If is_letterbox is true, then the value specifies the
last line of a horizontal letterbox bar area at top of reconstructed frame.
Otherwise, it specifies the last horizontal luminance sample of a vertical pillarbox
bar area at the left 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.
0 for progressive or field 1 and 1 for field 2
if true then bar data specifies letterbox, otherwise pillarbox
parent #GstMeta
Bar data should be included in video user data whenever the rectangular picture area containing useful information does not extend to the full height or width of the coded frame and AFD alone is insufficient to describe the extent of the image.
Note: either vertical or horizontal bars are specified, but not both.
For more details, see:
https://www.atsc.org/wp-content/uploads/2015/03/a_53-Part-4-2009.pdf
and SMPTE ST2016-1