Drawing an arrow between two rectangle shapes

Question

i am trying to draw an arrow between two rectangle shapes. the arrow will start from the center of one rectangle and end with the arrow tip at the edge of the other rectangle. i actually draw the arrow first, and draw the rectangles last so the effect of where the arrow starts will seem to come from the edge and not the center.
i have code using some trigonmetry that works for squares, but as soon as the shape becomes a rectangle (i.e. width and height are not the same), the drawing breaks.
can i detect where a line intersects with a shape through clipping and use that point location to draw my arrow head? if so, how?

Answer 1

Here's one way to do this using the rule of similar triangles.
import java.awt.*;
import java.awt.geom.*;
import javax.swing.*;
public class Pointers extends JPanel {
    Rectangle r1 = new Rectangle(40,60,100,150);
    Rectangle r2 = new Rectangle(200,250,175,100);
    int barb = 20;
    double phi = Math.toRadians(20);
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        Graphics2D g2 = (Graphics2D)g;
        g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
                            RenderingHints.VALUE_ANTIALIAS_ON);
        g2.setPaint(Color.blue);
        g2.draw(r1);
        g2.draw(r2);
        g2.setPaint(Color.red);
        g2.draw(getPath());
    private GeneralPath getPath() {
        double x1 = r1.getCenterX();
        double y1 = r1.getCenterY();
        double x2 = r2.getCenterX();
        double y2 = r2.getCenterY();
        double theta = Math.atan2(y2 - y1, x2 - x1);
        Point2D.Double p1 = getPoint(theta, r1);
        Point2D.Double p2 = getPoint(theta+Math.PI, r2);
        GeneralPath path = new GeneralPath(new Line2D.Float(p1, p2));
        // Add an arrow head at p2.
        double x = p2.x + barb*Math.cos(theta+Math.PI-phi);
        double y = p2.y + barb*Math.sin(theta+Math.PI-phi);
        path.moveTo((float)x, (float)y);
        path.lineTo((float)p2.x, (float)p2.y);
        x = p2.x + barb*Math.cos(theta+Math.PI+phi);
        y = p2.y + barb*Math.sin(theta+Math.PI+phi);
        path.lineTo((float)x, (float)y);
        return path;
    private Point2D.Double getPoint(double theta, Rectangle r) {
        double cx = r.getCenterX();
        double cy = r.getCenterY();
        double w = r.width/2;
        double h = r.height/2;
        double d = Point2D.distance(cx, cy, cx+w, cy+h);
        double x = cx + d*Math.cos(theta);
        double y = cy + d*Math.sin(theta);
        Point2D.Double p = new Point2D.Double();
        int outcode = r.outcode(x, y);
        switch(outcode) {
            case Rectangle.OUT_TOP:
                p.x = cx - h*((x-cx)/(y-cy));
                p.y = cy - h;
                break;
            case Rectangle.OUT_LEFT:
                p.x = cx - w;
                p.y = cy - w*((y-cy)/(x-cx));
                break;
            case Rectangle.OUT_BOTTOM:
                p.x = cx + h*((x-cx)/(y-cy));
                p.y = cy + h;
                break;
            case Rectangle.OUT_RIGHT:
                p.x = cx + w;
                p.y = cy + w*((y-cy)/(x-cx));
                break;
            default:
                System.out.println("Non-cardinal outcode: " + outcode);
        return p;
    public static void main(String[] args) {
        JFrame f = new JFrame();
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.getContentPane().add(new Pointers());
        f.setSize(400,400);
        f.setLocation(200,200);
        f.setVisible(true);
}