NPike - 6 months ago 88

Java Question

I have a convex polygon (typically just a rotated square), and I know all of 4 points. How do I determine if a given point (yellow/green) is *inside* the polygon?

EDIT: For this particular project, I don't have access to all of the libraries of the JDK, such as AWT.

Answer

This page: http://www.ecse.rpi.edu/Homepages/wrf/Research/Short_Notes/pnpoly.html shows how to do this for any polygon.

I have a Java implementation of this, but it is too big to post here in its entirety. However, you should be able to work it out:

```
class Boundary {
private final Point[] points; // Points making up the boundary
...
/**
* Return true if the given point is contained inside the boundary.
* See: http://www.ecse.rpi.edu/Homepages/wrf/Research/Short_Notes/pnpoly.html
* @param test The point to check
* @return true if the point is inside the boundary, false otherwise
*
*/
public boolean contains(Point test) {
int i;
int j;
boolean result = false;
for (i = 0, j = points.length - 1; i < points.length; j = i++) {
if ((points[i].y > test.y) != (points[j].y > test.y) &&
(test.x < (points[j].x - points[i].x) * (test.y - points[i].y) / (points[j].y-points[i].y) + points[i].x)) {
result = !result;
}
}
return result;
}
}
```

And here is a sketch of the Point class

```
/**
* Two dimensional cartesian point.
*/
public class Point {
public final double x;
public final double y;
...
}
```