Comment redimensionner pngs avec transparence en PHP?

Voici une solution qui fonctionne bien pour moi.

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;
}

le remplissage de la nouvelle image avec une couleur transparente est également nécessaire (comme dycey codé mais je devine oublié de mentionner :)), pas seulement l'économie "stratégique" par lui-même.

IIRC, vous devez également être sûr PNGs sont 24bit, i.e. truecolor, et non 8bit pour éviter un comportement buggy.

vieux fil, mais juste au cas où-l'exemple de Dycey devrait fonctionner, si vous nommez les choses correctement. Voici une version modifiée utilisée dans ma classe de redimensionnement d'image. Notez le contrôle pour vous assurer que imagecolorallocatealpha() est défini, ce qui ne sera pas le cas si vous utilisez 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);
            }
        }
    }

il est probablement lié aux nouvelles versions de PHP (j'ai testé avec PHP 5.6) mais cela fonctionne maintenant sans la nécessité de remplir l'image avec un fond transparent:

$image_p = imagecreatetruecolor(480, 270);
imageAlphaBlending($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);

ça ne marche pas non plus pour moi. :( c'est ma solution.. mais j'ai aussi un fond noir et l'image n'est pas transparente

    <?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" ?>" />

exemple complet. Notez que pour certaines images png trouvées sur internet il fonctionne incorrectement, mais pour mon propre créé avec photoshop il fonctionne très bien.

    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

voici le code complet qui fonctionne pour les fichiers png avec la préservation de la transparence de leurs images..

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);
}

ni la solution ci-dessus n'a fonctionné pour moi. C'est le moyen que j'ai trouvé pour résoudre le problème.

// 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);
}