46

I'm attempting to resize pngs with transparent backgrounds in PHP and the code samples I've found online don't work for me. Here's the code I'm using, advice will be much appreciated!

$this->image = imagecreatefrompng($filename);

imagesavealpha($this->image, true);
$newImage = imagecreatetruecolor($width, $height);

// Make a new transparent image and turn off alpha blending to keep the alpha channel
$background = imagecolorallocatealpha($newImage, 255, 255, 255, 127);
imagecolortransparent($newImage, $background);
imagealphablending($newImage, false);
imagesavealpha($newImage, true);

imagecopyresampled($newImage, $this->image, 0, 0, 0, 0, $width, $height,  $this->getWidth(), $this->getHeight());
$this->image = $newImage;  
imagepng($this->image,$filename);


Update By 'not working' I meant to say the background color changes to black when I resize pngs.

Ryan Doherty
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11 Answers11

89

From what I can tell, you need to set the blending mode to false, and the save alpha channel flag to true before you do the imagecolorallocatealpha()

<?php
/**
 * https://stackoverflow.com/a/279310/470749
 * 
 * @param resource $image
 * @param int $newWidth
 * @param int $newHeight
 * @return resource
 */
public function getImageResized($image, int $newWidth, int $newHeight) {
    $newImg = imagecreatetruecolor($newWidth, $newHeight);
    imagealphablending($newImg, false);
    imagesavealpha($newImg, true);
    $transparent = imagecolorallocatealpha($newImg, 255, 255, 255, 127);
    imagefilledrectangle($newImg, 0, 0, $newWidth, $newHeight, $transparent);
    $src_w = imagesx($image);
    $src_h = imagesy($image);
    imagecopyresampled($newImg, $image, 0, 0, 0, 0, $newWidth, $newHeight, $src_w, $src_h);
    return $newImg;
}
?>

UPDATE : This code is working only on background transparent with opacity = 0. If your image have 0 < opacity < 100 it'll be black background.

Ryan
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Dycey
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  • The answer was something completely unrelated, but this is the correct way to resize with transparency. – Ryan Doherty Nov 30 '09 at 16:28
  • This is definitely the RIGHT way to create/copy transparent PNG images. – tftd Nov 30 '11 at 22:09
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    @RyanDoherty if this **wasn't** the answer then can you provide your solution? – Steve Robbins Mar 29 '12 at 20:22
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    this does not work for me as well... background stays black. when i do imagecopyresampled it will get black again also if it was transparent by using the previous commands... – Andreas Linden Sep 24 '12 at 14:20
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    A million different methods tried from a million different answers and suggestions and this is the only one that worked. Thanks @Dycey – Onimusha Jan 26 '14 at 00:18
  • Remember that the resource keeps the transparency. So if you copy a transparent onto the image you've just resized it wont be right. I couldn't find anything on unallocating transparency so I used imagecopy on the resized resource to clear it. Just saying. – JSG Jun 20 '20 at 00:53
14

Here is a final solution that is working fine for me.

function resizePng($im, $dst_width, $dst_height) {
    $width = imagesx($im);
    $height = imagesy($im);

    $newImg = imagecreatetruecolor($dst_width, $dst_height);

    imagealphablending($newImg, false);
    imagesavealpha($newImg, true);
    $transparent = imagecolorallocatealpha($newImg, 255, 255, 255, 127);
    imagefilledrectangle($newImg, 0, 0, $width, $height, $transparent);
    imagecopyresampled($newImg, $im, 0, 0, 0, 0, $dst_width, $dst_height, $width, $height);

    return $newImg;
}
Michael
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6

using imagescale is better compared to imagecopyresampled. No empty image resource required for the resized image, requires only two arguments compared to the ten required by imagecopyresampled. Also produces better quality with smaller sizes. If using PHP 5.5.18 or earlier, or PHP 5.6.2 or earlier, you should provide the height which is the 3rd argument as the aspect ratio calculation was incorrect.

$this->image =   imagecreatefrompng($filename);
$scaled = imagescale($this->image, $width);
imagealphablending($scaled, false);
imagesavealpha($scaled, true);
imagepng($scaled, $filename);
hkiame
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  • Thanks for this. This worked for me in late 2020. I used `imagecreatetruecolor` instead of `imagecreatefrompng` and skipped the `imagescale` part. – salsbury Dec 10 '20 at 21:17
4

old thread, but just in case - Dycey's example should work, if you name things correctly. Here is a modified version used in my image resizing class. Notice the check to make sure imagecolorallocatealpha() is defined, which it won't be if you are using GD <2.0.8

