[FFmpeg-devel] [PATCH v2 2/2] ffprobe: Fix incorrect display of closed_captions property

[James Almer]

On 9/19/2021 12:28 AM, Soft Works wrote:
> 
> 
>> -----Original Message-----
>> From: ffmpeg-devel <ffmpeg-devel-bounces at ffmpeg.org> On Behalf Of James Almer
>> Sent: Sunday, 19 September 2021 05:05
>> To: ffmpeg-devel at ffmpeg.org
>> Subject: Re: [FFmpeg-devel] [PATCH v2 2/2] ffprobe: Fix incorrect display of
>> closed_captions property
>>
>> On 9/18/2021 7:10 PM, Soft Works wrote:
>>> Repro Example:
>>> ffprobe -show_entries stream=closed_captions:disposition=:side_data=
>> "http://streams.videolan.org/streams/ts/CC/NewsStream-608-ac3.ts"
>>>
>>> While the codec string includes "Closed Captions",
>>> the stream data is showing: closed_captions=0
>>>
>>> The test ref was incorrect as the test media file
>>> actually does have cc.
>>>
>>> Signed-off-by: softworkz <softworkz at hotmail.com>
>>> ---
>>> v2: Fix test result. It was undiscovered locally as make fate
>>>       does not seem to properly rebuild ffprobe.
>>>
>>>    fftools/ffprobe.c       | 5 +++--
>>>    tests/ref/fate/ts-demux | 2 +-
>>>    2 files changed, 4 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/fftools/ffprobe.c b/fftools/ffprobe.c
>>> index 880f05a6c2..9425c1a2e3 100644
>>> --- a/fftools/ffprobe.c
>>> +++ b/fftools/ffprobe.c
>>> @@ -2678,10 +2678,11 @@ static int show_stream(WriterContext *w,
>> AVFormatContext *fmt_ctx, int stream_id
>>>            print_int("width",        par->width);
>>>            print_int("height",       par->height);
>>>            if (dec_ctx) {
>>> +            unsigned codec_properties =
>> av_stream_get_codec_properties(ist->st);
>>
>> Library users are meant to use their own decoder contexts if they want
>> this kind of information. AVStream->internal->avctx is internal to lavf.
>> Accessors like this should be avoided.
> 
> If you look further up in util
>>
>> ffprobe is already decoding frames for the purpose of show_frames, and
>> in fact if you add -show_frames to the above command line example you
>> gave, the output of show_streams will report closed_captions=1.
> 
> Yes, I know that, but if you don't - the output is wrong.

Yeah, and fixing that would be ideal. The output of show_streams should 
(if possible) not depend on other options also being passed.
Your patch will only do it for properties, and it requires new public 
API, whereas my suggestion will do it for all other fields, like 
coded_{width,height}, and requires changes contained entirely within 
ffprobe with proper usage of existing API.

> 
>> So the correct approach here is to make ffprobe decode a few frames when
>> you request show_streams, even if you don't request show_frames.
> 
> This is something that I would want to avoid. It has already happened
> and the result is available already. The "Closed Captions" part in
> the video codec string is coming from that result.
> 
> It doesn't make sense IMO, to perform another decoding run to replicate
> that result, because:
> 
> - It can't be taken for granted that this will lead to the same result
>    that already exists
>    How many frames do you think would be the right amount to process?
>    What if the file start with a bunch of audio frames?

You decode until you get the info you need, which is what 
avformat_find_stream_info() does to fill its own internal 
AVCodecContext. The simplest way would be to ensure one frame per stream 
is decoded.

I'm not going to approve more accessors to specific fields in 
AVStream->internal->avctx (Ideally, we'd remove the few that 
unfortunately already exist) because they will just not stop being added 
after that. There will always be another avctx field someone wants to 
use, and eventually, we'll just end up with AVStream->codec all over again.

> - It takes additional processing time, which could have a huge impact
>    depending on how the input is being processed or a rather small
>    impact.
>    But even when the impact would be small - it could matter a lot
>    when you are probing a library of like 10k or 100k files.
> 
> Kind regards,
> softworkz
> 
> 
> 
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