intersectsLine() function is not working as expected

Question

I am trying to check if a line intersects a set of rectangles.

This is my code:

public class Test {

    public static void main(String args[]) {
        Rectangle2D.Double rectangle1 = new Rectangle2D.Double(32, 64, 32, 32);     
        Rectangle2D.Double rectangle2 = new Rectangle2D.Double(0, 32, 32, 32);      

        Line2D.Double line = new Line2D.Double(36, 63, 5, 12);

        System.out.println(rectangle1.intersectsLine(line)); // outputs false. Why?
        System.out.println(rectangle2.intersectsLine(line)); // outputs true as expected
    }
}

As you can see, the start point (36, 63) is within rectangle1, but apparently this line segment doesn't intersect the rectangle that it starts in.

However, it intersects the rectangle that stores the end point (5, 12).

Any idea why?

How do I get it to detect intersection?

Answer 1

The line does not intersect rectangle1, and that's why Java is giving you back the correct answer. Draw the lines in a GUI and see for yourself. Perhaps you're confused on the Rectangle2D.Double constructor -- the last parameters are width and height.

Draw the GUI and see; they almost touch, but not quite:

Drawing both rectangles:

import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.RenderingHints;
import java.awt.geom.*;

import javax.swing.*;

public class Test extends JPanel {
    private static final int PREF_W = 850;
    private static final int PREF_H = PREF_W;
    Rectangle2D rectangle1 = new Rectangle2D.Double(32, 64, 32, 32);     
    // Rectangle2D rectangle2 = new Rectangle2D.Double(0, 32, 32, 32);      

    Line2D line = new Line2D.Double(36, 63, 5, 12);

    public Test() {
        // TODO Auto-generated constructor stub
    }

    @Override
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        Graphics2D g2 = (Graphics2D) g;
        double scale = 8.0;
        g2.scale(scale, scale); // make it big to show it for you
        g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
        g2.draw(rectangle1);
        // g2.draw(rectangle2);
        g2.draw(line);
    }

    @Override
    public Dimension getPreferredSize() {
        if (isPreferredSizeSet()) {
            return super.getPreferredSize();
        }
        return new Dimension(PREF_W, PREF_H);
    }

    private static void createAndShowGui() {
        Test mainPanel = new Test();

        JFrame frame = new JFrame("Test");
        frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
        frame.getContentPane().add(mainPanel);
        frame.pack();
        frame.setLocationRelativeTo(null);
        frame.setVisible(true);
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                createAndShowGui();
            }
        });
    }
}