jcrop正在保存图像,这只是黑色背景

I am using jcrop to crop and save image but what i get is black image , what is the problem in below code:

Second :: also it navigate to some other page , i dont want navigation . how to do that ?

<?php

/**
 * Jcrop image cropping plugin for jQuery
 * Example cropping script
 * @copyright 2008-2009 Kelly Hallman
 * More info: http://deepliquid.com/content/Jcrop_Implementation_Theory.html
 */

if ($_SERVER['REQUEST_METHOD'] == 'POST')
{
    $targ_w = $targ_h = 150;
    $jpeg_quality = 90;

    $src = 'demo_files/pool.jpg';
    $img_r = imagecreatefromjpeg($src);
    $dst_r = ImageCreateTrueColor( $targ_w, $targ_h );

    imagecopyresampled($dst_r,$img_r,0,0,$_POST['x'],$_POST['y'],
    $targ_w,$targ_h,$_POST['w'],$_POST['h']);


    imagejpeg($dst_r,'null.jpg',$jpeg_quality);

    exit;
}

// If not a POST request, display page below:

?>

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>Untitled Document</title>
        <script src="../js/jquery.min.js"></script>
        <script src="../js/jquery.Jcrop.js"></script>
        <link rel="stylesheet" href="../css/jquery.Jcrop.css" type="text/css" />
        <script language="Javascript">

            $(function(){

                $('#cropbox').Jcrop({
                    aspectRatio: 1,
                    onSelect: updateCoords
                });

            });

            function updateCoords(c)
            {
                $('#x').val(c.x);
                $('#y').val(c.y);
                $('#w').val(c.w);
                $('#h').val(c.h);
            };

            function checkCoords()
            {
                if (parseInt($('#w').val())) return true;
                alert('Please select a crop region then press submit.');
                return false;
            };

        </script>

</head>

<body>
<!-- This is the image we're attaching Jcrop to -->
        <img src="demo_files/pool.jpg" id="cropbox" />

        <!-- This is the form that our event handler fills -->
        <form action="crop.php" method="post" onsubmit="return checkCoords();">
            <input type="hidden" id="x" name="x" />
            <input type="hidden" id="y" name="y" />
            <input type="hidden" id="w" name="w" />
            <input type="hidden" id="h" name="h" />
            <input type="submit" value="Crop Image" />
        </form>
</body>
</html>

I solved by changing it to following code,

<?php


if ($_SERVER['REQUEST_METHOD'] == 'POST')
{
    $targ_w = $targ_h = 150;
    $jpeg_quality = 90;

    $src = 'demo_files/pool.jpg';
    $img_r = imagecreatefromjpeg($src);
    $dst_r = ImageCreateTrueColor( $targ_w, $targ_h );

    imagecopyresampled($dst_r,$img_r,0,0,$_POST['x'],$_POST['y'],
    $targ_w,$targ_h,$_POST['w'],$_POST['h']);


    imagejpeg($dst_r,'hello.jpg',$jpeg_quality);

    exit;
}

// If not a POST request, display page below:

?>