Re: [FFmpeg-devel] [PATCH] avformat/mpegts: Add AVFMT_FLAG_ALLOW_CODEC_CHANGES (v2)

From: Scott Theisen <scott.the.elm-at-gmail.com@ffmpeg.org>
To: ffmpeg-devel@ffmpeg.org
Subject: Re: [FFmpeg-devel] [PATCH] avformat/mpegts: Add AVFMT_FLAG_ALLOW_CODEC_CHANGES (v2)
Date: Tue, 1 Jul 2025 10:08:57 -0400
Message-ID: <18489e8f-bd18-41ef-901c-9ee5e3abecfb@gmail.com> (raw)
In-Reply-To: <CAJgjuoxT44JPHd0rAPHnnH=Y2Otxud66XM_W05f3ODUvp2V_Ag@mail.gmail.com>

On 2025/06/30 21:32, Pavel Koshevoy wrote:
> On Mon, Jun 30, 2025 at 3:46 PM Scott Theisen <
> scott.the.elm-at-gmail.com@ffmpeg.org> wrote:
>
>> On 2025/06/29 23:55, 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.
>> Is this signaled in any way or is the client expected to check if
>> codecpar has changed?
>>
> The client is expected to check codecpar for changes ...
> I've described it a bit more here
> https://ffmpeg.org/pipermail/ffmpeg-devel/2025-June/344921.html
>
>
>
>> I ask because MythTV added checking and a callback for streams changing
>> between PMT versions, including adding or removing streams.
>>
>>
> In my code (outside ffmpeg) I've also added a similar mechanism.
> BTW, in mpeg-ts captures that I have the PMT version doesn't always change,
> even when there are significant changes in the program structure.

If the PMT version_number is not incremented, that doesn't comply with 
the standard ("The version number shall be incremented by 1 modulo 32 
when a change in the information carried within the section occurs." 
ISO/IEC 13818-1:2021 §2.4.4.10 Semantic definition of fields in 
transport stream program map section).

Since the CRC would be different, the non-compliant PMT is parsed and 
used (see skip_identical() in mpegts.c).

I had tried to just check the version_number (and 
current_next_indicator), but the Blu-ray for Rio (2011) has a new PMT 
version_number for (I think) every frame, which broke playback of that 
Blu-ray in MythTV since the decoder was resetting every frame, so we had 
to revert back to comparing the PMT data (ignoring the descriptors). 
https://github.com/MythTV/mythtv/commit/616074a0a5f4fa9ad7241e673d6693cbd7b5e310

Regards,

Scott Theisen
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