Problem using RescaleOp to brighten image

Question

Hi, I'm trying to use java.awt.image.RescaleOp to brighten or darken an image. My code:
(Using java.awt.image.BufferedImage, java.awt.RenderingHints)
     protected static Image createBrightImage(Graphics g, BufferedImage bi)
          float scaleFactor = 1.3f;
          RenderingHints hints = new RenderingHints(RenderingHints.KEY_ANTIALIASING,
          RenderingHints.VALUE_ANTIALIAS_ON);
          hints.put(RenderingHints.KEY_RENDERING,
          RenderingHints.VALUE_RENDER_QUALITY);
          RescaleOp op = new RescaleOp(scaleFactor, 0, hints);
          return op.filter(bi, null);
There is no error, but when I try to display the image I don't see anything. The image I have (a .PNG) is a coloured circle on a square background; the background is 100% transparent (alpha transparency). I also tried setting hints to null (which is perfectly legal AFAIK) but no change.
It also doesn't work with a .JPG.
Any idea what I've done wrong with this filter?
I found if I create a new BufferedImage and write the image data to it (in the context of the BufferedImage) then perform this code, then the filter DOES work (and I see an image)... unfortunately the transparent background (alpha channel) was converted to a solid black background. So this technique is flawed (regardless of the fact that it shouldn't be necessary IMO).
BTW, the code for making this new image:
     BufferedImage bi = new BufferedImage(image.getWidth(null), image.getHeight(null), BufferedImage.TYPE_INT_RGB);
     java.awt.Graphics2D g2d = bi.createGraphics();
     g2d.drawImage(image, 0, 0, null);
     g2d.dispose();
Any ideas how to make this work? BTW, I've had success with the ColorSpace filter to create grayscale images...
TIA
Phil

Answer 1

import java.awt.*;
import java.awt.geom.*;
import java.awt.image.*;
import java.io.*;
import java.net.*;
import javax.imageio.*;
import javax.swing.*;
public class RescaleExample {
    public static void main(String[] args) throws IOException {
        File file = new File("junk.png");
        createPNGFile(file);
        BufferedImage image = copy(ImageIO.read(file), BufferedImage.TYPE_INT_ARGB);
        BufferedImage copy = copy(image, BufferedImage.TYPE_INT_ARGB);
        brighten(copy);
        JPanel cp = new JPanel(new GridLayout(1,2));
        URL url = new URL("http://today.java.net/jag/bio/JagHeadshot.jpg");
        cp.setBorder(new CentredBackgroundBorder(ImageIO.read(url)));
        cp.add(new JLabel(new ImageIcon(image)));
        cp.add(new JLabel(new ImageIcon(copy)));
        final JFrame f = new JFrame("RescaleExample");
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.setContentPane(cp);
        f.pack();
        SwingUtilities.invokeLater(new Runnable(){
            public void run() {
                f.setLocationRelativeTo(null);
                f.setVisible(true);
    public static void createPNGFile(File file) throws IOException {
        file.delete();
        BufferedImage bi = new BufferedImage(250,250, BufferedImage.TYPE_INT_ARGB);
        //starts out transparent
        Graphics2D g = bi.createGraphics();
        //add some dark colors, suitable for rescaling.
        g.setColor(new Color(128,0,0));
        g.fillRect(50,50,50,150);
        g.setColor(new Color(0,128,0));
        g.fillRect(100,50,50,150);
        g.setColor(new Color(0,0,128));
        g.fillRect(150,50,50,150);
        g.dispose();
        ImageIO.write(bi, "png", file);
    public static BufferedImage copy(BufferedImage bi, int type) {
        BufferedImage result = new BufferedImage(bi.getWidth(), bi.getHeight(), type);
        Graphics2D g = result.createGraphics();
        g.drawRenderedImage(bi, null);
        g.dispose();
        return result;
    public static void brighten(BufferedImage bi) {
        float scaleFactor = 1.5f;
        RenderingHints hints = new RenderingHints(RenderingHints.KEY_COLOR_RENDERING, RenderingHints.VALUE_COLOR_RENDER_QUALITY);
        RescaleOp op = new RescaleOp(scaleFactor, 0, hints);
        op.filter(bi, bi);
class CentredBackgroundBorder implements javax.swing.border.Border {
    private final BufferedImage image;
    public CentredBackgroundBorder(BufferedImage image) {
        this.image = image;
    public void paintBorder(Component c, Graphics g, int x, int y, int width, int height) {
        x += (width-image.getWidth())/2;
        y += (height-image.getHeight())/2;
        ((Graphics2D) g).drawRenderedImage(image, AffineTransform.getTranslateInstance(x,y));
    public Insets getBorderInsets(Component c) {
        return new Insets(0,0,0,0);
    public boolean isBorderOpaque() {
        return true;
Notes
1. RescaleOp seems to have problems with images that have an alpha channel. The fix I finally found
was to pass the same bufferedimage as both args to filter:op.filter(bi, bi);As the API states:
... in-place operation is allowed (i.e. the source and destination can be the same object).
Just be aware that you need to make a copy first, If you want to retain the original image.
2. Be aware of the difference between TYPE_INT_RGB and TYPE_INT_ARGB.
3. This is not an error, but realize only certain hints are relevant. Again, from the API:
If a RenderingHints object is specified in the constructor, the color rendering hint and the dithering
hint may be used when color conversion is required.