video_image:
    providers:
        - sonata.media.provider.image
    formats:
        medium: { width: 1306, quality: 100 }

<?php

namespace AppBundleResizer;

use GaufretteFile;
use ImagineGdImage;
use SonataMediaBundleModelMediaInterface;
use SonataMediaBundleResizerSimpleResizer;

class PngCustomResizer extends SimpleResizer
{
    public function resize(MediaInterface $media, File $in, File $out, $format, array $settings)
    {
        /** @var Image $image */
        $image = $this->adapter->load($in->getContent());

        $thumbnail = $image->thumbnail($this->getBox($media, $settings), $this->mode);

        $resource = $thumbnail->getGdResource();

        $width = $thumbnail->getSize()->getWidth();
        $height = $thumbnail->getSize()->getHeight();

        // convert to png8 with alpha
        $img = imagecreatetruecolor($width, $height);
        $bga = imagecolorallocatealpha($img, 0, 0, 0, 127);
        imagecolortransparent($img, $bga);
        imagefill($img, 0, 0, $bga);
        imagecopy($img, $resource, 0, 0, 0, 0, $width, $height);
        imagetruecolortopalette($img, false, 255);
        imagealphablending($img, false);
        imagesavealpha($img, true);

        $optimizedImage = new Image($img, $image->palette(), $image->metadata());

        // set quality 0 to set png compression = 9
        $content = $optimizedImage->get($format, ['quality' => 0]);

        $out->setContent($content, $this->metadata->get($media, $out->getName()));
    }
}