Timmos Timmos - 2 months ago 14
Java Question

Java Graphics.fillXxx() in combination with Graphics2D.scale()

I have this program:

import java.awt.Color;
import java.awt.Dimension;
import java.awt.Font;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.geom.Rectangle2D;

import javax.swing.JComponent;
import javax.swing.JFrame;
import javax.swing.SwingUtilities;
import javax.swing.WindowConstants;


public class TestLine {

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

@Override
public void run() {
new TestLine().start();
}
});
}

private static void start() {
JFrame frame = new JFrame();
frame.setContentPane(new CarthPanel());
frame.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
frame.pack();
frame.setVisible(true);
}

private static class CarthPanel extends JComponent {

@Override
public Dimension getPreferredSize() {
return new Dimension(200, 200);
}

@Override
protected void paintComponent(Graphics g) {
Graphics2D gg = (Graphics2D) g;

int w = gg.getClipBounds().width;
int h = gg.getClipBounds().height;
System.out.println("(w,h)=(" + w + "," + h + ")");

gg.translate(w - 1, 0); // <<< when uncommenting both lines, mirroring applies
gg.scale(-1.0, 1.0); //

paintTest(gg, w, h);
}

private static void paintTest(Graphics2D g, int w, int h) {
// black background
g.setColor(Color.black);
g.fillRect(0, 0, w, h);

// colored corners
g.setColor(Color.RED);
g.drawLine(0, 0, 10, 0);
g.drawLine(0, 0, 0, 10);
g.setColor(Color.RED);
g.drawLine(0, 199, 10, 199);
g.drawLine(0, 199, 0, 189);
g.setColor(Color.CYAN);
g.drawLine(189, 0, 199, 0);
g.drawLine(199, 0, 199, 10);
g.setColor(Color.CYAN);
g.drawLine(189, 199, 199, 199);
g.drawLine(199, 199, 199, 189);

// yellow squares
g.setColor(Color.yellow);
g.drawRect(3, 3, 10, 10);
g.fillRect(186, 3, 11, 11);
g.fillRect(3, 186, 11, 11);
g.drawRect(186, 186, 10, 10);

String chars = "ABC";
g.setFont(Font.decode("Arial 72"));
Rectangle2D rect = g.getFontMetrics().getStringBounds(chars, g);
g.drawString(chars, (int) (200 - rect.getWidth()) / 2, (int) (200 - rect.getHeight()));
}
}
}


If you run this program once with two particular lines commented in and then once with the same lines commented out (see the code), then you get to see this:

[Graphics.scale() and Graphics.translate()[1]

If you didn't spot the problem, here's a zoomed pic:

Zoomed

One would expect the image to be perfectly mirrored. This is true for all strokes and hollow shapes (= drawXxx() methods). All filled shapes however (= fillXxx() methods) are drawn exactly one pixel to the left of where I expect them; e.g. the filled yellow rectangles, and you can also notice that the black background has shifted, as seen by the white line at the right border.

Is this a bug or is this intended? I suspect that it has something to do with the difference how "width" and "height" are being handled in drawXxx() and fillXxx() methods:


  • drawRect(x,y,w,h) results in a hollow rectangle with X-axis boundaries x and x+w, so the rectangle is w+1 pixels wide.

  • fillRect(x,y,w,h) results in a filled rectangle with X-axis boundaries x and x+w-1, so the rectangle is w pixels wide.



What am I missing?

Answer

It is a bug, it is not a feature :) Surely it is not intended, the mirrored image should be perfect, there's no reason why it shouldn't.

There is a way to use BufferedImage to render on it normally, and then drawing this BufferedImage flipped, the only drawback is a little performance influence and not well looking text drawn using LCD subpixeling.

import java.awt.Color;
import java.awt.Dimension;
import java.awt.Font;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.geom.Rectangle2D;
import java.awt.image.BufferedImage;

import javax.swing.JComponent;
import javax.swing.JFrame;
import javax.swing.SwingUtilities;
import javax.swing.WindowConstants;


public class TestLine {

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

            @Override
            public void run() {
                new TestLine().start();
            }
        });
    }

    private static void start() {
        JFrame frame = new JFrame();
        frame.setContentPane(new CarthPanel());
        frame.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
        frame.pack();
        frame.setVisible(true);
    }

    private static class CarthPanel extends JComponent {

        @Override
        public Dimension getPreferredSize() {
            return new Dimension(200, 200);
        }

        @Override
        protected void paintComponent(Graphics g) {
            Graphics2D gg = (Graphics2D) g;

//            System.out.println("(w,h)=(" + w + "," + h + ")");

            gg.translate(getWidth(), 0); // <<< when uncommenting both lines, mirroring applies
            gg.scale(-1.0, 1.0);    //

            BufferedImage img = new BufferedImage(getWidth(), getHeight(), BufferedImage.TYPE_INT_RGB);

            try {
                paintTest(img.createGraphics());
            } catch (Exception e) {
                e.printStackTrace();
            }

            g.drawImage(img, 0, 0, null);
        }



        private void paintTest(Graphics2D g) {
            // black background
            g.setColor(Color.black);
            g.fillRect(0, 0, getWidth(), getHeight());

            // colored corners
            g.setColor(Color.RED);
            g.drawLine(0, 0, 10, 0);
            g.drawLine(0, 0, 0, 10);
            g.setColor(Color.RED);
            g.drawLine(0, 199, 10, 199);
            g.drawLine(0, 199, 0, 189);
            g.setColor(Color.CYAN);
            g.drawLine(189, 0, 199, 0);
            g.drawLine(199, 0, 199, 10);
            g.setColor(Color.CYAN);
            g.drawLine(189, 199, 199, 199);
            g.drawLine(199, 199, 199, 189);

            // yellow squares
            g.setColor(Color.yellow);
            g.drawRect(3, 3, 10, 10);
            g.fillRect(186, 3, 11, 11);
            g.fillRect(3, 186, 11, 11);
            g.drawRect(186, 186, 10, 10);

            String chars = "ABC";
            g.setFont(Font.decode("Arial 72"));
            Rectangle2D rect = g.getFontMetrics().getStringBounds(chars, g);
            g.drawString(chars, (int) (200 - rect.getWidth()) / 2, (int) (200 - rect.getHeight()));
        }
    }
}