imagecreatefrompng()를 사용할 때 PNG 썸네일의 투명도가 떨어지는 이유는 무엇입니까?

<code class="php">$dimg = imagecreatetruecolor($width_new, $height_new);

// Start changes for PNG transparency
switch ($stype) {

    case 'gif':
    case 'png':
        // Define black as a color
        $background = imagecolorallocate($dimg, 0, 0, 0);
        // Make black transparent
        imagecolortransparent($dimg, $background);
        // Disable blending to avoid mixing black with the image
        imagealphablending($dimg, false);
        // Enable alpha channel preservation
        imagesavealpha($dimg, true);
        break;

    default:
        break;
}
// End changes

$wm = $w/$nw;
$hm = $h/$nh;</code>