How can output data-url?

[cuevven]

Hi!
I have some problem, I try to output the GIF data-url, but throw an exception "fseek(): stream does not support seeking" .
My code look like this

    public function getPreview($width = 120, $height = null)
    {
        if($this->mime_type != 'image/gif'){
            $img = Image::make( asset($this->guid) )->resize($width, $height, function($constraint){
                $constraint->aspectRatio();
            });
        }else{
            $img = GluImage::get( asset($this->guid) )
                ->resize($width, $height);
        }

        return $img->encode('data-url');
    }

If I want to output the data-url, what should I do?