Straight Line Region in Java | Detect When an Object Crosses a Line

Detecting Regions Divided by a Diagonal Line | Maths Explanation for Java Kids

In this tutorial, you'll learn how to detect when an object crosses a straight line in Java. We'll explore how to check which side of a line a point lies on, and how to apply this logic to a Java canvas animation.

When working with graphics or simulations in Java, you might need to check which side of a line a ball or object is on. This technique is common in Java canvas collision detection and helps define regions divided by a line.

Consider a moving body (circle). When it moves across a diagonal straight line, we can perform an action - such as changing the ball's colour - after it crosses from one region to another.

Figure: Java diagram showing object crossing a straight line region

Understanding the Straight Line Equation in Java

We use the slope-intercept formula (y = mx + c) to define boundaries for region detection in Java.

To implement straight line region detection, we'll compare the ball's x-position with the position of the line at the same y-coordinate, using the line equation in Java.
If the ball's midpoint is (x_b, y_b) and the line at that height is (x_l, y_l), then:
x_l = my_d + c
The ball crosses the line when: x_d >= x_l.

Figure: Java straight line region diagram showing object coordinates for line detection

This logic uses the line equation in Java to check when an object moves from one region to another.
This can also help with collision detection and boundary demarcation in Java graphics or HTML canvas.

Java Code: Detecting Line Crossing in Canvas

To determine which side of a diagonal boundary a point belongs to, we use a simple line equation. This Java example demonstrates line region detection using the slope-intercept method.

Create a new Java project; call it Dymetric.
Create 3 new Java classes; File, New.
Call them Facet, PanelsStraightLineRegion and StraightLineRegion.
Type out the adjoining Java code for detecting the instance a travelling body crosses the path of a straight line.
This Java line crossing detection example changes the ball's colour once it moves across the line.

Summary: Detecting Line Boundaries with Java

You've learned how to use the line equation in Java to **detect regions divided by a straight line and trigger actions when objects cross the line**. This simple logic forms the foundation of Java graphics and animation algorithms.

By now, you can use Java to detect when an object crosses a straight line and determine which region it belongs to. This simple mathematical approach is useful for animations, physics simulations, and canvas line region detection.

Applying the Line Region Detection Logic in Java

This tutorial teaches you to:

Detect when a ball crosses a straight or diagonal line.
Identify which side of a line a point lies on.
Create interactive Java canvas projects with region-based logic.

You can extend this principle to handle collision detection, line-segment intersection, or more complex 2D graphics region detection.

So! Java Fun Practice Exercise - Detect Straight Line Boundary

As a fun practice exercise, try modifying the Java code to explore different coordinates and intercepts. This will be a great way to connect mathematics and programming, and help you understand more about Java animations and linear boundaries.

Java Straight Line Boundary Code - Main Class

package dymetric;

public class Dymetric {

    public static void main(String[] args) {
        Facet lcd = new Facet();
        lcd.setVisible(true);
    }

}

Java Straight Line Boundary Window Setup - Facet Class

package dymetric;

import java.awt.*;
import javax.swing.*;

/**
* This class just creates a display window to attach our canvas to.
*/
public class Facet extends JFrame {

    public Container face;
    public ButtonandCanvasPanels components;
    public ImageIcon logo;

    public Facet() {
        // Give our window a title
        super("A window that will hold a Canvas and Button");
        // set window start points and dimensions
        setSize(780,400);
        // close the window when told to
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setResizable(true); // should this window be resizable?

        // It'll be nice to have our own logo (feel free to use yours)
        logo = new ImageIcon(getClass().getResource("studyingPays.png"));
        this.setIconImage(logo.getImage());

        face = this.getContentPane();
        // background colour - may not be necessay since our canvas will have a colour of its own
        face.setBackground(Color.PINK);


        components = new ButtonandCanvasPanels();
        // using the default layout manager
        face.add(components.button_panel, BorderLayout.NORTH);
        // attach appropriate drawing component
        face.add(components.canvas_panel, BorderLayout.CENTER);
    }
}

Java Straight Line Boundary Canvas and Button Controls Class

package dymetric;

import java.awt.event.*;
import java.awt.*;
import javax.swing.*;

public class ButtonandCanvasPanels implements ActionListener {
    public JPanel button_panel, canvas_panel;
    public JButton motion_bttn;
    public StraightLineRegion boundary;
    public ButtonandCanvasPanels() {
        button_panel = new JPanel();
        // pick a background colour
        button_panel.setBackground(Color.PINK);
        button_panel.setLayout(new FlowLayout(FlowLayout.CENTER, 0, 0));
        // O my; but for convenience sake let's add our control button here
        motion_bttn = new JButton("Move");
        motion_bttn.setBackground(new Color(255, 0, 255));
        motion_bttn.addActionListener(this);
        // using the default layout manager
        button_panel.add(motion_bttn);


        canvas_panel = new JPanel();
        canvas_panel.setLayout(new BorderLayout());
        boundary = new StraightLineRegion();
        // attach appropriate drawing component
        canvas_panel.add(boundary, BorderLayout.CENTER);
    }

    /**
     * Respond to the button click event
     */
    public void actionPerformed(ActionEvent evt) {
        boundary.checkBoundary();
    }
}

Java Animation Code for Straight Line Region class

package dymetric;

import java.awt.*;

public class StraightLineRegion extends Canvas {

    protected Color ball_colour;
    // coordinates for the ball(circle)
    protected int x_ball = 50;
    protected int y_ball = 100;
    protected int previous_x = x_ball;
    protected int previous_y = y_ball;

    protected int x1 = 400;
    protected int x2 = 600;
    protected int y1 = 310;
    protected int y2 = 10;
    // x-coordinate for diagonal line
    protected double x_line;
    protected double m, c; // slope and y-intercept of a straight line
    protected final int aWIDTH, aHEIGHT;

    public StraightLineRegion() {
        setBackground(Color.LIGHT_GRAY); // canvas color
        ball_colour = Color.YELLOW;
        aWIDTH = aHEIGHT = 80;

        m = (double) (y2 - y1) / (x2 - x1); // slope
        c = (double) (x2 * y1 - x1 * y2) / (x2 - x1); // y-intercept

        x_line = (double) (y_ball - c) / m;
    }

    // Feel free to double buffer if flickering occurs
    public void paint(Graphics g) {
        //draw diagonal line
        g.setColor(Color.BLACK);
        g.drawLine(x1, y1, x2, y2);

        // erase previous circle
        g.setColor(Color.LIGHT_GRAY);
        g.fillOval(previous_x, previous_y, aWIDTH, aHEIGHT);
        g.setColor(ball_colour);
        // draw a circle
        g.fillOval(x_ball, y_ball, aWIDTH, aHEIGHT);
        previous_x = x_ball;
    }

    public void checkBoundary() {
        // condition for continuing motion
        while (x_ball <= 650) {
            if (x_ball >= x_line) {
                ball_colour = Color.GREEN; // color for our moving body(circle)
            }
            paint(this.getGraphics());

            x_ball += 10;
            // introduce a delay between renderings
            try {
                Thread.sleep(50);
            } catch (InterruptedException e) {
            }
        }
    }
}

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