converting gif image to pixels and then back to gif

My real goal is to read the values from a graph in GIF format, into some meaningful data structure but in order to get started I need to be able to read the colour of each pixel of the GIF in question.

In order to test this i want to save the segment of the GIF i am reading to file for visual analysis, but am having trouble.

after reading this post I attempted to do something similar, however my output GIF always comes out completely black.

can anyone tell me what i've misunderstood?

BufferedImage bi = Image开发者_StackOverflow中文版IO.read(new URL("http://upload.wikimedia.org/wikipedia/commons/3/36/Sunflower_as_GIF.gif"));
int x = 100;
int y = 100;
int width = 100;
int height = 100;
int[] data = grabPixels(bi, x, y, width, height);
BufferedImage img = createImage(data, width, height);
ImageIO.write(img, "gif", new File("part.gif"));

...

private int[] grabPixels(BufferedImage img, int x, int y, int width, int height)
{
    try
    {
        PixelGrabber pg = new PixelGrabber(img, x, y, width, height, true);
        pg.grabPixels();
        if ((pg.getStatus() & ImageObserver.ABORT) != 0)
            throw new RuntimeException("image fetch aborted or errored");
        return convertPixels((int[]) pg.getPixels(), width, height);
    }
    catch (InterruptedException e)
    {
        throw new RuntimeException("interrupted waiting for pixels", e);
    }
}

public int[] convertPixels(int[] pixels, int width, int height)
{
    int[] newPix = new int[width * height * 3];
    int n = 0;
    for (int j = 0; j < height; j++)
    {
        for (int i = 0; i < width; i++)
        {
            int pixel = pixels[j * width + i];
            newPix[n++] = (pixel >> 16) & 0xff;
            newPix[n++] = (pixel >> 8) & 0xff;
            newPix[n++] = (pixel) & 0xff;
        }
    }
    return newPix;
}

private BufferedImage createImage(int[] pixels, int width, int height)
{
    BufferedImage image = new BufferedImage(width, height, BufferedImage.TYPE_INT_RGB);
    WritableRaster raster = (WritableRaster) image.getData();
    raster.setPixels(0, 0, width, height, pixels);
    return image;
}

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All black sounds like zeros, as if the image hadn't loaded yet. You might check the result returned by grabPixels() or specify a timeout. Once you have a BufferedImage, you could use getRaster() and work with the WritableRaster.