Why can't I use getScaledInstance to scale images?

[Howard Ting]

The statement I tried is as follows:

g.drawImage(new ImageIcon("chi_wei.jpg").getImage().getScaledInstance(100,100,Image.SCALE_DEFAULT),0,0,Color.WHITE,this);

But it ended up drawing nothing out.
Could somebody tell me if there's something wrong with the statement above or something I forget?

[Ernest Friedman-Hill]

This sounds like a Swing/AWT question to me, so I'm going to move this to the Swing/AWT forum.

[Craig Wood]

Yes — two things:
1 - You load a new image each time your paintComponent method calls drawImage. You only need to load the image one time so this line

    image = new ImageIcon("chi_wei.jpg").getImage();

belongs in your class constructor or a method called from it.

2 - in the test app below I originally put this line

                Image scaledImage = image.getScaledInstance((int)imageWidth,
                                                            (int)imageHeight,
                                                            Image.SCALE_DEFAULT);

inside paintComponent here

            case GET_SCALED_INSTANCE:
                // here . . .
                g.drawImage(scaledImage, (int)x, (int)y, this);

and the (System.out.println()) marker showed that paintComponent was being called repeatedly, so much so that the app would/could not repaint itself after being partially covered/uncovered by another window. The marker output in the console was

C:\jexp>java ImageScaling
scalingMethod = 1
scalingMethod = 0
scalingMethod = 0
scalingMethod = 0
scalingMethod = 0
. . .

suggesting that the second instance of ImageScalingPanel loaded first and the first one went spastic.

So I looked in the getScaledInstance method detail to find this line: The new Image object may be loaded asynchronously even if the original source image has already been loaded completely. Apparently the attempt to load the scaled instance inside paintComponent was causing a runaway loop.

The code below works okay. Note what happens to the image quality for scale factors over 1.0.

import java.awt.*;
import java.awt.event.*;
import java.awt.geom.*;
import java.io.*;
import java.net.*;
import javax.imageio.ImageIO;
import javax.swing.*;
 
public class ImageScaling
{
    public static void main(String[] args)
    {
        JPanel panel = new JPanel(new GridBagLayout());
        panel.setBackground(Color.white);
        GridBagConstraints gbc = new GridBagConstraints();
        gbc.insets = new Insets(5,5,5,5);
        gbc.weighty = 1.0;
        gbc.gridwidth = gbc.REMAINDER;
        Image image = loadImage();
        panel.add(new ImageScalingPanel(image, 0), gbc);
        panel.add(new ImageScalingPanel(image, 1), gbc);
        JFrame f = new JFrame();
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.getContentPane().add(new JScrollPane(panel));
        f.setSize(400,400);
        f.setLocation(200,200);
        f.setVisible(true);
    }
 
    private static Image loadImage()
    {
        String fileName = "images/redfox.jpg";
        Image image = null;
        try
        {
            URL url = ImageScaling.class.getResource(fileName);
            image = ImageIO.read(url);
        }
        catch(MalformedURLException mue)
        {
            System.err.println("url: " + mue.getMessage());
        }
        catch(IOException ioe)
        {
            System.err.println("read: " + ioe.getMessage());
        }
        return image;
    }
}
 
class ImageScalingPanel extends JPanel
{
    Image image, scaledImage;
    int scalingMethod;
    double scale;
    final int
        GET_SCALED_INSTANCE = 0,
        AFFINE_TRANSFORM    = 1;
 
    public ImageScalingPanel(Image image, int index)
    {
        this.image = image;
        scalingMethod = index;
        scale = 0.5;
        scaledImage = image.getScaledInstance((int)(scale * image.getWidth(this)),
                                              (int)(scale * image.getHeight(this)),
                                              Image.SCALE_DEFAULT);
    }
 
    protected void paintComponent(Graphics g)
    {
        super.paintComponent(g);
        Graphics2D g2 = (Graphics2D)g;
        g2.setRenderingHint(RenderingHints.KEY_INTERPOLATION,
                            RenderingHints.VALUE_INTERPOLATION_BICUBIC);
        int w = getWidth();
        int h = getHeight();
        double imageWidth = scale * image.getWidth(this);
        double imageHeight = scale * image.getHeight(this);
        double x = (w - imageWidth)/2;
        double y = (h - imageHeight)/2;
        //System.out.println("scalingMethod = " + scalingMethod);
        switch(scalingMethod)
        {
            case GET_SCALED_INSTANCE:
                g.drawImage(scaledImage, (int)x, (int)y, this);
                break;
            case AFFINE_TRANSFORM:
                AffineTransform at = AffineTransform.getTranslateInstance(x, y);
                at.scale(scale, scale);
                g2.drawImage(image, at, this);
        }
    }
 
    public Dimension getPreferredSize()
    {
        int w = (int)(scale * image.getWidth(this));
        int h = (int)(scale * image.getHeight(this));
        return new Dimension(w, h);
    }
}

[ August 10, 2004: Message edited by: Craig Wood ]

[Howard Ting]

Thanks for your reply, yet there's still the problem.
I copied the complete code to compile and run, but it can't still show the scaled picture. There was nothing in the window, just as before.
How in the world can I solve this problem? Is it because of my compiler?