Why is resized PNG image so much bigger than original image?

Question

I am quite confused why PNG images that are resized using GD library are much bigger in size than the original.

This is the code I am using to resize the image:

// create image from posted file
$src = imagecreatefrompng($file['tmp_name']);
// get original size of uploaded image
list($width,$height) = getimagesize($file['tmp_name']);
if($width>$maxImgWidth) {
    // resize the image to maxImgWidth, maintain the original aspect ratio
    $newwidth = $maxImgWidth;
    $newheight=($height/$width)*$newwidth;
    $newImage=imagecreatetruecolor($newwidth,$newheight);

    // fill transparent with white
    /*$white=imagecolorallocate($newImage, 255, 255, 255); 
    imagefill($newImage, 0, 0, $white);*/

    // the following is to keep PNG's alpha channels
    // turn off transparency blending temporarily
    imagealphablending($newImage, false);
    // Fill the image with transparent color
    $color = imagecolorallocatealpha($newImage,255,255,255,127);
    imagefill($newImage, 0, 0, $color); 
    // restore transparency blending
    imagesavealpha($newImage, true);

    // do the image resizing by copying from the original into $newImage image
    imagecopyresampled($newImage,$src,0,0,0,0,$newwidth,$newheight,$width,$height);

    // write image to buffer and save in variable
    ob_start(); // Stdout --> buffer
    imagepng($newImage,NULL,5); // last parameter is compression 0-none 9-best (slow), see also http://www.php.net/manual/en/function.imagepng.php
    $newImageToSave = ob_get_contents(); // store stdout in $newImageToSave
    ob_end_clean(); // clear buffer
    // remove images from php buffer
    imagedestroy($src);
    imagedestroy($newImage);
    $resizedFlag = true;
}

Then I save $newImageToSave as blob in mysql database.

I tried to prevent alpha channel and just set white background, no significant change in file size. I tried setting the "compression" parameters (0 to 9), but still bigger then the original.

Example
I took this image (1058px*1296px) and resized it to 900px * 1102px. These are the results:

Original File: 328 KB
PNG (0): 3,79 MB
PNG (5): 564 KB
PNG (9): 503 KB

Any tip how to get the resized image smaller in file size is appreciated.

--

PS: I thought it could be bit depth, but as you can see, the example image above has 32 bits, whereas the resized image is 24 bits.

Answer 1

You don't most of the functions you are calling to reduce the image , imagefill , imagealphablending etc can result to higher file size.

To Maintain the transparent use imagecreate instead of imagecreatetruecolor and just do a simple resize

$file['tmp_name'] = "wiki.png";
$maxImgWidth = 900;
// create image from posted file
$src = imagecreatefrompng($file['tmp_name']);
// get original size of uploaded image
list($width, $height) = getimagesize($file['tmp_name']);
if ($width > $maxImgWidth) {
    $newwidth = $maxImgWidth;
    $newheight = ($height / $width) * $newwidth;
    $newImage = imagecreate($newwidth, $newheight);
    imagecopyresampled($newImage, $src, 0, 0, 0, 0, $newwidth, $newheight, $width, $height);
    imagepng($newImage, "wiki2.png", 5); 
    imagedestroy($src);
    imagedestroy($newImage);
    $resizedFlag = true;
}

Final Size : 164KB