[FFmpeg-devel] [PATCH v4] avcodec/mpeg12dec: extract only one type of CC substream

[Stefano Sabatini]

On date Tuesday 2024-03-12 10:43:11 -0500, Marth64 wrote:
> In MPEG-2 user data, there can be different types of Closed Captions
> formats embedded (A53, SCTE-20, or DVD). The current behavior of the
> CC extraction code in the MPEG-2 decoder is to not be aware of
> multiple formats if multiple exist, therefore allowing one format
> to overwrite the other during the extraction process since the CC
> extraction shares one output buffer for the normalized bytes.
> 
> This causes sources that have two CC formats to produce flawed output.
> There exist real-world samples which contain both A53 and SCTE-20 captions
> in the same MPEG-2 stream, and that manifest this problem. Example of symptom:
> THANK YOU (expected) --> THTHANANK K YOYOUU (actual)
> 
> The solution is to pick only the first CC substream observed with valid bytes,
> and ignore the other types. Additionally, provide an option for users
> to manually "force" a type in the event that this matters for a particular
> source.
> 
> Signed-off-by: Marth64 <marth64 at proxyid.net>
> ---
>  libavcodec/mpeg12dec.c | 64 ++++++++++++++++++++++++++++++++++++++++--
>  1 file changed, 61 insertions(+), 3 deletions(-)

LGTM.