%--------------------------
 % @Author: Jingqiao Hu
 % @Date: 2021-03-18 18:17:09
 % @LastEditTime: 2021-03-18 21:42:10

 %                  vtk:             edofMat:
 %   y              4-----5          3-----2
 %   |             /|    /|         /|    /|
 %   0---x        7-0---6 1        7-0---6 1
 %  /             |     |/         |     |/ 
 % z              3-----2          4-----5
%--------------------------
function voxel2vtk(filename, nodes, eles)

    delete(filename); 
    fidOUT = fopen(filename, 'w');

    fprintf(fidOUT, '# vtk DataFile Version 2.0\n');
    fprintf(fidOUT, 'Unstructured Grid Example\n'); 
    fprintf(fidOUT, 'ASCII\n');
    fprintf(fidOUT, 'DATASET UNSTRUCTURED_GRID\n');

    nodes_num = size(nodes, 1);
    fprintf(fidOUT, 'POINTS %i float\n', nodes_num);
    for i = 1:nodes_num
        fprintf(fidOUT, '%f %f %f\n', nodes(i,1), nodes(i,2), nodes(i,3));
    end

    eles_num = size(eles, 1);
    eles_size = eles_num * 9;
    fprintf(fidOUT, 'CELLS %i %i\n', eles_num, eles_size);
    for ele = 1:eles_num
        nid0 = eles(ele, :);
        nid  = vtk_order(nid0) - 1;
        fprintf(fidOUT, '%i %i %i %i %i %i %i %i %i\n', 8, nid(1), nid(2), nid(3), nid(4), nid(5), nid(6), nid(7), nid(8));
    end

    fprintf(fidOUT, 'CELL_TYPES %i\n', eles_num);
    for ele = 1:eles_num
        fprintf(fidOUT, '12\n');
    end

    fprintf(fidOUT, 'POINT_DATA %i\n', nodes_num);

    fclose(fidOUT);
end

% change the nodes's order of edofMat to vtk 
function node = vtk_order(node_id)
    node = node_id;
    node([0,1,6,7]+1) = node_id([0,1,6,7]+1);
    node([3,2]+1)     = node_id([4,5]+1);
    node([4,5]+1)     = node_id([3,2]+1);
end