Fly-path planning consists of steps: (1) segmenting the colon wall (top
left), (2) shrinking the original space to a small scale (top middle),
(3) thinning for the trend of a pipe (top right), (4) determining a
smooth path in the small scale space (bottom left), and (5) mapping
back to the original space for the fly path (bottom right). An excellent
fly path is very important to achieve virtual colonoscopy.