Replacing av_read_frame() to reduce delay

Question

I am implementing a (very) low latency video streaming C++ application using ffmpeg. The client receives a video which is encoded with x264’s zerolatency preset, so there is no need for buffering. As described here, if you use av_read_frame() to read packets of the encoded video stream, you will always have at least one frame delay because of internal buffering done in ffmpeg. So when I call av_read_frame() after frame n+1 has been sent to the client, the function will return frame n.

Getting rid of this buffering by setting the AVFormatContext flags AVFMT_FLAG_NOPARSE | AVFMT_FLAG_NOFILLIN as suggested in the source disables packet parsing and therefore breaks decoding, as noted in the source.

Therefore, I am writing my own packet receiver and parser. First, here are the relevant steps of the working solution (including one frame delay) using av_read_frame():

AVFormatContext *fctx;
AVCodecContext *cctx;
AVPacket *pkt;
AVFrame *frm;

//Initialization of AV structures
//…

//Main Loop
while(true){

    //Receive packet
    av_read_frame(fctx, pkt);

    //Decode:
    avcodec_send_packet(cctx, pkt);
    avcodec_receive_frame(cctx, frm);

    //Display frame
    //…
}

And below is my solution, which mimics the behavior of av_read_frame(), as far as I could reproduce it. I was able to track the source code of av_read_frame() down to ff_read_packet(),but I cannot find the source of AVInputformat.read_packet().

int tcpsocket;
AVCodecContext *cctx;
AVPacket *pkt;
AVFrame *frm;
uint8_t recvbuf[(int)10e5];
memset(recvbuf,0,10e5);
int pos = 0;

AVCodecParserContext * parser = av_parser_init(AV_CODEC_ID_H264);
parser->flags |= PARSER_FLAG_COMPLETE_FRAMES;
parser->flags |= PARSER_FLAG_USE_CODEC_TS;

//Initialization of AV structures and the tcpsocket
//…

//Main Loop
while(true){

    //Receive packet
    int length = read(tcpsocket, recvbuf, 10e5);
    if (length >= 0) {

        //Creating temporary packet
        AVPacket * tempPacket = new AVPacket;
        av_init_packet(tempPacket);
        av_new_packet(tempPacket, length);
        memcpy(tempPacket->data, recvbuf, length);
        tempPacket->pos = pos;
        pos += length;
        memset(recvbuf,0,length);

        //Parsing temporary packet into pkt
        av_init_packet(pkt);
        av_parser_parse2(parser, cctx,
            &(pkt->data), &(pkt->size),
            tempPacket->data, tempPacket->size,
            tempPacket->pts, tempPacket->dts, tempPacket->pos
            );

        pkt->pts = parser->pts;
        pkt->dts = parser->dts;
        pkt->pos = parser->pos;

        //Set keyframe flag
        if (parser->key_frame == 1 ||
            (parser->key_frame == -1 &&
            parser->pict_type == AV_PICTURE_TYPE_I))
            pkt->flags |= AV_PKT_FLAG_KEY;
        if (parser->key_frame == -1 && parser->pict_type == AV_PICTURE_TYPE_NONE && (pkt->flags & AV_PKT_FLAG_KEY))
            pkt->flags |= AV_PKT_FLAG_KEY;
        pkt->duration = 96000; //Same result as in av_read_frame()

        //Decode:
        avcodec_send_packet(cctx, pkt);
        avcodec_receive_frame(cctx, frm);
        //Display frame
        //…
    }
}

I checked the fields of the resulting packet (pkt) just before avcodec_send_packet() in both solutions. They are as far as I can tell identical. The only difference might be the actual content of pkt->data. My solution decodes I-Frames fine, but the references in P-Frames seem to be broken, causing heavy artifacts and error messages such as “invalid level prefix”, “error while decoding MB xx”, and similar.

I would be very grateful for any hints.

Edit 1: I have developed a workaround for the time being: in the video server, after sending the packet containing the encoded data of a frame, I send one dummy packet which only contains the delimiters marking beginning and end of the packet. This way, I push the actual video data frames through av_read_frame(). I discard the dummy packets immediately after av_frame_read().

Edit 2: Solved here by rom1v, as written in his comment to this question.

Answer 1

av_parser_parse2() does not neccessarily consume your tempPacket in one go. You have to call it in another loop and check its return value, like in the API docs.