🚀 go-pugleaf

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.