The frontier is a curve that separates the visible from invisible
region - at least locally - for a given camera motion. In the
following video we show the visible region sweeping out the surface. But
notice that near the top of the surface the contour generators do not
sweep across the surface but in fact form an envelope.
Frontier point for these two viewpoints.
We can of course consider many frontier points.
Profiles of complex scene MPEG
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