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• h264_omx = mostly black videos, libx264 too slow, audio slows everything down

  11 août 2017, par Brendan Grant

  I’m attempting to port a live tv transcoding app from Windows (on .net core) to work on a Raspberry Pi 3.

  Under the hood I uses ffmpeg to take in an mpeg2 stream from a local networked tv tuner and spit out a local HLS stream which is exposed via a custom http server.

  FFmpeg on Linux is configured for build as following :

  sudo ./configure --arch=armhf --target-os=linux --enable-gpl --enable-libx264 --enable-nonfree --enable-mmal --enable-omx --enable-omx-rpi

  Seeing perf issues early on, I tried to boil it down to a simpler form and started using a VOB from a DVD (also mpeg2), take the following example :

  ffmpeg -i VTS_01_2.VOB -c:a aac -c:v h264_omx -b:v 256k -hls_time 3 -hls_wrap 30 "/media/usb1/foo.m3u8"

  Within a matter of seconds, this drops to not only under 30 FPS, but down to the low teens.

  The following runs plenty fast, and is able to keep up when pointed to the tv stream... only all but the first ts file will end up being black (but with audio)... which it turns out the previous line also suffers from :

  ffmpeg -i VTS_01_2.VOB -c:a copy -c:v h264_omx -b:v 256k -hls_time 3 -hls_wrap 30 "/media/usb1/foo.m3u8"

  Aside from the oddity of my choice of audio codecs (ie accepting what is already there vs choosing something else) would so slow down the stream, I also discovered that in order to have visible video... I have to go without hardware assistance.

  ffmpeg -i VTS_01_2.VOB -c:a aac -c:v libx264 -b:v 256k -hls_time 3 -hls_wrap 30 "/media/usb1/foo.m3u8"

  and

  ffmpeg -i VTS_01_2.VOB -c:a copy -c:v libx264 -b:v 256k -hls_time 3 -hls_wrap 30 "/media/usb1/foo.m3u8"

  both turn out video which shows up... and with good audio, except both are too slow in terms of FPS to be able to keep up with a live tv stream.

  Is there something I am missing here to be able to get ffmpeg to be able to handle turning an mpeg2 stream into an h.264 one... and have the resulting files have video ?

  To save space here, I’ve put the console logs on pastebin.

  h264_omx : https://pastebin.com/GK3zN4aB
  libx264 : https://pastebin.com/uEdCwk47

  As I think about it more, the "[mpegts @ 0x2fe0be0] H.264 bitstream error, startcode missing, size 082 drop=0 speed=3.07x" error may be related, but when looking at a longer session we do not see them as regular (as I would think) to obliterate the entire video stream on later files whose sizes are inline with the first : https://pastebin.com/J1EzTqgK

• I get black screen when i record the headless selenium driver with xvfb and ffmpeg [closed]

  28 juillet 2022, par Saran Raj

  xvfb-run —listen-tcp —server-num 44 -s "-ac -screen 0 1920x1080x24" mvn clean test &
  
export DISPLAY=:44
  
ffmpeg -f x11grab -video_size 1920x1080 -i :44 -codec:v libx264 -r 12 video.mp4

  


  This is the bash script i am using to run the selenium test cases(in headless mode) with maven as the build tool in port 44 and capture it.
But when i execute it i only get a black screen. What to do ?

  


• simple way to confirm if a video has a black or transparent background [closed]

  10 juillet 2022, par TheForgot3n1

  What is a method of determining if you are seeing a black or transparent background when working with videos without using anything other than the Windows OS ?