The former case gives an impression of three dimensionality to the flat circles, the latter is useful for reducing dependence upon color for encoding information. A spline curve can be constructed from any set of atoms, though the usual set is that of alpha carbons. B-splines are used for smoothness.
Linear interpolation is used to extend the splines to the first and last atom within a spline.
Surfaces can be constructed as accessible or molecular surfaces. Construction times are typically of the order of five seconds on an R4000 SGI machine. This is independent of molecular size as the method is grid based where the number of grid points is constant, i.e larger molecules have courser surfaces. This grid size can be adjusted. The method of surface construction from a lattice representation is a form of marching cubes, though the original algorithm predate the description of that method.