virtual Frame video_length () const = 0;
/** @return video sample aspect ratio (if known); must not be called if has_video() == false */
virtual boost::optional<double> sample_aspect_ratio () const {
virtual Frame video_length () const = 0;
/** @return video sample aspect ratio (if known); must not be called if has_video() == false */
virtual boost::optional<double> sample_aspect_ratio () const {