gui_comp_h_tet4( id, ix, x)

function H = gui_comp_h_tet4( id, ix, x)
% H = gui_comp_h_tet4( id, ix, x)
%-----------------------------------------------------------
% compute the heights for the nodes of selected elements 
% WZ. Shan, 23/01/09                                     
%-----------------------------------------------------------
% input:
%   ix : elem. con. mat. [p1, p2, p3, p4]
%   id : indices of selected elements
%   nel : number of elements
%   x   : nodal coordinates, [x, y, z]
% output:
%   H : heights [h1, h2, h3, h4]

ix1 = ix( id, :)';

p1 = [2 1 1 1];
p2 = [3 3 2 2];
p3 = [4 4 4 3];

id1 = ix1( p1, :);
id2 = ix1( p2, :);
id3 = ix1( p3, :);
id1 = id1(:);
id2 = id2(:);
id3 = id3(:);

vec1 = x( id2, :) - x( id1, :);
vec2 = x( id3, :) - x( id1, :);

% normals
N = cross( vec1, vec2, 2);
Nnorm = sqrt(N(:, 1).^2 + N(:, 2).^2 + N( :, 3).^2);
N     = N./repmat( Nnorm, 1, 3);

% mid point distance
mid  = (x( id1, :) + x( id2, :) + x( id3, :))/3;
dmid = x( ix1(:), :) - mid;

% heights
H = abs(dot( N, dmid, 2));

% reshape
H = reshape( H, 4, length(id))';