  /**
     * usually when people use PNGs, it's because they need alpha channel 
     * support (that means transparency kids). So here we jump through some 
     * hoops to create a big transparent rectangle which the resampled image 
     * will be copied on top of. This will prevent GD from using its default 
     * background, which is black, and almost never correct. Why GD doesn't do 
     * this automatically, is a good question.
     *
     * @param $w int width of target image
     * @param $h int height of target image
     * @return void
     * @private
     */
    function _preallocate_transparency($w, $h) {
        if (!empty($this->filetype) && !empty($this->new_img) && $this->filetype == 'image/png')) {
            if (function_exists('imagecolorallocatealpha')) {
                imagealphablending($this->new_img, false);
                imagesavealpha($this->new_img, true);
                $transparent = imagecolorallocatealpha($this->new_img, 255, 255, 255, 127);
                imagefilledrectangle($this->new_img, 0, 0, $tw, $th, $transparent);
            }
        }
    }
sbeam
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  • tw - Target Width th - Target Height – Beniston May 18 '12 at 10:47
  • I think what he implied was: tw and th are undefined. The function parameters are called $w and $h, but imagefilledrectangle() is called with $tw and $th. That's a bug. – Gerben Oct 18 '18 at 13:20
4

The filling of the new image with a transparent colour is also required (as Dycey coded but I'm guessing forgot to mention :)), not just the 'strategic' saving by itself.

IIRC, you also need to be sure PNGs are 24bit, ie truecolor, and not 8bit to avoid buggy behaviour.

jTresidder
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4

It's probably related to the newer versions of PHP (I tested with PHP 5.6) but this now works without the need to fill the image with a transparent background:

$image_p = imagecreatetruecolor(480, 270);
imageAlphaBlending($image_p, false);
imageSaveAlpha($image_p, true);
$image = imagecreatefrompng('image_with_some_transaprency.png');
imagecopyresampled($image_p, $image, 0, 0, 0, 0, 480, 270, 1920, 1080);
imagepng($image_p, 'resized.png', 0);
Capsule
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2

this is also not working for me :( thisis my solution.. but i also get a black background and the image is not transparent

    <?php
$img_id = 153;

$source = "images/".$img_id.".png";
$source = imagecreatefrompng($source);
$o_w = imagesx($source);
$o_h = imagesy($source);

$w = 200;
$h = 200;

$newImg = imagecreatetruecolor($w, $h);
imagealphablending($newImg, false);
imagesavealpha($newImg,true);
$transparent = imagecolorallocatealpha($newImg, 255, 255, 255, 127);
imagefilledrectangle($newImg, 0, 0, $w, $h, $transparent);
imagecopyresampled($newImg, $source, 0, 0, 0, 0, $w, $h, $o_w, $o_h);

imagepng($newImg, $img_id.".png");

?>
<img src="<?php echo $img_id.".png" ?>" />
Alexandr
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2

Here is full code working for png files with preserving their images transparency..

list($width, $height) = getimagesize($filepath);
$new_width = "300";
$new_height = "100";

if($width>$new_width && $height>$new_height)
{
    $image_p = imagecreatetruecolor($new_width, $new_height);
    imagealphablending($image_p, false);
    imagesavealpha($image_p, true);
    $image = imagecreatefrompng($filepath);
    imagecopyresampled($image_p, $image, 0, 0, 0, 0, $new_width, $new_height, $width, $height);
    imagepng($image_p,$filepath,5);
}
Kaushal Sachan
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1

Full example. Notice that for some png images found on internet it works incorrect, but for my own created with photoshop it works fine.

    header('Content-Type: image/png');

$filename = "url to some image";

$newWidth = 300;
$newHeight = 300;

$imageInfo = getimagesize($filename);

$image = imagecreatefrompng($filename); //create source image resource
imagesavealpha($image, true); //saving transparency

$newImg = imagecreatetruecolor($newWidth, $newHeight); //creating conteiner for new image
imagealphablending($newImg, false);
imagesavealpha($newImg,true);
$transparent = imagecolorallocatealpha($newImg, 255, 255, 255, 127); //seting transparent background
imagefilledrectangle($newImg, 0, 0, $newWidth, $newHeight, $transparent);
imagecopyresampled($newImg, $image, 0, 0, 0, 0, $newWidth, $newHeight,  $imageInfo[0], $imageInfo[1]);

imagepng($newImg); //printout image string
Don Vaidoso
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1

You can simply resize the image with imagescale()
This code works fine for me:

$image = imagescale($image, 720, -1, IMG_BICUBIC);
imagepng($image);
Lukas Schramm
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0

Nor the above solution worked for me. This is the way what i found out to solve the issue.

