[FFmpeg-user] DNxHD / MOV: what is the extra data in stsd?

[Thomas Worth]

I noticed that encoding a DNxHD encoded MOV with FFmpeg results in some
extra data in the stsd atom for the video track. This data isn't there after
encoding to other codecs, say Apple Animation/qtrle for example. At its most
basic structure, stsd is 102 bytes, but with DNxHD material is is I think
275 bytes. Furthermore, this data is apparently needed to render the video
correctly, because if I truncate stsd to 102, the resulting video is black
in the QuickTime player (it still plays in VLC, though).

This is some of what I'm seeing (as text):

ACLRACLR0001
APRGAPRG0001
ARESARES0001

What is this data and what is referencing it? I have some additional stsd
parameters I need to insert, but I don't know if putting stuff in before the
"DNxHD extra data" (what we'll call it) will confuse whatever is looking for
this extra data.

Baptiste?