PHP：使用GD库时图像上的黑条

我在 PHP中使用GD库有点问题 – 我调整图像大小,然后我想将其裁剪为320px(宽度)/ 240px(高度).让我说调整大小的图像是320px / 300px.当我裁剪时,图像底部会出现1-px黑条 – 我不知道为什么.
我正在使用imagecrop,imagecreatefromjpeg和imagecopyresampled

这是一个例子：

谢谢你的时间.

代码

$filename = '../store/projects/project-123.jpg';
$mime = mime_content_type($filename);
list($w,$h) = getimagesize($filename);

$prop = $w / $h;
$new_w = 0;
$new_h = 0;

if ($prop <= 4/3) {
    $new_w = 320;
    $new_h = (int)floor($h*($new_w/$w));
} else {
    $new_h = 240;
    $new_w = (int)floor($w*($new_h/$h));
}

$thumb = imagecreatetruecolor($new_w,$new_h);

if (strcmp($mime,'image/png') == 0) {
    header('Content-Type: image/png');
    $source = imagecreatefrompng($filename);
} else {
    header('Content-Type: image/jpeg');
    $source = imagecreatefromjpeg($filename);
}

imagecopyresampled($thumb,$source,$new_w,$new_h,$w,$h);

$filename = '../store/projects-thumbs/project-123.jpg';

$crop_data = array('x' => 0,'y' => 0,'width' => 320,'height'=> 240);
$thumb = imagecrop($thumb,$crop_data);

imagejpeg($thumb,$filename,100);  


imagedestroy($thumb);
imagedestroy($source);