[FFmpeg-devel] [PATCH 1/4] fftools/ffmpeg_enc: strip DOVI config record for AV1

[Niklas Haas]

On Fri, 22 Mar 2024 10:41:13 +0100 Anton Khirnov <anton at khirnov.net> wrote:
> Quoting Niklas Haas (2024-03-21 13:11:32)
> > On Thu, 21 Mar 2024 11:16:57 +0100 Anton Khirnov <anton at khirnov.net> wrote:
> > > Quoting Niklas Haas (2024-03-19 20:16:39)
> > > > From: Niklas Haas <git at haasn.dev>
> > > > 
> > > > AV1 streams don't use configuration records, so delete them when
> > > > encoding to AV1. Ideally this would be, as the comment suggests, handled
> > > > at the frame-level (and stripped by the av1 encoder), but given the
> > > > status quo of copying the packet-level data here directly, we should
> > > > definitely make an effort to strip it.
> > > > ---
> > > >  fftools/ffmpeg_enc.c | 25 ++++++++++++++-----------
> > > >  1 file changed, 14 insertions(+), 11 deletions(-)
> > > 
> > > I'm very much not a fan of having codec-specific code in ffmpeg CLI. It
> > > implies that every single caller must now be aware of this
> > > (undocumented?) interaction of this specific side data with this
> > > specific codec ID.
> > 
> > Note: This is an existing bug, not introduced by this series. This
> > series just makes it obvious. The status quo is that, beacuse of this
> > logic in ffmpeg_enc.c, we incorrectly forward dolby vision configuration
> > records when transcoding to AV1.
> 
> I know pretty much nothing about dolby vision, so could you please
> explain why precisely is this incorrect? And at what point in the
> transcoding chain does the side data become invalid?

Dolby Vision basically consists of two separate pieces of metadata:

1. The (per-stream) configuration struct, AV_PKT_DATA_DOVI_CONF
2. The per-frame structs (RPUs), AV_FRAME_DATA_DOVI_METADATA
     (ditto AV_FRAME_DATA_DOVI_RPU_BUFFER, which is the same)

A valid HEVC dolby vision file should contain both - the configuration
struct tells the decoder that hey, this file is dolby vision (and what
profile to expect, whether there's an enhancement layer, etc.). The RPUs
contain the actual DV-specific details of how each frame is encoded.

A valid AV1 dolby vision file, on the other hand, only uses the
per-frame RPUs, it does not have a configuration struct at all.

The current logic in ffmpeg_enc.c copies over all stream-level metadata,
including the DOVI_CONF struct, to the output file. This generates
a stream which is *marked* as being Dolby Vision, but in which none of
the frames actually contain DV RPUs. This *probably* violates some spec
somewhere, and at the very least is not desirable behavior. (And for
AV1, the configuration struct's presence is definitely a no-go)

Basically, we want to handle all of these scenarios:

1. When transcoding DV HEVC (profile 8) to DV AV1 (profile 10), we need
   to strip the configuration struct somewhere
2. When transcoding DV HEVC (profile 8) to HEVC, we need to strip the
   configuration struct IFF we're also stripping the per-frame RPUs
   (e.g. as a result of filtering).
3. When transcoding DV AV1 (profile 10) to HEVC, we need to *synthesize*
   a configuration struct containing the correct values.

I think the best way forward for now is:

1. Always strip the dovi configuration record when transcoding
2. Have the encoder generate (and attach to avctx.coded_side_data) the
   correct configuration record.

I will write a patch for #2.

> > Or, indeed, when transcoding to *any* format - since current FFmpeg also
> > does not propagate dolby vision RPUs, we generate broken files pretty
> > much always when transcoding dolby vision. So we definitely need to
> > strip the metadata from the stream muxer *somewhere*. Where else comes
> > to mind?
> > 
> > This also gets into another topic I wanted to touch on, which is that
> > the presence of dynamic dolby vision metadata currently hinders the
> > ability of libavfilter to treat the video primaries/gamma as
> > a negotiable colorspace property (the way it is done currently for YUV
> > matrix/range). This is because when interpreted as such, DV metadata
> > fundamentally changes the colorspace of the incoming video stream.
> > Ideally we would like some way to negotiate DV metadata on the
> > query_formats() level.
> > 
> > Ideally, we'd want something like AVCOL_SPC_DOLBYVISION, but we can't
> > easily introduce that without breaking ISO/IEC 23091 compatibility..
> 
> In principle it could be yet another negotiated field, could it not? You
> just added a bunch of those recently, what's another one?

Adding more fields to this negotiation process is a very obnoxious and
tedious process, with lots of boilerplate for each new field added.
Maybe we can come up with some better mechanism first?