Fig. 2.

Left: the approach taken with deformable models. A surface of known topology but incorrect geometry (a sphere) is driven by an energy functional towards the desired pial surface shown in red. The difficulty stems from finding terms that will generate a smooth surface but will allow it to pass through an intermediate representation that can push enough surface area into e.g. the sylvian fissure. Right: in contrast, deforming the topologically incorrect surface model outwards to the surface of the sphere is a relatively simple computational problem.