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• ffmpeg create screenshots full of artefacts and completly illegible

  17 septembre 2020, par Hekimen

  I am using ffmpeg to create 15 screenshots from video with simple command

  


  ffmpeg -ss 10:00 -y -i 'video.mp4' -f mjpeg -vframes 1 -an 'image.jpg'


  


  command is executed 15x in row with different -ss time. But sometimes, completly randomly, images - all 15, even each of them is created by own process, are basicaly generated full of artefacts and completly illegible :


  


  I believe is not problem with video, as they are almost always different - codecs, bitrate, lenght, quality, resolution, etc. I was trying search similar problem, but only same problem i found was when images was created from real-time stream over UDP (problem with UDP transfer protocol), which is not my case as ffmpeg and videos are installed and stored on same HDD (centos OS). Otuput from ffmpeg command is also without any error. I am also unable reproduce this behavior, when i run process for screenshots again, all screenshots are created properly. My only suspicion is server load, screenshots are created on encoding server which have average load around 50% on CPU : 
Is it possible that when the CPU is fully loaded, ffmpeg can create broken images ?

  


• ffmpeg - audio sync issue

  27 août 2020, par Tuxa

  First, I decode a video into single frames :

  


  ffmpeg -vsync 0 -i video.mp4 -y frames/%d.png -f mkvtimestamp_v2 pts.txt

  


  Then, those frames are processed and overwritten using another application and an mp4 video is created again. Note that the original video is reused to copy the audio stream.

  


  ffmpeg -vsync 0 -i frames/%d.png -vsync 0 -i video.mp4 -map 0 -map 1:a output.mp4

  


  At the end mp4fpsmod is used to transfer the pts to get the exact same frame timestamps :

  


  mp4fpsmod output.mp4 -t pts.txt -o output_fixed.mp4

  


  This works fine for most videos.

  


  However, there are some videos that are fine but after processing, audio and video are slightly out of sync. It seems this is related to those warnings when decoding the video :

  


  


  [mkvtimestamp_v2 @ 0x55f1000e7ec0] Non-monotonous DTS in output stream 1:0 ; previous : 4400, current : 4400 ; changing to 4401. This may result in incorrect timestamps in the output file.

  


  


  


  [image2 @ 0x55f1000ee040] Application provided invalid, non monotonically increasing dts to muxer in stream 0 : 132 >= 132

  


  


  Why does this happen ? Can this be fixed somehow ?

  


• PyAv av.open() specify used codec

  7 septembre 2020, par Kazooie

  When PyAv is used to open the alsa audio device. How I can specify used codec and not the ffmpeg default one because that is wrong. By default it will use pcm_s16le and I need to use pcm_s32le. I can record from my device with following ffmpeg command :

  


  ffmpeg -f alsa -acodec pcm_s32le -i dmic_sv alsaout.wav


  


  but not with

  


  ffmpeg -f alsa -i dmic_sv alsaout.wav


  


  Which will give me following error :

  


  [alsa @ 0x12061c0] cannot set sample format 0x10000 2 (Invalid argument)
dmic_sv: Input/output error


  


  How to transfer the working command to PyAv av.open() function ? There is stream_options but it doesn't seem to work. I tried

  


  stream_options = [{'-acodec': 'pcm_s32le'}]
av.open('dmic_sv', format='alsa', mode='r', stream_options=stream_options)


  


  And I get the same as above.

  


  av.error.OSError: [Errno 5] Input/output error: 'dmic_sv'; last error log: [alsa] cannot set sample format 0x10000 2 (Invalid argument)


  


  How to do this ?