doubts about Raster and BufferedImage

hi,
what is the purpose of the optionally preallocated double array in the
method getPixels of class Raster ?

public double[] getPixels(int x,int y,int w, int h,double[] dArray)

I was playing around with the api to create a BufferedImage using a
double array,andwas trying to normalise the pixel values as follows.
I expected to find the normalised values in the final array ,but it
still contains the original image data. Obviously the
this.bi.getRaster().setPixels(0,0,this.getWidth(),this.getHeight(),ndata)
doesn't do what I thought it would do.
if somebody can tell me where I messed it up ,it will be great.

thanks and regards,

mark

<code>
import java.awt.image.BufferedImage;
import java.awt.image.ColorConvertOp;
import java.io.File;
import java.io.IOException;
import javax.imageio.ImageIO;

public class MyImage {

    private BufferedImage bi;//of type BufferedImage.TYPE_BYTE_GRAY
    private String fileName;
    public MyImage(String imageFileName) throws IOException{
        this.fileName = imageFileName;
        File imageFile = new File(imageFileName);
        this.bi=makeGrayScaleImage(ImageIO.read(imageFile));
    }
    public MyImage(String imageName,int width,int height,double[] data){
        if (data.length != width*height){
            throw new IllegalArgumentException("data size must be equal to
"+width*height);
        }
        this.bi = new BufferedImage(width, height,
BufferedImage.TYPE_BYTE_GRAY);
        this.bi.getRaster().setPixels(0, 0, width, height, data);
        this.fileName = imageName;
    }
    private BufferedImage makeGrayScaleImage(BufferedImage img) {
        BufferedImage gray = null;
        try{
                  gray = new BufferedImage(img.getWidth(),img.getHeight(),
                      BufferedImage.TYPE_BYTE_GRAY);
                  ColorConvertOp ccop = new ColorConvertOp(
                      img.getColorModel().getColorSpace(),
                      gray.getColorModel().getColorSpace(),null);
                  ccop.filter(img,gray);

           }catch(Exception e){
            System.err.println("grayscale conversion failed");
                }
        return gray;
    }
    public int getWidth(){
        return this.bi.getWidth();
    }
    public int getHeight(){
        return this.bi.getHeight();
    }
    public double[] getData(){
        int h = getHeight();
        int w = getWidth();
        double[] data = new double[h*w];
        double[] pdata=this.bi.getData().getPixels(0,0,w,h,data);
        return pdata;
    }
    public void normaliseImageData(){
        double[] ndata = getNormalisedData();

this.bi.getRaster().setPixels(0,0,this.getWidth(),this.getHeight(),ndata);
    }
    private double[] getNormalisedData(){
        double[] d=getData();
        double maxval=max(d);
        for (int i=0;i<d.length;i++){
            d[i]/=maxval;
        }
        return d;
    }
    private static double max(double[] arr){
        double m=Double.MIN_VALUE;
        for(int i=0;i<arr.length;i++){
            m=Math.max(m,arr[i]);
        }
        return m;
    }
    private static void printArray(double[] a){
        for (double x:a){
            System.out.print(x+" ");
        }
        System.out.println();
    }
    private static void debug(String msg) {
        System.out.println(msg);
    }
    public static void main(String[] args) {
        MyImage my = new MyImage("dummy.png", 4, 5, new double[]
{23,32,13,55,65,36,46,64,27,43,71,58,38,25,62,47,19,72,37,55});
        double[] data = my.getData();
        debug("original image data:");
        printArray(data);
        double[] ndata = my.getNormalisedData();
        debug("normalised data:");
        printArray(ndata);
        debug("image data after normalisation:");
        double[] newdata = my.getData();
        printArray(newdata);

    }
}

</code>