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• C++ ffmpeg video missing frames and won't play in Quicktime

  5 décembre 2019, par Oliver Dain

  I wrote some C++ code that uses ffmpeg to encode a video. I’m having two strange issues :

  1. The final video is always missing 1 frame. That is, if I have it encode 10 frames the final video only has 9 (at least that’s what ffprobe -show_frames -pretty $VIDEO | grep -F '[FRAME]' | wc -l tells me.
  2. The final video plays fine in some players (mpv and vlc) but not in Quicktime. Quicktime just shows a completely black screen.

  My code is roughly this (modified a bit to remove types that are unique to our code base) :

  First, I open the video file, write the headers and initialize things :

  template <class ptrt="ptrt">
  
  using UniquePtrWithDeleteFunction = std::unique_ptr>;
  
  
  
  
  
  std::unique_ptr<ffmpegencodingframesink> FfmpegEncodingFrameSink::Create(
  
      const std::string&amp; dest_url) {
  
    AVFormatContext* tmp_format_ctxt;
  
    auto alloc_format_res = avformat_alloc_output_context2(&amp;tmp_format_ctxt, nullptr, "mp4", dest_url.c_str());
  
    if (alloc_format_res &lt; 0) {
  
      throw FfmpegException("Error opening output file.");
  
    }
  
    auto format_ctxt = UniquePtrWithDeleteFunction<avformatcontext>(
  
        tmp_format_ctxt, CloseAvFormatContext);
  
  
  
    AVStream* out_stream_video = avformat_new_stream(format_ctxt.get(), nullptr);
  
    if (out_stream_video == nullptr) {
  
      throw FfmpegException("Could not create outputstream");
  
    }
  
  
  
    auto codec_context = GetCodecContext(options);
  
    out_stream_video->time_base = codec_context->time_base;
  
  
  
    auto ret = avcodec_parameters_from_context(out_stream_video->codecpar, codec_context.get());
  
    if (ret &lt; 0) {
  
      throw FfmpegException("Failed to copy encoder parameters to outputstream");
  
    }
  
  
  
    if (!(format_ctxt->oformat->flags &amp; AVFMT_NOFILE)) {
  
      ret = avio_open(&amp;format_ctxt->pb, dest_url.c_str(), AVIO_FLAG_WRITE);
  
      if (ret &lt; 0) {
  
        throw VideoDecodeException("Could not open output file: " + dest_url);
  
      }
  
    }
  
  
  
    ret = avformat_init_output(format_ctxt.get(), nullptr);
  
    if (ret &lt; 0) {
  
      throw FfmpegException("Unable to initialize the codec.");
  
    }
  
  
  
    ret = avformat_write_header(format_ctxt.get(), nullptr);
  
    if (ret &lt; 0) {
  
      throw FfmpegException("Error occurred writing format header");
  
    }
  
  
  
    return std::unique_ptr<ffmpegencodingframesink>(
  
        new FfmpegEncodingFrameSink(std::move(format_ctxt), std::move(codec_context)));
  
  }
  
  </ffmpegencodingframesink></avformatcontext></ffmpegencodingframesink></class>

  Then, every time I get a new frame to encode I pass it to this function (the frames are being decoded via ffmpeg from another mp4 file which Quicktime plays just fine) :

  // If frame == nullptr then we're done and we're just flushing the encoder
  
  // otherwise encode an actual frame
  
  void FfmpegEncodingFrameSink::EncodeAndWriteFrame(
  
      const AVFrame* frame) {
  
    auto ret = avcodec_send_frame(codec_ctxt_.get(), frame);
  
    if (ret < 0) {
  
      throw FfmpegException("Error encoding the frame.");
  
    }
  
  
  
    AVPacket enc_packet;
  
    enc_packet.data = nullptr;
  
    enc_packet.size = 0;
  
    av_init_packet(&enc_packet);
  
  
  
    do {
  
      ret = avcodec_receive_packet(codec_ctxt_.get(), &enc_packet);
  
      if (ret ==  AVERROR(EAGAIN)) {
  
        CHECK(frame != nullptr);
  
        break;
  
      } else if (ret ==  AVERROR_EOF) {
  
        CHECK(frame == nullptr);
  
        break;
  
      } else if (ret < 0) {
  
        throw FfmpegException("Error putting the encoded frame into the packet.");
  
      }
  
  
  
      assert(ret == 0);
  
      enc_packet.stream_index = 0;
  
