Prevent drawing over the same area of a Graphics2D

Question

Language: Java.

Hi, I need to prevent drawing over the same location of a Graphics2D more than once. For example, if the user draws using a BasicStroke with a round cap and a width of 10 over a certain area, that same area cannot be drawn on a second time.

The reason I want to do this is so that the user can draw (free-hand) translucent colours over an image without drawing over the same stroke (thus increasing the density of the colour and reducing its translucency).

I've tried storing the shapes of all the strokes made by the user (as Area objects that subtract the shape) and then clipping the Graphics2D by the intersection of all those Area objects.

This almost works, but the 'shape' obtained by the clip is not quite the same as the 'shape' drawn by the stroke - it is out by a couple of pixels.

Does anyone have any other ideas that might work?

Answer 1

The concept is relatively simple, you need to have multiple layers onto which you can render...

There are multiple different ways to approach the problem. You could maintain a list of Points and on each paint cycle, render these points to a backing buffer, which you would then draw over the main content using a AlphaComposite.

You could (as this example does) draw directly to the backing buffer and repaint the content, again, using a AlphaComposite to render the higher layer.

You could have any number of layers...

import java.awt.AlphaComposite;
import java.awt.BasicStroke;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Point;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import java.awt.geom.Line2D;
import java.awt.image.BufferedImage;
import javax.imageio.ImageIO;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.UIManager;

public class PaintOver {

    public static void main(String[] args) {
        new PaintOver();
    }

    public PaintOver() {
        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                try {
                    UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
                } catch (Exception ex) {
                }

                JFrame frame = new JFrame("Test");
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.add(new MapPane());
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);

            }
        });
    }

    public class MapPane extends JPanel {

        private BufferedImage background;
        private BufferedImage foreground;

        public MapPane() {
            try {
                background = ImageIO.read(getClass().getResource("/TreasureMap.png"));
                foreground = new BufferedImage(background.getWidth(), background.getHeight(), BufferedImage.TYPE_INT_ARGB);
            } catch (Exception e) {
                e.printStackTrace();
            }

            MouseAdapter mouseHandler = new MouseAdapter() {
                private Point startPoint;

                @Override
                public void mousePressed(MouseEvent e) {
                    startPoint = e.getPoint();
                }

                @Override
                public void mouseReleased(MouseEvent e) {
                    startPoint = null;
                }

                @Override
                public void mouseDragged(MouseEvent e) {
                    Point endPoint = e.getPoint();
                    Graphics2D g2d = foreground.createGraphics();

                    Point offset = getOffset();

                    Point from = new Point(startPoint);
                    from.translate(-offset.x, -offset.y);
                    Point to = new Point(endPoint);
                    to.translate(-offset.x, -offset.y);
                    g2d.setColor(Color.RED);
                    g2d.setStroke(new BasicStroke(4, BasicStroke.CAP_ROUND, BasicStroke.JOIN_ROUND));
                    g2d.draw(new Line2D.Float(from, to));
                    g2d.dispose();
                    startPoint = endPoint;
                    repaint();
                }
            };

            addMouseListener(mouseHandler);
            addMouseMotionListener(mouseHandler);

        }

        @Override
        public Dimension getPreferredSize() {
            return background == null ? super.getPreferredSize() : new Dimension(background.getWidth(), background.getHeight());
        }

        protected Point getOffset() {
            Point p = new Point();
            if (background != null) {
                p.x = (getWidth() - background.getWidth()) / 2;
                p.y = (getHeight() - background.getHeight()) / 2;
            }
            return p;
        }

        @Override
        protected void paintComponent(Graphics g) {
            super.paintComponent(g);
            if (background != null) {
                Graphics2D g2d = (Graphics2D) g.create();
                Point offset = getOffset();

                g2d.drawImage(background, offset.x, offset.y, this);
                g2d.setComposite(AlphaComposite.SrcOver.derive(0.5f));
                g2d.drawImage(foreground, offset.x, offset.y, this);
                g2d.dispose();
            }
        }
    }
}