How to find visible facets of a convex hull

[repp]

Hi All,

How do I determine the visible facets of a 3D object patch? I am
assuming for now I can reduce the shape of the object to a convex
hull. How do I know which facets of the hull are visible viewed from
a specific point in space?

Thanks in Advanced

[Doug Schwarz]

Post by repp
Hi All,
How do I determine the visible facets of a 3D object patch? I am
assuming for now I can reduce the shape of the object to a convex
hull. How do I know which facets of the hull are visible viewed from
a specific point in space?
Thanks in Advanced

1. Generate a normal vector, n, pointing out for each facet. This
requires that given the three vertices you know whether they're ordered
clockwise or counterclockwise. If the vertices are A, B and C ordered
clockwise then calculate the cross product, n = (C - A) x (B - A) where
all are vector quantities.

2. Pick a point inside the hull, P. Call your viewpoint V. Take the
dot product, s = n . (V - P) for each facet. If s > 0 then the facet is
visible from V. (Or maybe it's s < 0 -- you'll figure it out.)