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How to draw a transparent background?

I am trying to make the JPanel part transparent, but I cannot get it to work. Can this be done?

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

public class ClearPanel extends JPanel{
public static void main(String[] args) {
    ClearPanel c = new ClearPanel();
    c.setPreferredSize(new Dimension(200, 200));
    c.setOpaque(false);

    JPanel backPanel = new JPanel();
    backPanel.setBackground(Color.CYAN);

    backPanel.add(c);

    JFrame f = new JFrame();
    f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    f.setContentPane(backPanel);
    f.pack();
    f.setLocationRelativeTo(null);
    f.setVisible(true);
}

@Override
protected void paintComponent(Graphics g) {
    super.paintComponent(g);
    g.fillOval(0, 0, 200, 200);
    g.clearRect(45, 45, 50, 50);

    Graphics2D g2 = (Graphics2D) g;
    g2.setComposite(AlphaComposite.getInstance(AlphaComposite.SRC_OVER, 0.0f));
    g2.fillRect(75, 75, 50, 50);
}
}

The oval should be opaque, but the rectangles that I would like to be transparent. By transparent, I mean that I should see the panel behind ClearPanel.

Starting from MadProgrammer, answer, is there a way to make this gray square where it is outside the area, but to remain transparent where it is in this area?

    @Override
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        Rectangle fill = new Rectangle(getWidth(), getHeight());
        Graphics2D g2d = (Graphics2D) g.create();

        Rectangle hole = new Rectangle(0, 0, 100, 100);

        Area area = new Area(fill);
        area.subtract(new Area(hole));
        g2d.setColor(getBackground());
        g2d.fill(area);

        g2d.setColor(Color.RED);
        g2d.setComposite(AlphaComposite.SrcOver.derive(0.0f));
        g2d.fill(hole);

        g2d.setComposite(AlphaComposite.SrcOver.derive(1.0f));
        g2d.setColor(Color.DARK_GRAY);
        if(area.contains(0,0,100,200))
            g2d.fillRect(0, 0, 100, 200);

        g2d.dispose();      
    }
+1
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1 answer

You have a problem, by default it is JPanelopaque, which means that repainting will NOT draw anything under it.

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enter image description here

import java.awt.AlphaComposite;
import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Rectangle;
import java.awt.geom.Area;
import java.awt.geom.Ellipse2D;
import java.awt.image.BufferedImage;
import java.io.File;
import java.io.IOException;
import java.util.logging.Level;
import java.util.logging.Logger;
import javax.imageio.ImageIO;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.UIManager;
import javax.swing.UnsupportedLookAndFeelException;

public class TransparentPane {

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

    public TransparentPane() {
        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                try {
                    UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
                } catch (ClassNotFoundException ex) {
                } catch (InstantiationException ex) {
                } catch (IllegalAccessException ex) {
                } catch (UnsupportedLookAndFeelException ex) {
                }

                BackgroundPane backgroundPane = new BackgroundPane();
                backgroundPane.setBackground(Color.RED);
                backgroundPane.setLayout(new BorderLayout());
                backgroundPane.add(new TranslucentPane());

                JFrame frame = new JFrame("Test");
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.setLayout(new BorderLayout());
                frame.add(backgroundPane);
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }
        });
    }

    public class BackgroundPane extends JPanel {

        private BufferedImage bg;

        public BackgroundPane() {
            try {
                bg = ImageIO.read(new File("/path/to/your/image.jpg"));
            } catch (IOException ex) {
                ex.printStackTrace();
            }
        }

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

        @Override
        protected void paintComponent(Graphics g) {
            super.paintComponent(g);
            if (bg != null) {
                int width = getWidth() - 1;
                int height = getHeight() - 1;
                int x = (width - bg.getWidth()) / 2;
                int y = (height - bg.getHeight()) / 2;
                g.drawImage(bg, x, y, this);
            }
        }

    }

    public class TranslucentPane extends JPanel {

        public TranslucentPane() {
            setOpaque(false);
        }

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

        @Override
        protected void paintComponent(Graphics g) {
            super.paintComponent(g);
            Rectangle fill = new Rectangle(getWidth(), getHeight());
            Graphics2D g2d = (Graphics2D) g.create();

            int width = getWidth() - 1;
            int height = getHeight() - 1;
            int radius = Math.min(width, height) / 2;
            int x = (width - radius) / 2;
            int y = (height - radius) / 2;

