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• FFMPEG documentation example for chromakey doesn't work for green background png file

  15 septembre 2021, par JPM

  Im new in FFMPEG and I try to make just simple green background chroma keying of png images.
In FFMPEG documentation in 38.16 Chromakey - there is an example script :

  



  ffmpeg -i input.png -vf chromakey=green out.png

  



  for which is said that "Make every green pixel in the input image transparent"
That exactly what i wanted.
I tried with many different png images but the result is always a copy of input image.
Also i replaced the word "green" to 0x00FF00 or 0x008000 and still no successes. Only when replacing green to black and then the result is as expected - transparent.
Im working on Windows with pre-build binaries of FFMpeg.
What am I missing ?
Thanks.

  


• Audiopulse record multiple xfbv screens with python

  7 octobre 2020, par Eric Lagarda

  I’m trying to do something complicated and I need some help.

  


  I’m able to record a session (video and audio) with python using xvfb and ffmpeg.

  


  I just want to visit a page using selenium and record the video and audio. It’s working but when I try to run multiple instances, the audio is mixing between records.

  


  How can achieve that ? I’m using pulseaudio. I know that with also you can set like a interface per instance, but I don’t know how to do it.

  


  Thanks for you support and comments.

  


• How to save ffmpeg segmets to disk immediately with sub-second intervals ?

  20 octobre 2023, par amfast

  I'm trying to record video on a raspberry and have it save as much as possible (sub-second resolution) in case of a power cutoff.

  


  I use -f segment to save the encoded stream in 100ms segments with the hope that all but the interrupted (by power cutoff) segment will be saved in memory. Unfortunately, when cutting off power, all the destination files (output_0001.mp4, output_0002.mp4, ...) are created, but empty.

  


  To save the files to disk immediately, I added the -strftime 1 option that allows formatting the output filename as time. It seems weird that this is the (only ?) way to trigger immediate saving of files, but it works - untill I try to have segments smaller than 1 second. The problem seems to be that the format string %d, that previously added a sequence number in my output filenames, now represents "day" (i.e. date) and the smallest resolution time format string is %S for second. I saw %f suggested somewhere for smaller resolutions, but it only prints "%f".

  


  The result is that the segmentation part of ffmpeg does create 100ms segments and save them to disk immediately, but the strftime feature gives the output files names that only change every second, so all the interim files are overwritten.

  


  Example of the failing command below. Without the -strftime option this creates nice segments, but does not save them to disk immediately.

  


  libcamera-vid --flush \
    --framerate ${FRAMERATE} \
    --width ${WIDTH} \
    --height ${HEIGHT} \
    -n \
    -t ${TIMEOUT} \
    --codec yuv420 \
    -o - | 
ffmpeg \
    -fflags nobuffer \
    -strict experimental \
    -loglevel debug \
    -flags low_delay \
    -f rawvideo \
    -pix_fmt yuv420p \
    -s:v ${WIDTH}x${HEIGHT} \
    -r ${FRAMERATE} \
    -i - \
    -c:v h264_v4l2m2m \
    -f segment \
    -segment_time 0.1 \
    -segment_format mp4 \
    -reset_timestamps 1 \
    -strftime 1 \
    -b:v ${ENCODING_BITRATE} \
    -g 1 \
    "output_%04d.mp4"


  


  Question :
  
Is there another way besides -strftime to trigger immediate saving ? Or is there a mechanism to feed finer resolution format strings to the output filename ?