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• ffmpeg record timelapse camera v4l2

  21 mars 2019, par Paul G.

  How can I instruct ffmpeg (v. : 3.4.5) to record only one frame per minute from my video camera at /dev/video0 and copy it to a new file with 30 frames per second ? Goal is timelapse video of some months and record only one frame per minute into the out video to save space and processing power instead of just capturing the camera at 30fps and then preprocess the huge video file for speed up...

  The camera delivers a 1920x1080 30fps stream with "mjpeg" selected in commandline. I would like to keep the raw frames and just copy them into the out file. I tried this command to get 1fps input to 30fps output but this seems not to work :

  ffmpeg -framerate 1 -input_format mjpeg -i /dev/video0 -r 30 -c:v copy -an out.mp4
  
  
  
  [video4linux2,v4l2 @ 0x55ece63ac360] The driver changed the time per frame from 1/1 to 1/30
  
  Input #0, video4linux2,v4l2, from '/dev/video0':
  
    Duration: N/A, start: 37064.594605, bitrate: N/A
  
      Stream #0:0: Video: mjpeg, yuvj422p(pc, bt470bg/unknown/unknown), 1920x1080, 30 fps, 30 tbr, 1000k tbn, 1000k tbc

  But this doesnt work.

• Synchronizing screencasting (ffmpeg) and capturing from the webcam (OpenCV)

  24 avril 2013, par lyuba

  As from my previous questions, I am trying to build a simple eye tracker. Decided to start from a Linux version (run Ubuntu).

  To complete this task one should organize screencasting and webcam capturing in such way that frames from both streams exactly match each other and there is the same number of frames in each of them totally.

  Screencasting fps fully depends on the camera's fps, so each time we get the image from the webcam we can potentially grab a screen frame and stay happy. However, all the tools for the fast screencasting, like ffmpeg, for example, return the .avi file as the result and require the fps already known to be started.

  From the other side, tools like Java+Robot or ImageMagick seem to require around 20ms to return the .jpg screenshot, which is pretty slow for the task. But they may be requested right after each time the webcam frame is grabbed and provide the needed synchronization.

  So the sub-questions are :

  1. Does the USD camera's frame rate vary during a single session ?
  2. Are there any tools which provide fast screencasting frame by frame ?
  3. Is there any way to make ffmpeg push a new frame to the .avi file only when program initiates this request ?

  For my task I may either use C++ or Java.

  I am, actually, an interface designer, not the driver programmer, and this task seems to be pretty low-level. I would be grateful for any suggestion and tip !

• Using GDIgrab in FFmpeg with dshow Audio produces black screen

  16 mars 2016, par Spreadys

  here is my command :

  ffmpeg -f gdigrab -framerate 25 -offset_x 10 -offset_y 10 -show_region 1 -draw_mouse 1 -video_size 1280x720 -i desktop -f dshow -i audio="Microphone (2- ATR USB microphone)" -r 25 -threads 4 -c:v libx264 -pix_fmt yuv422p -preset superfast -tune fastdecode -x264opts keyint=25:min-keyint=1 -crf 4 -c:a aac -profile:a aac_low -async 25 "C:\Users\david\Desktop\%output%.mp4"

  The gdigrab video works great when it is on its own (no audio). The audio works fine when it is on its own (no video). When I join the two commands to capture both together, as soon as I move a window within my capture area, the area goes black.

  In Windows 7, I used to get around this by stopping the desktop composition service prior to capture, (SC stop uxsms), but this is now not possible in Win10.

  I thought it may be something graphics card related.
  My main monitor is on an Nvidia card, with my second running from the onboard Intel. This is setup for Quicksync H264 playback and encoding with my NLE.

  I know that I could use a dshow screen capture driver such as UScreen but am trying to avoid that as I need the capture area to be specified each time from a simple batch.

  Any help appreciated to solve this black area problem- its driving me crazy !
  David