🚀 go-pugleaf

In article <thompson.713120498@daphne.socsci.umn.edu>, thompson@atlas.socsci.umn.edu (T. Scott Thompson) writes:
|> morley@camins.camosun.bc.ca writes:
|> 
|> >Ok, this should be an easy one...  Given 3 X,Y points that form a triangle,
|> >what is the simplest (fastest) way to determine if they are clock-wise or
|> >counter-clock-wise?  I'm not interested in general polygons of N points,
|> >just plain old triangles.  Yes, this is for use in a graphics app.
|> 
|> Try this.  Calculate
|> 
|>       (X1 - X3)*(Y2 - Y3) - (X2 - X3)*(Y1 - Y3)
|> 
|> If the result is
|> 
|>     positive then 1 -> 2 -> 3 traverses the triangle counter-clockwise.
|> 
|>     negative then 1 -> 2 -> 3 traverses the triangle clockwise
|> 
|>     zero     then 1, 2, and 3 lie on a straight line.

This is a good test, but subject to numerical instability when the triangle is a "sliver." There isn't anything better. Just be careful. 
-- 

        -- jwh