Hi,

Well that was botched. Amazing what you think about

at 2:00am. Sorry.

Replace polyhedron with polygon. A polygon is a

2-D object, a polyhedron is a 3-D object made up of

a number of polygonal faces. Think of a cube made up

of squares. In 3-D graphics, the polyhedron is usually

called a mesh and is made up of triangles.

A normal is a fancy (?) term for a perpendicular vector.

For a plane that is simple to envision. As Dave said, for

a polygon, you can take the cross product of two edges

to produce a perpendicular vector, and normalize to get

the normal.

About the spinning: If you got a pizza pan and stuck

an arrow in it, you could rotate the fixed object around

the arrow. If you painted the left side of the arrow green

and the right side red, you would see the arrow rotate

with the pan.

But paint does not work with a one dimensional vector.

So when rotating (spinning) a plane about its normal, the

normal remains unchanged. Even though you can pass it

through all the math of the transform. So it is normally

(sorry) said to not rotate as it is identical before and after.

Hopefully this is better than yesterday's.

...,

Steve N.