Point inside a polygon defined by four points

Question

Hello everyone.
In my java aplication, I need to know if a point is inside a polygon defined by four points. These points can have any value.
A solution is:
1)trace four lines: since the point to analyze until all the four points of the polygon.
2)Calculate the angles formed between these lines.
3)If the sum of the four angles is equal to 360�, the point is inside of the polygon.
I think this could work, but i don�t know how to make this in java. Can anyone help me? Other solutions are welcome.
Thanks.
Fatima

Answer 1

This is the code from the jazz api:
    static public boolean isInsidePolygon(double x, double y, int np, double[] xp, double[] yp) {
        int i;
        double angle = 0.0;
        boolean inside = false;
        for (i = 0; i < (np - 1); i++) {
            angle += ZUtil.angleBetweenPoints(x, y, xp, yp[i], xp[i+1], yp[i+1]);
angle += ZUtil.angleBetweenPoints(x, y, xp[np-1], yp[np-1], xp[0], yp[0]);
// Allow for a bit of rounding
// Ideally, angle should be 2*pi.
if (java.lang.Math.abs(angle) > 6.2) {
inside = true;
} else {
inside = false;
return inside;
Here is the API, the above method is in ZUtil class.
You can download the source code from the jazz api web site and examine it.