toJSON(obj) {
        var json = Object3DSerializer.prototype.toJSON.call(this, obj);

        json.center = obj.center;
        json.material = new MaterialsSerializer().toJSON(obj.material);
        json.z = obj.z;
        json.isSprite = obj.isSprite;

        return json;
    }
SpriteSerializer
@author tengge / https://github.com/tengge1