I'm using libavcodec library and h264 codec to prepare the video stream on one end, transmit the encoded frames to the other PC and there decode it.

我使用libavcodec库和h264编解码器在一端准备视频流，将编码帧传输到另一台PC并在那里解码。

What I noticed after receiving very first packet (first encoded video frame) and feeding decoder with it, it is not possible to decode that frame. Only when I receive another frame the first one can be decoded but 'current' one not. So in the end I have constantly one frame delay on the decoder side.

我注意到，在收到第一个包(第一个编码的视频帧)并将其提供给解码器后，无法解码该帧。只有当我收到另一个帧时，第一个帧才能被解码，但“当前”帧不能。所以最后，我在解码端一直有一个帧延迟。

I was trying different presets (focusing rather on 'ultrafast'), also 'zerolatency' tune, also whole variety of bit_rate values of AVCodecContext.

我尝试了不同的预置(重点是‘超快’)，还有‘零度’调谐，还有AVCodecContext的各种码率值。

I also tried to flush (with nullptr packet) after injecting first frame data, just to check if it is maybe because of some internal buffers optimization - the frame still not decoded.
Experimenting with other codecs (like mpeg4) gives even worse 'dalay' in number of frames to the point when when first frames can become decodable.

我也试图在注入第一帧数据后刷新(使用nullptr包)，只是为了检查是否可能是因为内部缓冲区优化-帧仍然没有解码。使用其他编解码器(如MPEG4)进行试验时，在帧数量上甚至会出现更差的延迟，直到第一帧变得可解码时。

Is it normal, unavoidable because of some internal mechanisms? Otherwise how I can achieve real zero latency.

这是正常的，是因为某些内在机制而不可避免的吗？否则我如何才能实现真正的零延迟。

Supplementary setup information:

补充设置信息：

• max_b_frames set to 0 (higher value gives even more delay)


• pix_fmt set to AV_PIX_FMT_YUV420P

edit:

编辑：

Answering some comment question:

回答一些评论问题：

(1) What is the decoder (or playback system)?

Custom decoder written using libavcodec, the decoded frames are later displayed on screen by OpenGL.

使用libavcodec编写的自定义解码器，解码后的帧将通过OpenGL显示在屏幕上。

• initialization:

parser_ = av_parser_init(AV_CODEC_ID_H264);
codec_ = avcodec_find_decoder(AV_CODEC_ID_H264);
context_ = avcodec_alloc_context3(codec_);
context_->width = 1024;
context_->height = 768;
context_->thread_count = 1;
if ((codec_->capabilities & AV_CODEC_CAP_TRUNCATED) == 0)
{
  context_->flags |= AV_CODEC_FLAG_TRUNCATED;
}
if (avcodec_open2(context_, codec_, nullptr) < 0)
{
  throw std::runtime_error{"avcodec_open2 failed"};
}
avcodec_flush_buffers(context_);

• then player periodically calls of the method of decoder that suppose to check if the another frame can be retrieved and displayed:

auto result = avcodec_receive_frame(context_, frame_);
if (!buffer_.empty())
{  // upload another packet for decoding
  int used;
  if (upload_package(buffer_.data(), buffer_.size(), &used))
  {
    buffer_.erase(buffer_.begin(), buffer_.begin() + used);
  }
}
if (result == AVERROR(EAGAIN) || result == AVERROR_EOF || result < 0)
{
  return false;
}
yuv_to_rgb();
return true;

boolean return value informs if the decoding succeeded, and every time the buffer where the incomming packets are stored is checked and uploaded to libavcodec decoder

布尔返回值通知解码是否成功，每次检查存储传入分组的缓冲区并将其上载到libavcodec解码器

• and that is how the method that uploads the buffer looks like:

bool upload_package(const void* data, const std::size_t size, int* used)
{
  auto result = av_parser_parse2(parser_, context_, &packet_->data, &packet_->size, reinterpret_cast<const std::uint8_t*>(data), size, AV_NOPTS_VALUE, AV_NOPTS_VALUE, 0);
  if (result < 0)
  {
    return false;
  }
  *used = result;
  if (packet_->size != 0)
  {
    result = avcodec_send_packet(context_, packet_);
    if (result < 0)
    {
      return false;
    }
  }
  return true;
}

(2) If possible, save each one as a .bin file and then share the links with us for testing.

I will try to figure out something...

我会努力想办法的..。

(3) Show example C++ code of your encoder settings for H264...

• initialization:

codec_ = avcodec_find_encoder(AV_CODEC_ID_H264);
context_ = avcodec_alloc_context3(codec_);

context_->bit_rate = 1048576; // 1xMbit;
context_->width = 1024;
context_->height = 768;
context_->time_base = {1, 30}; // 30 fps
context_->pix_fmt = AV_PIX_FMT_YUV420P;
context_->thread_count = 1;

av_opt_set(context_->priv_data, "preset", "ultrafast", 0);
av_opt_set(context_->priv_data, "tune", "zerolatency", 0);
avcodec_open2(context_, codec_, nullptr);

frame_->format = AV_PIX_FMT_YUV420P;
frame_->width = 1024;
frame_->height = 768;
av_image_alloc(frame_->data, frame_->linesize, 1024, 768, AV_PIX_FMT_YUV420P, 32);

• frame encoding:

rgb_to_yuv();
frame_->pts = frame_num_++;
auto result = avcodec_send_frame(context_, frame_);
while (result >= 0)
{
  result = avcodec_receive_packet(context_, packet_);
  if (result == AVERROR(EAGAIN) || result == AVERROR_EOF)
  {
    return;
  }
  else if (result < 0)
  {
    throw std::runtime_error{"avcodec_receive_packet failed"};
  }
  // here the packet is send to the decoder, the whole packet is stored on the mentioned before buffer_ and uploaded with avcodec_send_packet
  // I can also add that the whole buffer/packet us uploaded at once
  stream_video_data(packet_->data, packet_->size); 
}
av_packet_unref(packet_);
}

edit2:

编辑2：

I think I figured out the issue that I had.

我想我找到了我的问题所在。

For every incoming data packet (encoded frame) I was calling first av_parser_parse2, and then I was sending the data through avcodec_send_packet.
And I was not recalling that procedure having empty buffer_, so for the first frame data the av_parser_parse2 was never called after uploading it through avcodec_send_packet, for the second frame it was called and the first frame was parsed, so it could be properly decoded, but for that (second) frame the parse2 was also not called, and so on ...

对于每个传入的数据分组(编码帧)，我首先调用av_parser_parse2，然后通过avcodec_end_Packet发送数据。我没有回想起缓冲区为空的过程，因此对于第一帧数据，在通过avcodec_end_Packet上传数据后，从未调用av_parser_parse2，对于第二帧，它被调用并解析第一帧，因此它可以被正确解码，但对于(第二)帧，也不调用parse2，依此类推……

So the issue in my case was wrong sequence of av_parser_parse2 and avcodec_send_packet to handle the encoded data.

因此，在我的例子中，问题是av_parser_parse2和avcodec_end_Packet处理编码数据的顺序错误。