Re: [FFmpeg-devel] [PATCH] avformat/mpegts: Add AVFMT_FLAG_ALLOW_CODEC_CHANGES

[Michael Niedermayer <michael@niedermayer.cc>]

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Hi

On Sun, Jun 22, 2025 at 12:10:30PM -0600, Pavel Koshevoy wrote:
> Make midstream AVStream.codecpar updates optional and disabled
> by default, so that avformat API clients can enable this feature
> explicitly when they add support for midstream codec changes.
> ---
>  doc/APIchanges              | 3 +++
>  doc/formats.texi            | 5 +++++
>  libavformat/avformat.h      | 2 ++
>  libavformat/mpegts.c        | 4 +++-
>  libavformat/options_table.h | 1 +
>  libavformat/version.h       | 2 +-
>  tests/fate/demux.mak        | 2 +-
>  7 files changed, 16 insertions(+), 3 deletions(-)
> 
> diff --git a/doc/APIchanges b/doc/APIchanges
> index 91710bb27d..43172fbcdd 100644
> --- a/doc/APIchanges
> +++ b/doc/APIchanges
> @@ -2,6 +2,9 @@ The last version increases of all libraries were on 2025-03-28
>  
>  API changes, most recent first:
>  
> +2025-06-17 - xxxxxxxxxx - lavf 62.2.100 - avformat.h
> +  Add AVFMT_FLAG_ALLOW_CODEC_CHANGES flag.
> +
>  2025-05-21 - xxxxxxxxxx - lavu 60.3.100 - avassert.h
>    Add av_unreachable() and av_assume() macros.
>  
> diff --git a/doc/formats.texi b/doc/formats.texi
> index 876a9e92b3..5a0d070247 100644
> --- a/doc/formats.texi
> +++ b/doc/formats.texi
> @@ -39,6 +39,11 @@ Set format flags. Some are implemented for a limited number of formats.
>  
>  Possible values for input files:
>  @table @samp
> +@item allow_codec_changes
> +Allow AVStream.codecpar to change midstream if input changes
> +(for example when MPEG-TS ES stream_type changes).
> +This is disabled by default, because most clients of avformat API
> +do not support random midstream codec changes.
>  @item discardcorrupt
>  Discard corrupted packets.
>  @item fastseek

should this document the relation to resolution & pixfmt changes ?


> diff --git a/libavformat/avformat.h b/libavformat/avformat.h
> index b6c63e2237..2e5232c96d 100644
> --- a/libavformat/avformat.h
> +++ b/libavformat/avformat.h
> @@ -1436,6 +1436,8 @@ typedef struct AVFormatContext {
>  #define AVFMT_FLAG_FAST_SEEK   0x80000 ///< Enable fast, but inaccurate seeks for some formats
>  #define AVFMT_FLAG_AUTO_BSF   0x200000 ///< Add bitstream filters as requested by the muxer
>  
> +#define AVFMT_FLAG_ALLOW_CODEC_CHANGES 0x400000 ///< Allow AVStream.codecpar to be updated midstream if input changes (e.g. MPEG-TS ES stream_type changes)
> +
>      /**
>       * Maximum number of bytes read from input in order to determine stream
>       * properties. Used when reading the global header and in
> diff --git a/libavformat/mpegts.c b/libavformat/mpegts.c
> index deb69a0548..ed4ff580e5 100644
> --- a/libavformat/mpegts.c
> +++ b/libavformat/mpegts.c
> @@ -2510,7 +2510,9 @@ static void pmt_cb(MpegTSFilter *filter, const uint8_t *section, int section_len
>          if (!st)
>              goto out;
>  
> -        if (pes && pes->stream_type != stream_type)
> +        if (pes && (!pes->stream_type ||
> +                    (pes->stream_type != stream_type &&
> +                     !!(ts->stream->flags & AVFMT_FLAG_ALLOW_CODEC_CHANGES))))
>              mpegts_set_stream_info(st, pes, stream_type, prog_reg_desc);
>  
>          add_pid_to_program(prg, pid);
> diff --git a/libavformat/options_table.h b/libavformat/options_table.h
> index e2e690fd2a..811dd342cc 100644
> --- a/libavformat/options_table.h
> +++ b/libavformat/options_table.h
> @@ -50,6 +50,7 @@ static const AVOption avformat_options[] = {
>  {"sortdts", "try to interleave outputted packets by dts", 0, AV_OPT_TYPE_CONST, {.i64 = AVFMT_FLAG_SORT_DTS }, INT_MIN, INT_MAX, D, .unit = "fflags"},
>  {"fastseek", "fast but inaccurate seeks", 0, AV_OPT_TYPE_CONST, {.i64 = AVFMT_FLAG_FAST_SEEK }, INT_MIN, INT_MAX, D, .unit = "fflags"},
>  {"nobuffer", "reduce the latency introduced by optional buffering", 0, AV_OPT_TYPE_CONST, {.i64 = AVFMT_FLAG_NOBUFFER }, 0, INT_MAX, D, .unit = "fflags"},
> +{"allow_codec_changes", "allow AVStream.codecpar to change at runtime if input changes", 0, AV_OPT_TYPE_CONST, { .i64 = AVFMT_FLAG_ALLOW_CODEC_CHANGES }, 0, 0, D, .unit = "fflags" },

If the option is user settable then ffmpeg/ffplay should possibly print a
warning, that they do not support this if the user forces the flag

thx

[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

The educated differ from the uneducated as much as the living from the
dead. -- Aristotle 

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