[Libav-user] How to pass custom information to AVPacket and receive it in AVFrame?

[Ben Harper]

I'm afraid I don't know good answers to your questions, but you do say that
you're manipulating the pos, PTS, etc fields. Maybe your manipulation of
them is causing the presentation order/buffering that you mention.

On Sun, 29 Sept 2024, 13:16 Wodzu, <brucedickinson at wp.pl> wrote:

> Hello,
>
> I am decoding a raw h264 stream based on example from:
> https://github.com/FFmpeg/FFmpeg/blob/master/doc/examples/decode_video.c
>
> The problem is that I need to somehow tie up a custom information to
> AVPacket and receive it in AVFrame. Because it is raw h264, I am sure that
> one AVPacket sent is equal to one AVFrame received.
>
> I've tried the following: AVPacket->opaque but this field is not passed to
> AVFrame->opaque.
> So I've tried some ugly hacks and use one of these fields:
>
> av_packet->duration =
> av_packet->pts =
> av_packet->dts =
> av_packet->pos =
>
> These can be read from frame after calling:
> avcodec_receive_frame(codec_context, frame);
>
> However, there are two strange things that are occurring:
>
>
>    1. When I am sending the very first IFrame (which also contains PPS
>    and SPS) via avcodec_send_packet(codec_context, packet) and then I want to
>    receive a corresponding frame via avcodec_receive_frame(), the function
>    will return me AVERROR(EAGAIN). I don't understand why it needs more data
>    if what it got is a full IFrame?. I need to send two more packets (each
>    containing exactly one frame) to get first frame out of
>    avcodec_receive_frame(). Why the decoder decides to store two frames in its
>    buffer before sending me anything.
>    2. Even though the frames are received in correct order (I've saved
>    them as images and checked them), the fields like av_packet->pos which was
>    passed to frame and can be read through frame->pkt_pos are in not correct
>    order. Take a look:
>
>
> avcodec_send_packet - (av_packet->pos): 0
> avcodec_send_packet - (av_packet->pos): 1
> avcodec_send_packet - (av_packet->pos): 2
> avcodec_receive_frame - (frame->pkt_pos): 0 <- first frame received after
> sending three packets, each containing one frame.
> avcodec_send_packet - (av_packet->pos): 3
> avcodec_receive_frame - (frame->pkt_pos): 2 <- mixed order, pkt_pos should
> be 1?
> avcodec_send_packet - (av_packet->pos): 4
> avcodec_receive_frame - (frame->pkt_pos): 1
> avcodec_send_packet - (av_packet->pos): 5
> avcodec_receive_frame - (frame->pkt_pos): 3
> avcodec_send_packet - (av_packet->pos): 6
> avcodec_receive_frame - (frame->pkt_pos): 4
> avcodec_send_packet - (av_packet->pos): 7
> avcodec_receive_frame - (frame->pkt_pos): 5
> avcodec_send_packet - (av_packet->pos): 8
> avcodec_receive_frame - (frame->pkt_pos): 6
> avcodec_send_packet - (av_packet->pos): 9
> flush called
> avcodec_receive_frame - (frame->pkt_pos): 8 <- again mixed order, should
> be 7.
> avcodec_receive_frame - (frame->pkt_pos): 7
> avcodec_receive_frame - (frame->pkt_pos): 9
>
> I would be grateful if you could:
> 1. Show me how can I force decoder to send me a frame immediately rather
> thanb keeping it in its buffer.
> 2. Show me how to fix that strange ordering.
> 3. Given any other advice how could I pass information in AVPacket and
> receive it in corresponding AVFrame.
>
> Thank you.
>
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