How to use pyav or opencv to decode a live stream of raw H.264 data?

Question

The data was received by socket ,with no more shell , they are pure I P B frames begin with NAL Header(something like 00 00 00 01). I am now using pyav to decode the frames ,but i can only decode the data after the second pps info(in key frame) was received(so the chunk of data I send to my decode thread can begin with pps and sps ), otherwise the decode() or demux() will return error "non-existing PPS 0 referenced decode_slice_header error" .

I want to feed data to a sustaining decoder which can remember the previous P frame , so after feeding one B frame, the decoder return a decoded video frame. Or someform of IO that can be opened as container and keep writing data into it by another thread.

Here is my key code:

#read thread... read until get a key frame, then make a new io.BytesIO() to store the new data.
rawFrames = io.BytesIO()
while flag_get_keyFrame:()
    ....
    content= socket.recv(2048)
    rawFrames.write(content)
    ....

#decode thread... decode content between two key frames
....
rawFrames.seek(0)
container = av.open(rawFrames)
for packet in container.demux():
    for frame in packet.decode():
        self.frames.append(frame)
....

My code will play the video but with a 3~4 seconds delay. So I am not putting all of it here, because I know it's not actually working for what I want to achieve. I want to play the video after receiving the first key frame and decode the following frames right after receiving them . Pyav opencv ffmpeg or something else ,how can I achieve my goal?

Answer 1

After hours of finding an answer for this as well. I figure this out myself.

For single thread, you can do the following:

rawData = io.BytesIO()
container = av.open(rawData, format="h264", mode='r')
cur_pos = 0
while True:
    data = await websocket.recv()
    rawData.write(data)
    rawData.seek(cur_pos)
    for packet in container.demux():
        if packet.size == 0:
            continue
        cur_pos += packet.size
        for frame in packet.decode():
            self.frames.append(frame)

That is the basic idea. I have worked out a generic version that has receiving thread and decoding thread separated. The code will also skip frames if the CPU does not keep up with the decoding speed and will start decoding from the next key frame (so you will not have the teared green screen effect). Here is the full version of the code:

import asyncio
import av
import cv2
import io
from multiprocessing import Process, Queue, Event
import time
import websockets

def display_frame(frame, start_time, pts_offset, frame_rate):
    if frame.pts is not None:
        play_time = (frame.pts - pts_offset) * frame.time_base.numerator / frame.time_base.denominator
        if start_time is not None:
            current_time = time.time() - start_time
            time_diff = play_time - current_time
            if time_diff > 1 / frame_rate:
                return False
            if time_diff > 0:
                time.sleep(time_diff)
    img = frame.to_ndarray(format='bgr24')
    cv2.imshow('Video', img)
    return True

def get_pts(frame):
    return frame.pts

def render(terminated, data_queue):
    rawData = io.BytesIO()
    cur_pos = 0
    frames_buffer = []
    start_time = None
    pts_offset = None
    got_key_frame = False
    while not terminated.is_set():
        try:
            data = data_queue.get_nowait()
        except:
            time.sleep(0.01)
            continue
        rawData.write(data)
        rawData.seek(cur_pos)
        if cur_pos == 0:
            container = av.open(rawData, mode='r')
            original_codec_ctx = container.streams.video[0].codec_context
            codec = av.codec.CodecContext.create(original_codec_ctx.name, 'r')
        cur_pos += len(data)
        dts = None
        for packet in container.demux():
            if packet.size == 0:
                continue
            dts = packet.dts
            if pts_offset is None:
                pts_offset = packet.pts
            if not got_key_frame and packet.is_keyframe:
                got_key_frame = True
            if data_queue.qsize() > 8 and not packet.is_keyframe:
                got_key_frame = False
                continue
            if not got_key_frame:
                continue
            frames = codec.decode(packet)
            if start_time is None:
                start_time = time.time()
            frames_buffer += frames
            frames_buffer.sort(key=get_pts)
            for frame in frames_buffer:
                if display_frame(frame, start_time, pts_offset, codec.framerate):
                    frames_buffer.remove(frame)
                if cv2.waitKey(1) & 0xFF == ord('q'):
                    break
        if dts is not None:
            container.seek(25000)
        rawData.seek(cur_pos)
        if cv2.waitKey(1) & 0xFF == ord('q'):
            break
    terminated.set()
    cv2.destroyAllWindows()

async def receive_encoded_video(websocket, path):
    data_queue = Queue()
    terminated = Event()
    p = Process(
        target=render,
        args=(terminated, data_queue)
    )
    p.start()
    while not terminated.is_set():
        try:
            data = await websocket.recv()
        except:
            break
        data_queue.put(data)
    terminated.set()

Answer 2

Its normal getting 3~4 seconds delay because you are reading encoded data and decoding it takes time via on CPU.

1. If you have GPU hardware, you can use FFMPEG to decode H264 by GPU. Here is an example.
2. If you don't have a GPU, decoding H264 on CPU always will cause delays. You can use FFMPEG for effective decoding but this will also decrease total delay almost 10%