private void extractImageData( DataInputStream is )
        throws IOException, AWTException
    {
        switch ( biBitCount )
        {
        case 1:
            throw new AWTException("Unhandled bits/pixel: " + biBitCount);
        case 4:
            extract4BitData(is); 
            break;
        case 8:  
            extract8BitData(is); 
            break;
        case 24:
            throw new AWTException("24-bit is Unhandled bits/pixel: " + biBitCount);
        default:
            throw new AWTException("Invalid bits per pixel: " + biBitCount);
        }
    }