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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.