Question

I am writing a program that displays Hadamard pattern using recursion.

A 1-by-1 Hadamard pattern is a single black square. In general a 2N-by-2N Hadamard pattern is obtained by aligning 4 copies of the N-by-N pattern in the form of a 2-by-2 grid, and then inverting the colors of all the squares in the lower right N-by-N copy.

I would like to produce the same picture as this one

My code is:

import java.awt.*;

import javax.swing.*;

@SuppressWarnings("serial")
public class Hadamard extends JPanel{


@SuppressWarnings("unused")
private void hadamard(Graphics g, int n, int x, int y, int width, int height){

    if(n == 0){

        g.fillRect(x/2, y/2, width, height);
        return;
    }
    else{

        hadamard(g, n-1, x, y, width/2, height/2);

        hadamard(g, n-1, x + width, y,  width/2, height/2);

        hadamard(g, n-1, x, y + height, width/2, height/2);

        g.setColor(Color.WHITE);
        hadamard(g, n-1, x + width, y + height, width/2, height/2);
        g.setColor(Color.BLACK);


    }
}

protected void paintComponent(Graphics g){
    super.paintComponent(g);

    hadamard(g, 2, getWidth() / 2, getHeight() / 2, getWidth() / 2, getHeight() / 2);

}

public static void main(String[] args) {

    Hadamard panel = new Hadamard();
    JFrame app = new JFrame();

    app.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    app.add(panel);
    app.setSize(516, 538);
    app.setVisible(true);

}

}

I don’t change properly the colors of the squares in the lower right corner. I’ve been stuck at this step for the last a couple of hours and am hoping someone would give me an idea how to do it correctly because I don’t know.

Thank you in advance.
Nath

Answer 1

Your code is not inverting the lower-right quadrant, it’s simply forcing it to be white.

I would say that the best solution is to add an extra parameter to your recursive method (let’s call it b), whose value can be true or false. You can then recurse as:

    hadamard(g, n-1, x, y, width/2, height/2, b);
    hadamard(g, n-1, x + width, y,  width/2, height/2, b);
    hadamard(g, n-1, x, y + height, width/2, height/2, b);
    hadamard(g, n-1, x + width, y + height, width/2, height/2, !b);

Then when you get to the fillRect calls, choose white or black depending on the value of b.