% illustration of a torus, obtained as an isosurfacefunction main()   % big and small radii of the torus   R = 3; r = 1;    Kb = R+r;     h = 0.1; % h is the grid size. Smaller h means prettier picture.      X = (-Kb-h):h:(Kb+h);   m = length(X);   Y = (-Kb-h):h:(Kb+h);   n = length(Y);   Z = (-r-h):h:(r+h);     k = length(Z);    W = zeros(m, n, k); % the zero level set of this function will be the desired shape    for i=1:m      for j=1:n         x = X(i);         y = Y(j);          W(i, j, :) = (sqrt(x^2+y^2)-R)^2 + Z.^2-r^2; % torus eqn, vectorize in Z      end   end   figure(4); clf; hold on; axis equal; axis off;   H = patch(isosurface(W, 0));   isonormals(W, H);         light_green=[184, 224, 98]/256;   % set some propeties   set(H, 'FaceColor', light_green, 'EdgeColor','none', 'FaceAlpha', 1);   set(H, 'SpecularColorReflectance', 0.1, 'DiffuseStrength', 0.8);   set(H, 'FaceLighting', 'phong', 'AmbientStrength', 0.3);   set(H, 'SpecularExponent', 108);   daspect([1 1 1]);   axis tight;   colormap(prism(28))        % viewing angle   view(-146, 32);  % add in a source of light   camlight (-10, 54); lighting phong;  %save as png  print('-dpng', '-r400', sprintf('Torus_illustration.png'));