https://bugzilla.gnome.org/show_bug.cgi?id=754567
/**
* gst_video_calculate_display_ratio:
- * @dar_n: Numerator of the calculated display_ratio
- * @dar_d: Denominator of the calculated display_ratio
+ * @dar_n: (out): Numerator of the calculated display_ratio
+ * @dar_d: (out): Denominator of the calculated display_ratio
* @video_width: Width of the video frame in pixels
* @video_height: Height of the video frame in pixels
* @video_par_n: Numerator of the pixel aspect ratio of the input video.