Rotate Rectangle in Java

Question

I need to create rectangles that are rotated around their center (so they don't need to be parallel to the axes of the coordinate system). So basicelly each rectangle can be defined by center-X, center-Y, width, height and angle. What I want to do then is to perform calculations on whether certain points are contained in these rectangles or not (so no drawing will be involved). I guess I cant use the Rectangle2D class because these rectangles will always be parallel to the x and y-axis of the coordinate system. Is the only way to get this functionality by writing my own rectangle class or is there anything existing (similar to Rectangle2D) I can use?

Answer 1

Rotate all the points you want to test and use contains(Point) method of the Rectangle2D as Mihai did.

But if you really want to rotate the rectangles you can do it like this (this is the integer version but probably you can do it with Rectangle2D aswell :)).

public class TestRotate {
    public static void main(String... args) {

        Rectangle r = new Rectangle(50, 50, 100, 100);
        Point check = new Point(100, 151); // clearly outside

        System.out.println("first: " + r.contains(check));

        AffineTransform at = AffineTransform.getRotateInstance(
                Math.PI/4, r.getCenterX(), r.getCenterY());

        Polygon p = new Polygon(); 

        PathIterator i = r.getPathIterator(at);
        while (!i.isDone()) {
            double[] xy = new double[2];
            i.currentSegment(xy);
            p.addPoint((int) xy[0], (int) xy[1]);
            System.out.println(Arrays.toString(xy));

            i.next();
        }

        // should now be inside :)
        System.out.println("second: " + p.contains(check));
    }
}

Answer 2

You can use Rectangle2D to check for containment, if instead of rotating your rectangle by an angle, say, counterclockwise, you rotate each of the points you need to check by the same angle clockwise, relative to the center of the rectangle. Something like

double dx = point.x - rectangleCenter.x;
double dy = point.y - rectangleCenter.y;
double newX = rectangleCenter.x - dx*Math.cos(angle) + dy*Math.sin(angle);
double newY = rectangleCenter.x - dx*Math.sin(angle) - dy*Math.cos(angle);