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• recording video in real-time with OpevCV VideoWriter

  19 février 2016, par nick topper

  I have a few OpenCV projects that analyze video over USB, and in certain conditions must record the video stream to a file. People using my software complain that 10+ minute recordings yield video files that are about 20 seconds longer than they should be.

  I’m using openCV’s VideoWriter. Iv’e tried things like setting CV2_CAP_PROP_FPS to a very low setting, and iv’e tried getting the average frame rate over a few seconds to find a good setting for my frame rate of the output file. Still not close enough to real time for my needs.

  Does anyone know of a good way to make sure my video is recording close to real time ? Should I use something like time.sleep (in python) to cap my framerate ? Or is there a better way to do this ?

• ffmpeg : Computing PSNR on RGB channels

  21 janvier 2016, par Mike

  I have 2 rgb48le tif images that I want to compare with ffmpeg to compute the PSNR

  ffmpeg automatically computes the PSNR on YUV channels instead of RGB.
  This is the command that I’m using on "ffmpeg-20160119-git-cc83177-win64-static" :

  ffmpeg -y -an -i image1.tif -i image2.tif -filter_complex "psnr" output.tif

  and this is the output :

  [tiff_pipe @ 045b36a0] Stream #0: not enough frames to estimate rate; consider increasing probesize
  
  Input #0, tiff_pipe, from 'image1.tif':
  
    Duration: N/A, bitrate: N/A
  
  Stream #0:0: Video: tiff, rgb48le, 7680x4320 [SAR 1:1 DAR 16:9], 25 tbr, 25 tbn, 25 tbc
  
  [tiff_pipe @ 045c53a0] Stream #0: not enough frames to estimate rate; consider increasing probesize
  
  Input #1, tiff_pipe, from 'image2.tif':
  
    Duration: N/A, bitrate: N/A
  
      Stream #1:0: Video: tiff, rgb48le, 7680x4320 [SAR 1:1 DAR 16:9], 25 tbr, 25 tbn, 25 tbc
  
  Output #0, image2, to 'output.tif':
  
    Metadata:
  
  encoder         : Lavf56.40.101
  
  Stream #0:0: Video: tiff, rgb48le, 7680x4320 [SAR 1:1 DAR 16:9], q=2-31, 200 kb/s, 25 fps, 25 tbn, 25 tbc (default)
  
  Metadata:
  
    encoder         : Lavc56.60.100 tiff
  
  Stream mapping:
  
    Stream #0:0 (tiff) -> psnr:main
  
    Stream #1:0 (tiff) -> psnr:reference
  
    psnr -> Stream #0:0 (tiff)
  
  Press [q] to stop, [?] for help
  
  frame=    1 fps=0.3 q=-0.0 Lsize=N/A time=00:00:00.04 bitrate=N/A
  
  video:195568kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: unknown
  
  [Parsed_psnr_0 @ 045cf760] PSNR y:46.00 u:49.32 v:50.34 average:48.14 min:48.14 max:48.14

  Is this normal ?
  Is there a way to force the PSNR to compute on RGB48 each channel ?

• Can ffmpeg periodically report statistics on a real-time audio stream (rather than file) ?

  19 janvier 2016, par Caius Jard

  I currently use ffmpeg to capture desktop screen and audio that the computer speakers are playing, something like a screencast. ffmpeg is started by an app that captures its console output, so I can have that app read the output and look for info

  I’d like to know if there are a set of switches I can supply to ffmpeg whereby it will periodically output some audio statistics that will directly report, or allow me to infer, that the audio stream has gone silent ?

  I see some audio statistics switches/filters but the help docs for these seem to imply they will collect their stats over the processing of an entire stream and then report them at the end.. I’d prefer something like "the average audio volume over the past 5 seconds" reported every 5 seconds. I could even deduce from the audio bitrate of the encoder I think, if it’s VBR and the rate consistently falls because it’s encoding nothing