How to convert 24bits png image into 8 bits image

Question

Hi,
I have gone through some of threads in this forum on changing color bits. But, I still can find right way to change my image (24bits) into different color bits(8 bits). The reason i want to change the color bits because I am getting a GIF image from map server, convert it into PNG format then rescale then into size of mobile phone screen.The PNG image is really has large data length , so the it took a while to load the whole image on screen.
Note : The following code is taken from few threads in this forum and i have modified it to suit my application.
I have no idea about Java2d...any help really appreciated...Thanks in advance!!!
My code is as follow:-
public File Scale(int width, int height, BufferedImage ImgSrc)
     try
               //BufferedImage bi = ImageIO.read(ImgSrc);
               //int w = bi.getWidth(), h = bi.getHeight();
     //Image oldRescale = bi.getScaledInstance(i, j, Image.SCALE_SMOOTH);
     BufferedImage Rescale = rescale(ImgSrc, width, height);
               //ColorSpace cs = ColorSpace.getInstance(ColorSpace.CS_GRAY);
               //ColorConvertOp op = new ColorConvertOp(cs, null);
               //Rescale = op.filter(Rescale, null);
     File outfile = new File("map.png");
               ImageIO.write(Rescale, "png", outfile);
               BufferedImage fromFile = ImageIO.read(outfile);
               ColorModel cm = fromFile.getColorModel();
               System.out.println(cm.getClass() + ", bits/pixel=" + cm.getPixelSize());
          catch(Exception e)
               e.printStackTrace();
          return new File("map.png");
     public static GraphicsConfiguration getDefaultConfiguration()
GraphicsEnvironment ge = GraphicsEnvironment.getLocalGraphicsEnvironment();
GraphicsDevice gd = ge.getDefaultScreenDevice();
return gd.getDefaultConfiguration();
     public static BufferedImage copyRescaled(BufferedImage tgt, BufferedImage src)
          Graphics2D g2 = tgt.createGraphics();
int w=tgt.getWidth(), h=tgt.getHeight();
hintForBestQuality(g2);
     g2.drawImage(src, 0, 0, w, h, null);
     g2.dispose();
return tgt;
public static Graphics2D hintForBestQuality(Graphics2D g2)
g2.setRenderingHint(RenderingHints.KEY_INTERPOLATION,
          RenderingHints.VALUE_INTERPOLATION_BILINEAR);
g2.setRenderingHint(RenderingHints.KEY_RENDERING,
          RenderingHints.VALUE_RENDER_QUALITY);
return g2;
     public static BufferedImage rescale(BufferedImage src, int w, int h)
int transparency = src.getColorModel().getTransparency();
GraphicsConfiguration gc = getDefaultConfiguration();
BufferedImage tgt = gc.createCompatibleImage(w, h, transparency);
return copyRescaled(tgt, src);

Answer 1

First, we need to be clear about what you're trying to do -- is it rescale an
image or insure that the image is saved with a 8bit pixel depth? Or both? You are also
using GraphicsConfiguration's createCompatibleImage, which is excellent if you are going
to display the image immediately because it might be hardware accelerated
or compatible with hardware format, but you mentioned mobile phone screens, so
there is no advantage to using GraphicsConfiguration in that case, and if fact, it's
probably changing your ColorModel when you're not looking.
import java.awt.*;
import java.awt.event.*;
import java.awt.geom.*;
import java.awt.image.*;
import java.io.*;
import java.net.*;
import javax.imageio.*;
import javax.swing.*;
public class Example {
    public static void main(String[] args) throws IOException {
        URL url = new URL("http://java.sun.com/developer/technicalArticles/GUI/JavaServerFaces/fig2.gif");
        BufferedImage image = ImageIO.read(url);
        System.out.println("image's colormodel = " + image.getColorModel().getClass());
        BufferedImage rescaled = scaleAndConvertColorModel(image, 0.75, null); //GIF => IndexColorModel
        System.out.println("rescaled's colormodel = " + image.getColorModel().getClass());
        File png = new File("junk.png");
        ImageIO.write(rescaled, "png", png);
        BufferedImage fromFile = ImageIO.read(png);
        System.out.println("fromFile's colormodel = " + image.getColorModel().getClass());
        JPanel cp = new JPanel(new GridLayout(0,1));
        addTo(cp, image, "original image");
        addTo(cp, rescaled, "rescaled image");
        addTo(cp, fromFile, "png file image");
        JFrame f = new JFrame("Example");
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.setContentPane(cp);
        f.pack();
        f.setLocationRelativeTo(null);
        f.setVisible(true);
        Will rescale and convert colormodel if necessary.
        ColorModel == null => don't convert. May return src if suitable.
    public static BufferedImage scaleAndConvertColorModel(BufferedImage src, double scale, ColorModel cm) {
        int w0 = src.getWidth();
        int h0 = src.getHeight();
        int w = (int) (w0 * scale);
        int h = (int) (h0 * scale);
        ColorModel cm0 = src.getColorModel();
        if (cm == null)
            cm = cm0;
        if (w==w0 && h==h0 && cm.equals(cm0))
            return src;
        BufferedImage tgt = createImage(w, h, cm);
        Graphics2D g = tgt.createGraphics();
        if (scale < 1) {
            Image temp = src.getScaledInstance(w, h, Image.SCALE_AREA_AVERAGING);
            g.drawImage(temp, null, null);
        } else {
            hintForBestQuality(g);
            g.drawRenderedImage(src, AffineTransform.getScaleInstance(scale, scale));
        g.dispose();
        return tgt;
    public static BufferedImage createImage(int w, int h, ColorModel cm) {
        if (w <= 0 || h <= 0)
            throw new IllegalArgumentException("...");
        boolean alphaPremultiplied = cm.isAlphaPremultiplied();
        WritableRaster raster = cm.createCompatibleWritableRaster(w, h);
        return new BufferedImage(cm, raster, alphaPremultiplied, null);
    public static Graphics2D hintForBestQuality(Graphics2D g) {
        g.setRenderingHint(RenderingHints.KEY_INTERPOLATION, RenderingHints.VALUE_INTERPOLATION_BICUBIC);
        g.setRenderingHint(RenderingHints.KEY_RENDERING, RenderingHints.VALUE_RENDER_QUALITY);
        return g;
    static void addTo(Container c, BufferedImage image, String title) {
        JLabel label = new JLabel(new ImageIcon(image));
        label.setBorder(BorderFactory.createTitledBorder(title));
        c.add(label);
}