How to offset polygon edges?

Question

I have a list of point2D that makes a closed polygon. Now I want to create another set of 2D points by offsetting the polygon given an option inside or outside and an offset value. How can I do it?

Answer 1

For every polygon vertex calculate outer bisector vector as sum of normalized normals na and nb of two neighbor edges), then normalize it

 bis = na + nb 
 bis = bis / Length(bis)

Then find needed length of bisector to provide offset distance as

 l = d / Sqrt((1 + dotproduct(na,nb))/2)

(derived from l=d/cos(fi/2) and half-angle cosine formula)

And get offset polygon vertex (use minus for inner offset!):

P' = P + l * bis

Added: python implementation here