缩放图像（做大）用GD库添加抗锯齿

I have a piece of code that takes an image as input and does a resize (scaling up, making it bigger). Input image does not contain any anti-alias, it means that it has sharp borders and corners. I want this scaling procedure to add anti-alias to have smooth borders and edges, like some graphic programs do (for example paint.NET, if I make the image bigger, anti-alias is automatically added). How can I do that?

Here is my code

private function resize($source_path, $dest_path, $dest_w, $dest_h)
{
    $source = imagecreatefrompng($source_path);
    $source_w = imagesx($source);
    $source_h = imagesy($source);

    $output = imagecreatetruecolor($dest_w, $dest_h);
    imagealphablending($output, false);
    $transparent = imagecolorallocatealpha($output, 0, 0, 0, 127);
    imagefilledrectangle($output, 0, 0, $dest_w, $dest_h, $transparent);
    imagesavealpha($output, true);

    if (imagecopyresampled($output, $source, 0, 0, 0, 0, $dest_w, $dest_h, $source_w, $source_h)) {
        if (imagepng($output, $dest_path)) {
            return true;
        }
    }

    return false;
}

Thank you for any help.

EDIT: i tried to add interpolation

imagesetinterpolation($output,IMG_BICUBIC);

but nothing changed.