Drawing multiple lines in a BufferedImage

Question

I am trying to draw horizontal and vertical lines on a bufferedimage. It should end up looking like a grid of cells. But when I run the code, I see only two lines: the leftmost line and the topmost line (ie. a line from 0,0 to 0,height of image & 0,0 to width of image,0) Heres the code snippet:

  BufferedImage mazeImage = new BufferedImage(imgDim.width, imgDim.height, BufferedImage.TYPE_INT_RGB);
  Graphics2D g2d = mazeImage.createGraphics();
  g2d.setBackground(Color.WHITE);
  g2d.fillRect(0, 0, imgDim.width, imgDim.height);
  g2d.setColor(Color.BLACK);
  BasicStroke bs = new BasicStroke(2);
  g2d.setStroke(bs);
  // draw the black vertical and horizontal lines
  for(int i=0;i<21;i++){
   g2d.drawLine((imgDim.width+2)*i, 0, (imgDim.width+2)*i,  imgDim.height-1);
   g2d.drawLine(0, (imgDim.height+2)*i, imgDim.width-1, (imgDim.height+2)*i);
  }

And the overriden paint method:

public void paint(Graphics g) {
  g.drawImage(mazeImage, 0, 0, this);
}

This is all in a class called RobotMaze that extends JPanel. Any help is appreciated.

Answer 1

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

class GridLines {

    public static void main(String[] args) {

        Dimension imgDim = new Dimension(200,200);
        BufferedImage mazeImage = new BufferedImage(imgDim.width, imgDim.height, BufferedImage.TYPE_INT_RGB);


        Graphics2D g2d = mazeImage.createGraphics();
        g2d.setBackground(Color.WHITE);
        g2d.fillRect(0, 0, imgDim.width, imgDim.height);
        g2d.setColor(Color.BLACK);
        BasicStroke bs = new BasicStroke(2);
        g2d.setStroke(bs);
        // draw the black vertical and horizontal lines
        for(int i=0;i<21;i++){
            // unless divided by some factor, these lines were being
            // drawn outside the bound of the image..
            g2d.drawLine((imgDim.width+2)/20*i, 0, (imgDim.width+2)/20*i,imgDim.height-1);
            g2d.drawLine(0, (imgDim.height+2)/20*i, imgDim.width-1, (imgDim.height+2)/20*i);
        }

        ImageIcon ii = new ImageIcon(mazeImage);
        JOptionPane.showMessageDialog(null, ii);
    }
}

Answer 2

Print out your coordinates and you will see that you are plotting points outside the width and height of the image:

System.out.printf("Vertical: (%d,%d)->(%d,%d)\n",(imgDim.width+2)*i, 0, (imgDim.width+2)*i,  imgDim.height-1);
System.out.printf("Horizontal: (%d,%d)->(%d,%d)\n",0, (imgDim.height+2)*i, imgDim.width-1, (imgDim.height+2)*i);

Answer 3

How do you expect the result of (imgDim.width+2)*i to be within the image boundaries if i > 0?