// 100x100 large image
$image = new Image('/path/to/image.jpg', $imagine);

$config = (new PictureConfiguration())
    ->setSize((new PictureConfigurationItem())
        ->setResizeConfig((new ResizeConfiguration())
            ->setWidth(50)
        )
        ->setDensities('1x, 2x, 3x, 4x')
    )
;

$pictureGenerator->generate($image, $config, new ResizeOptions())->getImg('/path/to')['srcset'];
// cache/dir/4/image-de332f09.jpg 1x, image.jpg 2x, image.jpg 2x, image.jpg 2x