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• h264 inside AVI, MP4 and "Raw" h264 streams. Different format of NAL units

  6 octobre 2017, par bukkojot

  TL ;DR : I want to read raw h264 streams from AVI files, even broken/incomplete.

  Almost every document about h264 tells me that it consists of NAL packets. Okay. Almost everywhere told to me that the package should start with a signature like 00 00 01 or 00 00 00 01. For example, https://stackoverflow.com/a/18638298/8167678, https://stackoverflow.com/a/17625537/8167678

  The format of H.264 is that it’s made up of NAL Units, each starting
  with a start prefix of three bytes with the values 0x00, 0x00, 0x01
  and each unit has a different type depending on the value of the 4th
  byte right after these 3 starting bytes. One NAL Unit IS NOT one frame
  in the video, each frame is made up of a number of NAL Units.

  Okay.

  I downloaded random_youtube_video.mp4 and split one frame from it :

  ffmpeg -ss 10 -i random_youtube_video.mp4 -frames 1 -c copy pic.avi

  And got :
  
  Red part - this is part of AVI container, other - actual data.
  As you can see, here I have 00 00 24 A9 instead of 00 00 00 01

  This AVI file plays perfectly

  I do same for mp4 container :
  

  As you can see, here exact same bytes.
  This MP4 file plays perfectly

  I try to strip out raw data :
  ffmpeg -i pic.avi -c copy pic.h264
  

  This file can’t play in VLC or even ffmpeg, which produced this file, can’t parse it :
  

  I downloaded mp4 stream analyzer and got :
  

  MP4Box tells me :

   Cannot find H264 start code
  
   Error importing pic.h264: BitStream Not Compliant

  It very hard to learn internals of h264, when nothing works.

  So, I have questions :

  1. What actual data inside mp4 ?
  2. What I must read to decode that data (I mean different annex-es)
  3. How to read image and get decoded image (even with ffmpeg) from this "broken" raw stream ?

  UPDATE :

  It seems bug in ffmpeg :

  When I do double conversion :

           ffmpeg -ss 10 -i random_youtube_video.mp4 -frames 1 -c copy pic.mp4
  
           ffmpeg pic.mp4 -c copy pic.h264

  

  But when I convert file directly :

  ffmpeg -ss 10 -i random_youtube_video.mp4 -frames 1 -c copy pic.h264
  

  I have NALs signatures and one extra NAL unit. Other bytes are same (selected).

  This is bug ?

• My webcam works fine, while using cv2.Videocapture() but while trying to read from hard drive it shows error cap.isOpened() returns false, why ?

  27 septembre 2017, par Hu Yang

  import cv2
  video_capture = cv2.VideoCapture("rtsp ://admin:Trust1986_@10.58.94.47:554/h264/ch1/main/av_stream")
  video_capture.isOpened()

  False

  Other information below here, and ffmpeg I had installed.

    Video I/O:
  
      DC1394 1.x:                  NO
  
      DC1394 2.x:                  NO
  
      FFMPEG:                      NO
  
        avcodec:                   NO
  
        avformat:                  NO
  
        avutil:                    NO
  
        swscale:                   NO
  
        avresample:                NO
  
      GStreamer:                   NO
  
      OpenNI:                      NO
  
      OpenNI PrimeSensor Modules:  NO
  
      OpenNI2:                     NO
  
      PvAPI:                       NO
  
      GigEVisionSDK:               NO
  
      Aravis SDK:                  NO
  
      UniCap:                      NO
  
      UniCap ucil:                 NO
  
      V4L/V4L2:                    NO/NO
  
      XIMEA:                       NO
  
      Xine:                        NO
  
      Intel Media SDK:             NO
  
      gPhoto2:                     NO
  
  
  
  root@ubuntu:/home/face_recognition/opencv/ffmpeg/ffmpeg-3.3.4# ffmpeg -version
  
  ffmpeg version 3.3.4 Copyright (c) 2000-2017 the FFmpeg developers
  
  built with gcc 4.8 (Ubuntu 4.8.4-2ubuntu1~14.04.3)
  
  configuration: 
  
  libavutil      55. 58.100 / 55. 58.100
  
  libavcodec     57. 89.100 / 57. 89.100
  
  libavformat    57. 71.100 / 57. 71.100
  
  libavdevice    57.  6.100 / 57.  6.100
  
  libavfilter     6. 82.100 /  6. 82.100
  
  libswscale      4.  6.100 /  4.  6.100
  
  libswresample   2.  7.100 /  2.  7.100
  
  root@ubuntu:/home/face_recognition/opencv/ffmpeg/ffmpeg-3.3.4# 
  
  root@ubuntu:/home/face_recognition/opencv/ffmpeg/ffmpeg-3.3.4# python3.6m -V
  
  Python 3.6.2
  
  root@ubuntu:/home/face_recognition/opencv/ffmpeg/ffmpeg-3.3.4# pkg-config --modversion opencv  
  
  3.3.0

  what shall I do next step ? Thanks.

• What's the equivalent of "MP4Box -hint" in ffmpeg ?

  7 janvier 2023, par Roman Gaufman

  I currently run my ffmpeg command to record the video then afterwards run MP4Box -hint file.mp4 to make the file stream quicker for the web. Is there any way for ffmpeg to "hint" my file straightaway without the extra step ?