Recursive longest-edge bisection

Finite element routine, to refine selected tetrahedron by bisecting their longest edges

Given element and nodal data, this routine can be used to refine the selected elements. Only 4-node tetrahedron elements are allowed. During the refinement, only the edge which is the longest one of all the elements that share it will be cut, otherwise, recursive search will be performed in surrounding elements until such refinement can be performed.

% [ix, nix, x, nx, u, r] = recr_bisc_tet4( id, ix, nix, x, nx, u, r)
% ------------------------------------------------------------------
% Recursively bisect the longest edge of targeted elements
% WZ. Shan, 03/03/2009
% ----------------------------------------------------------------
% Input:
% id: indices of targeted elements
% ix: element connectivity, [p1, p2, p3, p4]
% nix : number of elements
% x : nodal coordinates, [x, y, z]
% nx : number of nodes
% u : nodal displacements, [ux, uy, uz]
% r : nodal force, [rx, ry, rz]
% Output:
% updated input value, See above