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# Finding inner normal of an edge using C

Posted on 1998-05-28

Hi,

How can we calculate the inner normal of a linear edge of a polygonal face ?

The data i have is some face data (its surface normal, face center and face boundary) and the vertices of the edge.

The vertices and other data are represented in the Cartesian form.

(only x,y coordinate is of the interest)

Your info is much appreciated.

I have a routine to calculate the normal vector** to the edge, but no idea how to make sure it

is an inner normal.

** note: edge vector = <x1,y1>

normal vector = <-y1,x1>

thanks,

Sam