The spec says

9: Interlaced with bottom field displayed first and top field stored first
14: Interlaced with top field displayed first and bottom field stored first

And avcodec.h states

AV_FIELD_TB,          //< Top coded first, bottom displayed first
AV_FIELD_BT,          //< Bottom coded first, top displayed first
Signed-off-by: James Almer <jamrial@gmail.com>
Signed-off-by: Anton Khirnov <anton@khirnov.net>