  
        bw = zeros(50,50,50); bw(25,25,25) = 1;
        D1 = bwdist(bw);
        D2 = bwdist(bw,'cityblock');
        D3 = bwdist(bw,'chessboard');
        D4 = bwdist(bw,'quasi-euclidean');
        figure
        subplot(2,2,1), isosurface(D1,15), axis equal, view(3)
        camlight, lighting gouraud, title('Euclidean')
        subplot(2,2,2), isosurface(D2,15), axis equal, view(3)
        camlight, lighting gouraud, title('City block')
        subplot(2,2,3), isosurface(D3,15), axis equal, view(3)
        camlight, lighting gouraud, title('Chessboard')
        subplot(2,2,4), isosurface(D4,15), axis equal, view(3)
        camlight, lighting gouraud, title('Quasi-Euclidean')