  
  
      LOG(INFO) << "Writing packet to stream.";
  
      av_interleaved_write_frame(format_ctxt_.get(), &enc_packet);
  
      av_packet_unref(&enc_packet);
  
    } while (ret == 0);
  
  }

  Finally, in my destructor I close everything up like so :

  FfmpegEncodingFrameSink::~FfmpegEncodingFrameSink() {
  
    // Pass a nullptr to EncodeAndWriteFrame so it flushes the encoder
  
    EncodeAndWriteFrame(nullptr);
  
    // write mp4 trailer
  
    av_write_trailer(format_ctxt_.get());
  
  }

  If I run this passing n frames to EncodeAndWriteFrame line LOG(INFO) << "Writing packet to stream."; gets run n times indicating the n packets were written to the stream. But ffprobe always shows only n - 1 frames int he video. And the final video doesn’t play on quicktime.

  What am I doing wrong ??

• How to fetch both live video frame and timestamp from ffmpeg to python on Windows

  8 mai 2018, par vijiboy

  Searching for an alternative as OpenCV would not provide timestamps for live camera stream (on Windows), which are required in my computer vision algorithm, I found ffmpeg and this excellent article https://zulko.github.io/blog/2013/09/27/read-and-write-video-frames-in-python-using-ffmpeg/
  The solution uses ffmpeg, accessing its standard output (stdout) stream. I extended it to read the standard error (stderr) stream as well.

  Working up the python code on windows, while I received the video frames from ffmpeg stdout, but the stderr freezes after delivering the showinfo videofilter details (timestamp) for first frame.

  I recollected seeing on ffmpeg forum somewhere that the video filters like showinfo are bypassed when redirected. Is this why the following code does not work as expected ?

  Expected : It should write video frames to disk as well as print timestamp details.
  Actual : It writes video files but does not get the timestamp (showinfo) details.

  Here’s the code I tried :

  import subprocess as sp
  
  import numpy
  
  import cv2
  
  
  
  command = [ 'ffmpeg', 
  
              '-i', 'e:\sample.wmv',
  
              '-pix_fmt', 'rgb24',
  
              '-vcodec', 'rawvideo',
  
              '-vf', 'showinfo', # video filter - showinfo will provide frame timestamps
  
              '-an','-sn', #-an, -sn disables audio and sub-title processing respectively
  
              '-f', 'image2pipe', '-'] # we need to output to a pipe
  
  
  
  pipe = sp.Popen(command, stdout = sp.PIPE, stderr = sp.PIPE) # TODO someone on ffmpeg forum said video filters (e.g. showinfo) are bypassed when stdout is redirected to pipes??? 
  
  
  
  for i in range(10):
  
      raw_image = pipe.stdout.read(1280*720*3)
  
      img_info = pipe.stderr.read(244) # 244 characters is the current output of showinfo video filter
  
      print "showinfo output", img_info
  
      image1 =  numpy.fromstring(raw_image, dtype='uint8')
  
      image2 = image1.reshape((720,1280,3))  
  
  
  
      # write video frame to file just to verify
  
      videoFrameName = 'Video_Frame{0}.png'.format(i)
  
      cv2.imwrite(videoFrameName,image2)
  
  
  
      # throw away the data in the pipe's buffer.
  
      pipe.stdout.flush()
  
      pipe.stderr.flush()

  So how to still get the frame timestamps from ffmpeg into python code so that it can be used in my computer vision algorithm...

• Async Javascript function returns undefined

  14 avril 2018, par Leon Wright

  I am trying to run this function but the it keeps returning undefined when I explicitly hardcode the return value.

  const splitVideo = async (sid, video, part) => {
  
      let framesLocation =`${process.cwd()}/${dirs.videoFrames}/${sid}_${part}`;
  
  
  
      console.log(fs.existsSync(framesLocation));
  
  
  
      if(!fs.existsSync(framesLocation)) {
  
          console.log("making dir");
  
          f.s.mkdirSync(framesLocation);
  
      }
  
  
  
      ffmpeg(video)
  
          .on('end', () => {
  
               return "done";
  
           })
  
           .on('error', (err) => {
  
               throw err;
  
           })
  
           .screenshots({
  
                timestamps: [1,2],
  
                filename: `${sid}_${part}/frame-%s.png`,
  
                folder: `${process.cwd()}/${dirs.videoFrames}`
  
           });
  
  };

  Please help this is very frustrating.