[FFmpeg-devel] [PATCH v2] avcodec: add realtime bitstream filter

[James Almer]

On 5/2/2019 7:42 AM, Moritz Barsnick wrote:
> On Wed, May 01, 2019 at 12:03:41 -0300, James Almer wrote:
>>> +    if (pkt->pts != AV_NOPTS_VALUE) {
>>> +        int64_t pts = av_rescale_q(pkt->pts, bsf->time_base_in, AV_TIME_BASE_Q) / ctx->speed;
>>> +        int64_t now = av_gettime_relative();
>>> +        int64_t sleep = pts - now + ctx->delta;
>>> +        if (!ctx->inited) {
>>> +            ctx->inited = 1;
>>> +            sleep = 0;
>>> +            ctx->delta = now - pts;
>>
>> If this is meant to be used for input, where seeking can take place,
>> wouldn't a flush() callback to set ctx->inited and ctx->delta back to 0
>> be needed?
> 
> Interesting point. I suppose you mean backward seeking. Is that a valid
> use case for a bitstream filter?

The flush() callback exists to reset the bsf into its initial state
without the need to destroy and recreate the context.
For example, some decoders auto-insert certain bitstream filters that
get applied by the generic lavc code before they are feed packets, like
the vp9 decoder and some hardware based h264/hevc decoders.
avcodec_flush_buffers(), which must be called every time seeking needs
to be done in a decoding scenario, flushes both the decoder and said
bitstream filters.

I don't think we'll ever have a decoder auto inserting this bsf, but we
don't know what an API user might want to do with bitstream filters in
their own scenarios. And if the bsf keeps a state, a flush() callback is
required to reset it.

> Would it also apply to the standard filter, where I took this code from?

There doesn't seem to be any kind of flush mechanism for filters, so i
suppose in those you're expected to uninit and reinit the filter every time?

> 
> I don't know, but I can try to add that, if required. Can you tell me
> how to emulate or test seeking from the command line? Would I create a
> file with backward jumps in PTS (and how)?

I guess the simplest way would be to auto insert this bsf into some
decoder (AVCodec.bsfs string), then seek using some libavcodec based
application that doesn't already decodes at realtime.

> 
> I also missed to bump libavcodec version, or to at least mention it,
> BTW.

Missing changelog entry and minor version bump, yeah.

> 
> Thanks,
> Moritz