Java: JPanel background not scaling

问题

I am trying to draw an image to a JPanel in java and I have that down, good. My problem now is I am attemping to make it scale to the full size of the window, but it is not doing so. I have tried methods, and they either make the image disappear, or there is no graphical change at all.

The code I am currently working with:

class ImagePanel extends JPanel {
    private static final long serialVersionUID = 1L;

    private Image img;

    public ImagePanel(String img) {
        this(new ImageIcon(img).getImage());
    }

    public ImagePanel(Image img) {
        this.img = img;
        Dimension size = new Dimension(img.getWidth(null), img.getHeight(null));
        setPreferredSize(size);
        setMinimumSize(size);
        setMaximumSize(size);
        setSize(size);
        setLayout(null);
    }

    public void paintComponent(Graphics g) {
        g.drawImage(img, 0, 0, null);
    }
}

Full class at: http://pastebin.com/CpcBt2j8

I have tried adding in code such at img = img.getScaledInstance(440, 440, Image.SCALE_SMOOTH) and that has no effect whatsoever. I have also tried making a new image called scaledImg and setting img's getScaledInstance value to that, and that completely removes the image from display.

How can I get the image to be scaled to the full size I want?

回答1:

To dynamically scale an image you use:

//g.drawImage(img, 0, 0, null);
g.drawImage(image, 0, 0, getWidth(), getHeight(), null);

See Background Panel for a complete implementation along with a tiling feature.

回答2:

I guess the "simplest" approach would be to do something like...

@Override
public void paintComponent(Graphics g) {

    super.paintComponent(g);
    Image scaled = img.getScaledInstance(getWidth(), getHeight(), Image.SCALE_SMOOTH);
    g.drawImage(scaled, 0, 0, null);

}

This is inefficient, costly and may not produce the desired results

To start with, take a look at The Perils of Image.getScaledInstance()

You could also take a look at Java: maintaining aspect ratio of JPanel background image for more details about maintaining the aspect ratio of the image and scale to fit vs scale to fill.

I would also buffer the result, so that you are not repeatedly scaling the image within the paintComponent method which might be caught often...

Updated with working example

This works fine for me...

import java.awt.BorderLayout;
import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.Graphics;
import java.awt.HeadlessException;
import java.awt.Image;
import java.io.File;
import java.io.IOException;
import javax.imageio.ImageIO;
import javax.swing.ImageIcon;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.UIManager;
import javax.swing.UnsupportedLookAndFeelException;

public class TestImagePane {

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

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

                try {
                    Image img = null;
                    img = ImageIO.read(new File("/path/to/image"));

                    JFrame frame = new JFrame("Testing");
                    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                    frame.setLayout(new BorderLayout());
                    frame.add(new ImagePanel(img));
                    frame.pack();
                    frame.setLocationRelativeTo(null);
                    frame.setVisible(true);
                } catch (IOException | HeadlessException exp) {
                    exp.printStackTrace();
                }
            }
        });
    }

    class ImagePanel extends JPanel {

        private static final long serialVersionUID = 1L;

        private Image img;
        private Image scaled;

        public ImagePanel(String img) {
            this(new ImageIcon(img).getImage());
        }

        public ImagePanel(Image img) {
            this.img = img;
        }

        @Override
        public void invalidate() {
            super.invalidate();
            int width = getWidth();
            int height = getHeight();

            if (width > 0 && height > 0) {
                scaled = img.getScaledInstance(getWidth(), getHeight(), Image.SCALE_SMOOTH);
            }
        }

        @Override
        public Dimension getPreferredSize() {
            return img == null ? new Dimension(200, 200) : new Dimension(img.getWidth(this), img.getHeight(this));
        }

        @Override
        public void paintComponent(Graphics g) {
            super.paintComponent(g);
            g.drawImage(scaled, 0, 0, null);
        }
    }

}

来源：https://stackoverflow.com/questions/19648833/java-jpanel-background-not-scaling