// upload directory
$upload_dir = "../uploads/";
// valid image formats
$valid_formats = array("jpg", "jpeg", "png");
// maximum image size 1 mb
$max_size = 1048576;
// crop image width, height
$nw = $nh = 800;
$nw1 = $nh1 = 400;
$nw3 = $nh3 = 200;
$nw2 = $nh2 = 100;
// checks that if upload_dir a directory/not
if (is_dir($upload_dir) && is_writeable($upload_dir)) {
    // not empty file
    if (!empty($_FILES['image'])) {
        // assign file name 
        $name = $_FILES['image']['name'];
        // $_FILES to execute all files within a loop
        if ($_FILES['image']['error'] == 4) {
            $message = "Empty FIle";
        }
        if ($_FILES['image']['error'] == 0) {
            if ($_FILES['image']['size'] > $max_size) {
                echo "E-Image is too large!<br>";
                $_SESSION['alert'] = "Image is too large!!";
            } else if (!in_array(pathinfo($name, PATHINFO_EXTENSION), $valid_formats)) {
                $_SESSION['alert'] = "This image is not a valid image format!!";
                echo "E-This image is not a valid image format<br>";
            } else if (file_exists($upload_dir . $name)) {
                $_SESSION['alert'] = "Image already exists!!";
                echo "E-Image already exists<br>";
            } else { // No error found! Move uploaded files 
                $size = getimagesize($_FILES['image']['tmp_name']);
                $x = (int) $_POST['x'];
                $y = (int) $_POST['y'];
                $w = (int) $_POST['w'] ? $_POST['w'] : $size[0];
                $h = (int) $_POST['h'] ? $_POST['h'] : $size[1];
                // path for big image
                $big_image_path = $upload_dir . "big/" . $name;
                // medium image path
                $medium_image_path = $upload_dir . "medium/" . $name;
                // small image path
                $small_image_path = $upload_dir . "small/" . $name;
                // check permission
                if (!is_dir($upload_dir . "big/") && !is_writeable($upload_dir . "big/")) {
                    mkdir($upload_dir . "big/", 0777, false);
                }
                if (!is_dir($upload_dir . "medium/") && !is_writeable($upload_dir . "medium/")) {
                    mkdir($upload_dir . "medium/", 0777, false);
                }
                if (!is_dir($upload_dir . "small/") && !is_writeable($upload_dir . "small/")) {
                    mkdir($upload_dir . "small/", 0777, false);
                }
                // image raw data from form
                $data = file_get_contents($_FILES["image"]["tmp_name"]);
                // create image
                $vImg = imagecreatefromstring($data);
                //create big image
                $dstImg = imagecreatetruecolor($nw, $nh);
                imagealphablending($dstImg, false);
                $trans_colour = imagecolorallocatealpha($dstImg, 0, 0, 0, 127);
                imagefilledrectangle($dstImg, 0, 0, $w, $h, $trans_colour);
                imagesavealpha($dstImg, true);
                imagecopyresampled($dstImg, $vImg, 0, 0, $x, $y, $nw, $nh, $w, $h);
                imagepng($dstImg, $big_image_path);
                //create medium thumb
                $dstImg1 = imagecreatetruecolor($nw1, $nh1);
                imagealphablending($dstImg1, false);
                $trans_colour1 = imagecolorallocatealpha($dstImg1, 0, 0, 0, 127);
                imagefilledrectangle($dstImg1, 0, 0, $w, $h, $trans_colour1);
                imagesavealpha($dstImg1, true);
                imagecopyresampled($dstImg1, $vImg, 0, 0, $x, $y, $nw1, $nh1, $w, $h);
                imagepng($dstImg1, $medium_image_path);
                // create smallest thumb
                $dstImg2 = imagecreatetruecolor($nw2, $nh2);
                imagealphablending($dstImg2, false);
                $trans_colour2 = imagecolorallocatealpha($dstImg2, 0, 0, 0, 127);
                imagefilledrectangle($dstImg2, 0, 0, $w, $h, $trans_colour2);
                imagesavealpha($dstImg2, true);
                imagecopyresampled($dstImg2, $vImg, 0, 0, $x, $y, $nw2, $nh2, $w, $h);
                imagepng($dstImg2, $small_image_path);
                /*
                 * Database insertion
                 */
                $sql = "INSERT INTO tbl_inksand_product_gallery ("
                        . "Product_Id,Gallery_Image_Big,Gallery_Image_Medium,Gallery_Image_Thumb,"
                        . "Gallery_Status,Created_By,Created_Datetime"
                        . ") VALUES ("
                        . "'{$Product_Id}','{$big_image_path}','{$medium_image_path}','{$small_image_path}',"
                        . "'A','$Created_By','{$time}'"
                        . ")";
                db_query($sql);
                if (db_affected_rows() == 1) {
                    if (imagedestroy($dstImg)) {
                        $_SESSION['success'] = "Image uploaded successfully.";
                        echo "S-Image uploaded successfully<br>";
                    } else {
                        $_SESSION['alert'] = "Image not uploaded!!";
                        echo "S-Image not uploaded";
                    }
                } else {
                    $_SESSION['alert'] = "Error in uploading image!!";
                    echo "E-Error in uploading image!!";
                }
            }
        }
    }
} else {
    mkdir($upload_dir, 0777);
}
Shivaram Mahapatro
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