[FFmpeg-devel] [PATCH] avformat/mxfenc: fix DNxHD GC ULs

[Nicolas Gaullier]

>Please add a reference to the relevant SMPTE document in the comment, or perhaps at the list of references at the start of the file
>
>/Tomas

I have added the reference to ST2019-4 for "VC3 mapping", so that should be ok for generic standard files.
It seems redundant for me, but if you want, I could add the link to the online register where the container ul is public ?
https://registry.smpte-ra.org/apps/pages/

Concerning the essence key, it is more tricky because of AVID in the place...
To start with, apart AVID, all frame-wrapped samples I have (I can share them with you but not all of them publicly), do respect the standard
- frame-wrapped : ARRI, Adobe Media Encoder, Harmonic : always 0x0C ("DNxHD" frame-mapping)
There are up to date publicly available ARRI samples where 0x0C is used here:
https://www.arri.com/en/learn-help/learn-help-camera-system/camera-sample-footage

But I also have an AVID Op1a file where the value 0x05 is used ("MPEG" frame-mapping, ie. s381m).
And concerning OPAtom, Philip de Nier has an AVID sample where the value 0x06 is used ("MPEG" clip-wrapping).

So, what is apparent at the end is that :
- apart from AVID, the standard values 0x0c/0x0d are used
- AVID uses the values from the older "MPEG mapping" (ie smpte 381m)

Now :
- currently ffmpeg uses 0x05 for OPatom which does not follow any implementation and seems bad
- it seems there is a consensus (incl. AVID) to always use 0x05 or 0x0C for frame-wrapping and 0x06 or 0x0d for clip-wrapping (OPAtom) => follow either s381m or st2019-4
- it seems clear ffmpeg shall take the "standard-flavor" for generic OP's, so 0x0C for frame-based wrapping
- it is less clear about OPAtom which is rather an AVID-hack-thing, but it should be moved to either 0x06 or 0x0d
- I have discussed this with philip de nier, and bmx (a reference software in my opinion) will stick to the AVID form, so 0x06. And I think it is reasonable, since OPAtom/Avid are almost the same damn thing

Note: no matter the essence key, the link between the tracks and the body with the TrackNumber always work, so it seems there are not much interoperability issues with it.

Nicolas