Detect if a set of points in an array that are the vertices of a complex polygon were defined in a clockwise or counterclockwise order?

Question

EDIT: I updated the program with the answer and it works great!

I am making a program (feel free to try it out) that lets users draw polygons which it then triangulates.

Answer 1

In case someone is using three.js the ShapeUtils comes with an inbuilt isClockWise method which internally uses the area method to determine the sign of the calculated area.

isClockWise: function ( pts ) {

    return ShapeUtils.area( pts ) < 0;

}

The ShapeUtils.isClockWise Method can be found here.

area: function ( contour ) {

    var n = contour.length;
    var a = 0.0;

    for ( var p = n - 1, q = 0; q < n; p = q ++ ) {

        a += contour[ p ].x * contour[ q ].y - contour[ q ].x * contour[ p ].y;

    }

    return a * 0.5;

},

The ShapeUtils.area Method can be found here.