            Ellipse2D hole = new Ellipse2D.Float(x, y, radius, radius);

            Area area = new Area(fill);
            area.subtract(new Area(hole));
            g2d.setColor(getBackground());
            g2d.fill(area);

            g2d.setColor(Color.RED);
            g2d.setComposite(AlphaComposite.SrcOver.derive(0.25f));
            g2d.fill(hole);

            g2d.dispose();
        }

    }

}

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enter image description here

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import java.awt.AlphaComposite;
import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Rectangle;
import java.awt.geom.Area;
import java.awt.geom.Ellipse2D;
import java.awt.image.BufferedImage;
import java.io.File;
import java.io.IOException;
import java.util.logging.Level;
import java.util.logging.Logger;
import javax.imageio.ImageIO;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.UIManager;
import javax.swing.UnsupportedLookAndFeelException;

public class TransparentPane {

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

    public TransparentPane() {
        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                try {
                    UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
                } catch (ClassNotFoundException ex) {
                } catch (InstantiationException ex) {
                } catch (IllegalAccessException ex) {
                } catch (UnsupportedLookAndFeelException ex) {
                }

                BackgroundPane backgroundPane = new BackgroundPane();
                backgroundPane.setBackground(Color.RED);
                backgroundPane.setLayout(new BorderLayout());
                backgroundPane.add(new TranslucentPane());

                JFrame frame = new JFrame("Test");
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.setLayout(new BorderLayout());
                frame.add(backgroundPane);
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }
        });
    }

    public class BackgroundPane extends JPanel {

        private BufferedImage bg;

        public BackgroundPane() {
            try {
                bg = ImageIO.read(new File("/Users/swhitehead/Dropbox/MegaTokyo/Evil_Small.jpg"));
            } catch (IOException ex) {
                ex.printStackTrace();
            }
        }

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

        @Override
        protected void paintComponent(Graphics g) {
            super.paintComponent(g);
            if (bg != null) {
                int width = getWidth() - 1;
                int height = getHeight() - 1;
                int x = (width - bg.getWidth()) / 2;
                int y = (height - bg.getHeight()) / 2;
                g.drawImage(bg, x, y, this);
            }
        }

    }

    public class TranslucentPane extends JPanel {

        public TranslucentPane() {
            setOpaque(false);
        }

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

        @Override
        protected void paintComponent(Graphics g) {
            super.paintComponent(g);
            Rectangle fill = new Rectangle(getWidth(), getHeight());
            Graphics2D g2d = (Graphics2D) g.create();

            int width = getWidth() - 1;
            int height = getHeight() - 1;
            int radius = Math.min(width, height) / 2;
            int x = (width - radius) / 2;
            int y = (height - radius) / 2;

            Ellipse2D hole = new Ellipse2D.Float(x, y, radius, radius);

            Area area = new Area(fill);
            area.subtract(new Area(hole));
            g2d.setColor(getBackground());
            g2d.fill(area);

            g2d.setColor(Color.RED);
            g2d.setComposite(AlphaComposite.SrcOver.derive(0.0f));
            g2d.fill(hole);
            g2d.dispose();

            g2d = (Graphics2D) g.create();

            // Basically, we create an area that is subtraction of the window/rectangle
            // from the whole.  This leaves us with a rectangle (with a hole in it)
            // that doesn't include the area where the whole is...
            Rectangle win = new Rectangle(
                            x + (radius / 2), 
                            y + (radius / 2), radius, (radius / 4));
            area = new Area(win);
            area.subtract(new Area(hole));

            // Then we create a area that is a subtraction of the original rectangle
            // from the one with a "hole" in it...
            Area actual = new Area(win);
            actual.subtract(area);
            g2d.setColor(Color.BLUE);
            g2d.setComposite(AlphaComposite.SrcOver.derive(0.5f));
            g2d.fill(actual);

            g2d.dispose();
        }        
    }
}
+5

Source: https://habr.com/ru/post/1625162/

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