If @is_letterbox is true, then the value specifies the
first line of a horizontal letterbox bar area at bottom of reconstructed frame.
Otherwise, it specifies the first horizontal
luminance sample of a vertical pillarbox bar area at the right side of the reconstructed frame.
If @is_letterbox is true, then the value specifies the first line of a horizontal letterbox bar area at bottom of reconstructed frame. Otherwise, it specifies the first horizontal luminance sample of a vertical pillarbox bar area at the right side of the reconstructed frame.