[FFmpeg-devel] [PATCH] avcodec: Allow to query number of consumed bytes using the new API

[jannis_wth]

30.01.19 21:02 - James Almer:
> On 1/30/2019 4:19 PM, jannis_wth wrote:
>> 30.01.19 19:51 James Almer:
>>> On 1/30/2019 3:43 PM, jannis_wth wrote:
>>>> Okay, so how about this one?
>>>> This functionality is important if you don't know the packet size you
>>>> are feeding. It allows you to reconstruct the size.
>>>
>>> In what scenario you can't know the size of the AVPacket you're feeding
>>> to avcodec_send_packet().
>>>
>> For example when a mp4 container lost its moov atom.
> 
> What value is stored in pkt->size in this scenario at the time you feed
> it to libavcodec? This function will return that value after all,
> provided it's valid data.
> 
> Also, AVCodecParsers are the usual way to reconstruct/assemble broken or
> incomplete packets.

pkt->size and its offset can be used to rebuild the moov atom.
I tried av_parser_parse2() but it I think it's not what I need. Its
return value does not match the true packet size, not even close.

>>> This is only used by decoders implementing the AVCodec.decode()
>>> callback. It will always be zero for any decoder using
>>> AVCodec.receive_frame(), like Bink Audio, libdav1d, etc.
>>
>> Well, what would be the correct way then?
>> Or is there no good way and thus you won't accept such interface?
> 
> Accepting it or not depends on what other developers think about such a
> function. Unlike the old API, the new one always fully consumes all
> packets you feed to it, so there's no real need to check how many bytes
> were consumed.
> Trying to expose the internals for this purpose is not really feasible,
> as you could see with all the different layers of internal buffering.
> 
> I guess avctx->internal->last_pkt_props could work for this, but it may
> not be consistent across decoders.

avctx->internal->last_pkt_props stores the properties of the last passed
packet, and does not get updated on decoding frames. Since I set the
input-packets size to a constant (size is unknown) this is not very
helpfull..

One could add a warning to avcodec_get_remaining_size() saying it only
works for some codecs.