Data Loss in JPEG/PNG resizing - php

Question

I've adapted this code that I found. It resizes .jpg and .png images, and maintains alpha layers.
When I resize an image, however, the reduction in file size is far more than I had anticipated. This isn't a problem for me, as I cannot see with my eyes any degradation or data loss.
What is causing the huge compression of the file, and if I ever needed to, how can I avoid it?
function thumbnail($image, $width, $height, $target) {
if($image[0] != "/") { // Decide where to look for the image if a full path is not given
if(!isset($_SERVER["HTTP_REFERER"])) { // Try to find image if accessed directly from this script in a browser
$image = $_SERVER["DOCUMENT_ROOT"].implode("/", (explode('/', $_SERVER["PHP_SELF"], -1)))."/".$image;
} else {
$image = implode("/", (explode('/', $_SERVER["HTTP_REFERER"], -1)))."/".$image;
}
} else {
$image = $_SERVER["DOCUMENT_ROOT"].$image;
}
$image_properties = getimagesize($image);
$image_width = $image_properties[0];
$image_height = $image_properties[1];
$image_ratio = $image_width / $image_height;
$type = $image_properties["mime"];
if(!$width && !$height) {
$width = $image_width;
$height = $image_height;
}
if(!$width) {
$width = round($height * $image_ratio);
}
if(!$height) {
$height = round($width / $image_ratio);
}
if($type == "image/jpeg") {
header('Content-type: image/jpeg');
$thumb = imagecreatefromjpeg($image);
} elseif($type == "image/png") {
header('Content-type: image/png');
$thumb = imagecreatefrompng($image);
} else {
return false;
}
$temp_image = imagecreatetruecolor($width, $height);
imagealphablending($temp_image, false);
imagesavealpha($temp_image,true);
$transparent = imagecolorallocatealpha($temp_image, 255, 255, 255, 127);
imagefilledrectangle($temp_image, 0, 0, $nWidth, $nHeight, $transparent);
imagecopyresampled($temp_image, $thumb, 0, 0, 0, 0, $width, $height, $image_width, $image_height);
//$thumbnail = imagecreatetruecolor($width, $height);
//imagecopyresampled($thumbnail, $temp_image, 0, 0, 0, 0, $width, $height, $width, $height);
if($type == "image/jpeg") {
imagejpeg($temp_image, 'img/'.$target.'.jpg');
} else {
imagepng($temp_image,'img/'.$target.'.png');
}
imagedestroy($temp_image);
//imagedestroy($thumbnail);
}

Answer 1

As showdev commented, both imagejpeg() and imagepng() accept a third optional parameter for the image quality. For imagejpeg() it runs from 0 to 100 and defaults to 75), for imagepng() it is from 0 to 9 and defaults to 6.
Also, 4.4Mb is a really big size for a ~2.4Mpx image. It problably contains a lot of metadata and most of it -like Photoshop thumbsnails- is not preserved when you do an imagecopyresampled() with PHP's GD library.
You should also run one of your test images through a lossless compression program like JStrip (http://davidcrowell.com/jstrip/), to check how much it weights without the bloat.