Recherche avancée

Sur d’autres sites (11460)

• Android encode video with ffmpeg while it is still recording

  30 décembre 2016, par Andreas Pabst

  I want to develop an android aplication which allows me to continuously record a video and upload parts of the video to a server without stopping the recording.
  It is crucial for the application that I can record up to 60 min without stopping the video.

  Initial approach

  Application consits of two parts :

  1. MediaRecorder which records a video continuously from the camera.
  2. Cutter/Copy - Part : While the video is recorded I have to take out certain segments and send them to a server.
     This part was implemented using http://ffmpeg4android.netcompss.com/
     libffmpeg.so. I used their VideoKit Wrapper which allows me to directly run ffmpeg with any params I need.

  My Problem

  I tried the ffmpeg command with the params

  ffmpeg -ss 00:00:03 -i  -t 00:00:05 -vcodec copy -acodec copy  

  which worked great for me as long as Android’s MediaRecorder finished recording.

  When I execute the same command, while the MediaRecorder is recording the file, ffmpeg exits with the error message "Operation not permitted".

  • I think that the error message doesn’t mean that android prevents the access to the file. I think that ffmpeg needs the "moov-atoms" to find the proper position in the video.

  For that reason I thought of other approaches (which don’t need the moov-atom) :

  1. Create a rtsp stream with android and access the rtsp stream later. The problem is that to my knowledge android SDK doesn’t support the recording to a rtsp stream.
  2. Maybe it is possible to access the camera directly with ffmpeg (/dev/video0 seems to be a video device ?!)
  3. I read about webm as an alternative for streaming, maybe android can record webm streams ?!

  TLDR : Too long didn’t read :

  I want to access a video file with ffmpeg (libffmpeg.so) while it is recording. Fffmpeg exits with the error message "Operation not permitted"

  Goal :

  My goal is to record a video (and audio) and take parts of the video while it is still recording and upload them to the server.

  Maybe you can help me solve the probelm or you have other ideas on how to approach my problem.

  Thanks a lot in advance.

• python3 openCV VideoCapture only black image (Raspbian Stretch)

  21 juin 2018, par PrimuS

  I have a little python function that analyzes a video, chunks it into one image per second and gives me the most dominant color for that image. (Code can be found here : https://github.com/primus852/python-movie-barcode)

  This works great on my Windows testing environment. However, on my Rasbian Stretch Raspberry Pi Setup it only produces a black image, as the source seems to be black.

  I compiled OpenCV (3.4.1) myself with this great article : https://www.pyimagesearch.com/2017/09/04/raspbian-stretch-install-opencv-3-python-on-your-raspberry-pi/, and it worked perfectly fine. I am using python3 and a virtualenv.

  I tried adding the ffmpeg package : apt install ffmpeg, to no avail.

  2 ideas

  • I compiled the openCV source without support for mkv/mp4/similar ? If so, how would I check that and can I just "re-compile" the whole package ?
  • I am missing codecs, where would I be able to check that ?

  The crucial code (I think) is this :

  cap = cv2.VideoCapture(full_path) are there other options that do not break the majority of my code ? I read about skvideo.io from scikit-video but that does not seem to work with my code...

  I am new to python, any hint is appreciated

  // EDIT, I think it is not a duplicate because I pass the I get no error that the capture cannot be opened and :

  OpenCV FFMPEG support :

  • python -c "import cv2; print(cv2.getBuildInformation())" | grep -i ffmpeg
    • returns : FFMPEG: YES

  FFMPEG Codec :

  • ffmpeg -codecs | grep -i avc (file is using AVC)
    • returns : DEV.LS h264 H.264 / AVC / MPEG-4 AVC / MPEG-4 part 10 (decoders: h264 h264_mmal h264_vdpau ) (encoders: libx264 libx264rgb h264_omx h264_vaapi )

  File PATH

  It exists and the path is correct...

  Any other ideas ? Could it possibly the virtualenv ?

  //EDIT2

  Tried with an AVI file

  works...

• How to force A/V sync using mkvmerge and external time-codes ?

  19 avril 2017, par b..

  Background

  I’m working on a project where video and audio are algorithmic interpretations of an MKV source file where I use ffmpeg -ss and -t to extract a particular region of audio and video to separate files. I use scene changes in the video in the audio process (i.e. the audio changes on video scene change), so sync is crucial.

  Audio is 48khz, using 512 sample blocks.
  Video is 23.976fps (I also tried 24).

  I store the frame onset of sceneChanges in a file in terms of cumulative blocks :

  blocksPerFrame = (48000 / 512) / 23.976
  
  sceneOnsetBlock = sceneOnsetFrame*blocksPerFrame

  I use these blocks in my audio code to treat the samples associated with each scene as a group.

  When I combine the audio and video back together (currently using ffmpeg to generate mp4(v) mp3(a) in an MKV container), the audio and video start off in sync but increasingly drifts until it ends up being 35 seconds off. The worst part is that the audio lag is nonlinear ! By non-linear, I mean that if I plot the lag against the location of that lag in time, I don’t get a line, but what you see in the image below). I can’t just shift or scale the audio to fit the video because of this nonlinearity. I cannot figure out the cause of this nonlinearly increasing audio delay ; I’ve double and triple checked my math.

  

  Since I know the exact timing of scene changes, I should be able to generate "external timecodes" (from the blocks above) for mkvmerge to perfectly sync the output !

  Subquestions :

  1. Is this the best approach (beyond trying to figure out what went wrong in the first place) ? As I’m using my video frames as a
     reference, if I use the scene changes as timecodes for the audio,
     will it force the video to match the audio or vice versa ? I’m much less concerned with the duration than the sync. The video was much more laborious to produce, so I’d rather loose some sound than some frames.

  2. I’m not clear on what numbers to use in the timecodes file.
     According to mkvmerge documentation "For video this is exactly
     one frame, for audio this is one packet of the specific audio type."
     Since I’m using MP3, what is the packet size ? Ideally, I could specify a packetsize (in the audio-encoder ?) that matches my block size (512) to keep things consistent and simple. Can I do this with ffmpeg ?

  Thank you !