多边形内的测试点

Question

I'm trying to test if a point is insde a polygon o not, and looking in SO. i found some codes that does it, but I tried, and I don't know what I'm doing wrong...

I got to Vectors to save x and y points:

Vector<Double> vxpoints;
Vector<Double> vxpoints;

thats my method "contains"

public boolean contains(double x, double y) {       
    int i,j = this.npoints - 1;  
    boolean oddNodes = false;  

    for(i=0;i<this.npoints;j=i++) {
        if ((((this.vypoints.get(i) <= y) && (y < this.vypoints.get(j))) ||
                ((this.vypoints.get(j) <= y) && (y < this.vypoints.get(i)))) &&
                (x < (this.vxpoints.get(j) - this.vxpoints.get(i)) * (y - this.vypoints.get(i)) / (this.vypoints.get(j) - this.vypoints.get(i)) + this.vxpoints.get(i)))
            oddNodes = !oddNodes;
    }   
    return oddNodes;

And when I test it, I do with "easy polygons": (There are to arrays os points, that I convert in vectors inside my class)

    double xpoints[] = {100,100,200,200}; //Square      
    double ypoints[] = {100,200,100,200};
    PolygonDouble test = new PolygonDouble(xpoints, ypoints);

    //System.out.println(test.getNumberOfCoordinates());
    if(test.contains(110,110))
        System.out.println("Inside");
    else
        System.out.println("Outside");

Output: --> Outside but if I try with the point (110,111) the Output --> Inside.

I don't kwow what's going on..... :S

Answer 1

The problem is in the definition of your square used in your test. The vertices are the wrong order. Change the order of the third and fourth vertex and it the test should work.

double xpoints[] = {100,100,200,200}; //Square      
double ypoints[] = {100,200,200,100};