Move points in the opposite direction

Question

I am working on a project in Java. I am trying to move the points p2 , p3 , p4 just outside the circumference of the circle in the opposite direction to the point p1. Below is the image, which describes the problem, I am trying to solve.

//given two points, calculates the angle
public static double calcAngle(Point2D.Double p1, Point2D.Double p2) {
    double deltaX = p2.x - p1.x;
    double deltaY = p2.y - p1.y;
    return (Math.atan2(deltaY, deltaX) * 180 / Math.PI);
}

//calculates a point on a circle given the angle, center of the circle and the radius
public static Point2D.Double pointOnCircle(Point2D.Double point, double radius , double angle) {
    double x = Math.abs(point.x + (radius * Math.cos(angle  * Math.PI / 180F)));
    double y = Math.abs(point.y + (radius * Math.sin(angle  * Math.PI / 180F)));
    return new Point2D.Double(x,y);
}

How do I calculate the angle in Java coordinate system and destination co-ordinates for each of the points p2 , p3 , p4 ?

I am yet to try the code above and would like to know if my approach is right before proceeding, since it is a part of the bigger project. Thanks in advance!

Answer 1

Your general idea seems workable but overly complicated. There is no need to convert from x/y-vector to angle and then back. SImply scaling vectors will be enough.

Point2D p = p2; // likewise for p3, p4
double factor = radius / p.distance(p1);
p.setLocation(p1.getX() + (p.getX() - p1.getX())*factor,
              p1.getY() + (p.getY() - p1.getY())*factor);

This takes the vector (p - p1) , ie the vector pointing from p1 towards p , scales it by factor and adds it to the position of p1 . The factor is chosen such that the new distance is equal to radius .

All of this will fail if p1 and p are the same, since in this case you'll have a division by zero. If this can be a problem for you, you might want to ensure that factor is a finite number, eg using Double.isFinite(double) .