Generated thumbnails are all black when uploading - php

Question

I'm generating thumbnail images from the original jpeg files when they are being uploaded. I could actually create and move those thumbnail files to another directory, but the problem is that those thumbnail files displays only the black color while uploading.
My code.
if(isset($_POST['upload'])){
$img = $_FILES['origin']['name'];
move_uploaded_file($_FILES['origin']['tmp_name'], 'image/'.$img);
define("SOURCE", 'image/');
define("DEST", 'thumb/');
define("MAXW", 120);
define("MAXH", 90);
$jpg = SOURCE.$img;
if($jpg){
list($width, $height, $type) = getimagesize($jpg); //$type will return the type of the image
if(MAXW >= $width && MAXH >= $height){
$ratio = 1;
}elseif($width > $height){
$ratio = MAXW / $width;
}else{
$ratio = MAXH / $height;
}
$thumb_width = round($width * $ratio); //get the smaller value from cal # floor()
$thumb_height = round($height * $ratio);
$thumb = imagecreatetruecolor($thumb_width, $thumb_height);
$path = DEST.$img."_thumb.jpg";
imagejpeg($thumb, $path);
echo "<img src='".$path."' alt='".$path."' />";
}
imagedestroy($thumb);
}
and the thumbnail file looks like this:

Answer 1

From php manual:
imagecreatetruecolor() returns an image identifier representing a black image of the specified size.
So the problem is that you actually create this black image and save it.
$thumb = imagecreatetruecolor($thumb_width, $thumb_height);
For solution on resizing, please refer to this question on stackoverflow.

Answer 2

Hmm, I just found my bug right now. The problem is that I use $jpg = SOURCE.$img; instead of $jpg = imagecreatefromjpeg($jpg); and also I need to copy the sample image to the new thumbnail image using
imagecopyresampled( $thumb, $jpg, 0, 0, 0, 0, $thumb_width, $thumb_height, $width, $height );
Then it works!!!
Thanks Alex for your answer which lead me to this solution.