[FFmpeg-devel] [PATCH 1/1] Added avs2 tags for MKV

[Ze Yuan]

>>The ordinary versions of these players don't use libavformat's Matroska
>>demuxer at all, so I presume that your patch intends to fix the problem
>>by changing how the files are muxed. 

Right. Without avs2 tag, MKV will use AVI compatible mode automatically I think.

>>Matroska does not support AVS2 natively at all atm. If you want to
>>change that, you should open an issue/pull request at [1]. And you
>>should refrain from already creating such files (or use a CodecID like
>>"V_AVS2/EXPERIMENTAL"). As long as there is no official support for AVS2
>>in Matroska, there won't be support for it in FFmpeg.

I think we can still try. It seems HEVC was supported by many tools before MKV mapping added. Also the encoder (ffmpeg) and the popular players(ffplay/vlc/mpc-hc) all work perfectly with this change.

More information:
Standard document: 
           1857.4-2018 - IEEE Standard for Second-Generation IEEE 1857 Video Coding (https://ieeexplore.ieee.org/document/8821610)

Encoding tool:
          Ffmpeg with avs2 enabled: https://github.com/xatabhk/FFmpeg-avs2-avs3/releases/tag/v4.3.1-avs-avs3-0812
          Command line: ffmpeg -i xxxx.mp4 -vcodec avs2 -acodec copy xxxx_avs2.mkv

Video payers:
(1) Ffmpeg with avs2 enabled: (https://github.com/xatabhk/FFmpeg-avs2-avs3/releases/tag/v4.3.1-avs-avs3-0812):
      Command line: ffplay xxxx_avs2.mkv
(2) VLC 3.0.x with avs2 enabled: https://github.com/xatabhk/vlc-3.0-avs2-avs3/releases/tag/v3.0-avs2-avs3-200812
(3) Mpc-hc 1.9.x with avs2 enabled: https://gitee.com/zhengtianbo/cavs-avs2-avs3_decoder_added_to_mpc_hc/releases

Avs2-MKV samples:
       https://github.com/xatabhk/avs2-avs3-video-samples

From: Andreas Rheinhardt 
Sent: Wednesday, August 19, 2020 6:19 AM
To: ffmpeg-devel at ffmpeg.org 
Subject: Re: [FFmpeg-devel] [PATCH 1/1] Added avs2 tags for MKV

Ze Yuan:
> From d4ae49d8fabdb70435c92457c153a869dc55a980 Mon Sep 17 00:00:00 2001
> From: =?UTF-8?q?=E9=83=91=E5=A4=A9=E6=B3=A2?= <naturalwalker at hotmail.com>
> Date: Wed, 19 Aug 2020 05:20:18 +0800
> Subject: [PATCH] Added avs2 tags for MKV container.
> 
> Before the patch, an avs2 encoded MKV video results in dropped frames in asv2 enabled video players.
> 
> Here are the asv2/avs3 enabled video players:
> VLC 3: https://github.com/xatabhk/vlc-3.0-avs2-avs3/releases/tag/v3.0-avs2-avs3-200812
> Mpc-hc: https://gitee.com/zhengtianbo/cavs-avs2-avs3_decoder_added_to_mpc_hc/releases

The ordinary versions of these players don't use libavformat's Matroska
demuxer at all, so I presume that your patch intends to fix the problem
by changing how the files are muxed. This information should actually be
directly included in the commit message so that one doesn't have to guess.

> 
> ---
> libavformat/isom.c     | 3 ++-
> libavformat/matroska.c | 1 +
> 2 files changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/libavformat/isom.c b/libavformat/isom.c
> index 209bbac5d1..080a457a4e 100644
> --- a/libavformat/isom.c
> +++ b/libavformat/isom.c
> @@ -255,7 +255,8 @@ const AVCodecTag ff_codec_movvideo_tags[] = {
>      { AV_CODEC_ID_PNG,   MKTAG('M', 'N', 'G', ' ') },
> 
>      { AV_CODEC_ID_VC1, MKTAG('v', 'c', '-', '1') }, /* SMPTE RP 2025 */
> -    { AV_CODEC_ID_CAVS, MKTAG('a', 'v', 's', '2') },
> +    { AV_CODEC_ID_CAVS, MKTAG('c', 'a', 'v', 's') },
> +    { AV_CODEC_ID_AVS2, MKTAG('a', 'v', 's', '2') },

By changing ff_codec_movvideo_tags you are changing more than just
Matroska in contradiction to what the commit message states.

> 
>      { AV_CODEC_ID_DIRAC,     MKTAG('d', 'r', 'a', 'c') },
>      { AV_CODEC_ID_DNXHD,     MKTAG('A', 'V', 'd', 'n') }, /* AVID DNxHD */
> diff --git a/libavformat/matroska.c b/libavformat/matroska.c
> index 7c56aba403..2ce60c6277 100644
> --- a/libavformat/matroska.c
> +++ b/libavformat/matroska.c
> @@ -78,6 +78,7 @@ const CodecTags ff_mkv_codec_tags[]={
>      {"S_HDMV/TEXTST"    , AV_CODEC_ID_HDMV_TEXT_SUBTITLE},
> 
>      {"V_AV1"            , AV_CODEC_ID_AV1},
> +    {"V_AVS2"           , AV_CODEC_ID_AVS2},

Matroska does not support AVS2 natively at all atm. If you want to
change that, you should open an issue/pull request at [1]. And you
should refrain from already creating such files (or use a CodecID like
"V_AVS2/EXPERIMENTAL"). As long as there is no official support for AVS2
in Matroska, there won't be support for it in FFmpeg.

- Andreas

[1]: https://github.com/cellar-wg/matroska-specification/
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