vb_calc_normal_variance

 Deviation angle from average normal vector in the neighbor of root vertex
   xxdev = vb_calc_normal_variance(member_id, xx)
 --- Input
 member_id{n} : member vertex index which belog to the n-th root point 
 xx(i,:)      : normal vector at the vertex-i in the original brain
 --- Output
 xxdev : deviation angle from average normal vector

 M. Sato  2006-7-14

 Copyright (C) 2011, ATR All Rights Reserved.
 License : New BSD License(see VBMEG_LICENSE.txt)

0001 function    xxdev = vb_calc_normal_variance(member_id, xx)
0002 % Deviation angle from average normal vector in the neighbor of root vertex
0003 %   xxdev = vb_calc_normal_variance(member_id, xx)
0004 % --- Input
0005 % member_id{n} : member vertex index which belog to the n-th root point
0006 % xx(i,:)      : normal vector at the vertex-i in the original brain
0007 % --- Output
0008 % xxdev : deviation angle from average normal vector
0009 %
0010 % M. Sato  2006-7-14
0011 %
0012 % Copyright (C) 2011, ATR All Rights Reserved.
0013 % License : New BSD License(see VBMEG_LICENSE.txt)
0014 
0015 % Number of target vertex for calculation
0016 Nroot = length(member_id);        
0017 
0018 % Deviation angle from average normal vector
0019 xxdev = zeros(Nroot,1);
0020 
0021 for n=1:Nroot,
0022     % neighbor point index
0023     inext = member_id{n};
0024     Nnext = length(inext);
0025     
0026     % normal vector of neighbor point
0027     xxnext = xx(inext,:);
0028     
0029     % mean normal vector
0030     xxn = sum(xxnext,1);
0031     xxs = max( sqrt(sum(xxn.^2)), eps );
0032     xxn = xxn/xxs;
0033     
0034     % mean angle from mean normal vector
0035     xxdev(n) = sum(acos(xxnext*xxn'))/Nnext;
0036 end;