Pintar pixel por pixel

import java.awt.Color;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.image.BufferedImage;
import java.awt.image.WritableRaster;

import javax.swing.JFrame;
import javax.swing.text.StyledEditorKit.ForegroundAction;

public class Principal extends JFrame{

	public Principal(){
		super();
		altera();
	}
	
	BufferedImage bi = new BufferedImage(200, 200, BufferedImage.TYPE_INT_RGB);
	public void altera(){
		Graphics2D g = bi.createGraphics();
		WritableRaster wr = bi.getRaster(); //começa aqui
		int r[] = new int [3];
		r[0]=255; r[1]=0; r[2]=0;
		for (int i = 0; i < 200; i++) {
			wr.setPixel(i, 75, r);	
		} //vai até aqui
		g.setColor(new Color (255, 200, 255, 200));
		g.fillRect(50, 50, 100, 100);
	}

	@Override
	public void paint(Graphics g) {
		super.paint(g);
		g.drawImage(bi, 0, 0, this);
	}

	public static void main(String[] args) {
		Principal p = new Principal();
		p.setBounds(100, 100, 200, 200);
		p.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
		p.setVisible(true);
	}
}