MATLAB guy but have to admit that python deals with this way more elegantly).

You may also wish to add a custom isa() method, to return true for both 'MyTri' and 'triangulation' objects in a fashion consistent with the application of isa() to a subclass. I've left that out here, because of course this isn't strictly a subclass.

Adding set.Points and set.ConnectivityList methods (very similar syntax to the getters example above) will allow you to modify your triangulation as normal, if that's what you require.

Finally, having done this myself, I have a bunch of methods to overload the triangulation class methods, just by passing variables through. To save anyone the unbelievable nuisance of duplicating this work, here they are (Caveat emptor: My unit tests don't have 100% coverage yet, so please test them as you use them!):

    % Overload all the triangulation class methods for this class
    function PC = barycentricToCartesian(obj, ti, B)
        PC = barycentricToCartesian(obj.TR, ti, B);
    end
    function B = cartesianToBarycentric(obj, ti, PC)
        B = cartesianToBarycentric(obj.TR, ti, PC);
    end
    function [CC, r] = circumcenters(obj, varargin)
        [CC,r] = circumcenters(obj.TR, varargin{:});
    end
    function ti = edgeAttachments(obj, varargin)
        ti = edgeAttachments(obj.TR, varargin{:});
    end
    function E = edges(obj)
        E = edges(obj.TR);
    end
    function FN = faceNormal(obj, varargin)
        FN = faceNormals(obj.TR, varargin{:});
    end
    function FE = featureEdges(obj, filterangle)
        FE = featureEdges(obj.TR, filterangle);
    end
    function [FBtri, FBpoints] = freeBoundary(obj)
        [FBtri, FBpoints] = freeBoundary(obj.TR);
    end
    function [IC,r] = incenter(obj,ti)
        [IC,r] = incenter(obj.TR,ti);
    end
    function [tf] = isConnected(obj,varargin)
        tf = isConnected(obj.TR,varargin{:});
    end
    function [vi,d] = nearestNeighbor(obj,varargin)
        [vi,d] = nearestNeighbour(obj.TR,varargin{:});
    end
    function N = neighbors(obj, varargin)
        N = neighbors(obj.TR, varargin{:});
    end
    function SZ = size(obj)
        SZ = size(obj.TR);
    end
    function ti = vertexAttachments(obj, varargin)
        ti = vertexAttachments(obj.TR, varargin{:});
    end
    function VN = vertexNormal(obj,varargin)
        VN = vertexNormal(obj.TR,varargin{:});
    end

Hope this helps, please upvote me if it does :)

Tom

[Carl Holmberg]

I have used this code and greatly apreaciate it! One can note that there is an "s" added to the end of the function call:

    faceNormals(obj.TR, varargin{:});

Answer 2

[Rebecca Krosnick]

It seems that we cannot create a subclass of "triangulation". Instead, you can create a class that has a "triangulation" object as a property rather than creating a subclass of "triangulation".

[Fabio Freschi]

Thanks Rebecca, my question is: to access the points the syntax will be

 mymesh.TR.Points

won't it?

[Tom Clark]

Yes it will. I've extended Rebecca's answer below to address this with a full example.

Answer 3

[per isakson]

You are too kind to Matlab. Matlab shall not crash whether or not you make a mistake. Now I crashed R2013b a couple of time with your code. I assume you need to ask support.

However, you might want to try this

P = [0 0 0; 0 1 0; 1 1 0; 1 1 1];
T = [1 2 3 4];
M = ones(size(T,1),1);
tr  = triangulation(T,P);
mym = mymesh(tr,M);
mym.TR.Points

outputs

ans =
     0     0     0
     0     1     0
     1     1     0
     1     1     1

where

classdef mymesh < handle
    properties
        Obj = [];
        TR
    end
    methods
        % constructor
        function primal = mymesh( tr, M )   % M is the object code
            primal.Obj = M;
            primal.TR  = tr;
        end
    end
end

[Fabio Freschi]

It works but its use is weird: Statements like

 mym.TR.Points

are effective but I see it as a workaround rather than a solution. I agree with you that Matlab should not crash. Anyway the bug seems to be fixed in 2015b.

Thank you for your support

Fabio

[per isakson]

It's not "weird"; it's composition

[Arabarra]

it's still weird that one should need to use composition instead of subclassing. TriRep (the predecessor of triangulation) used to allow subclassing (till R2016b)