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• Use ffmpeg libraries to convert stream formats

  17 septembre 2021, par Syrinx

  I'm attempting to write a small program and link it to a minimal set of ffmpeg libraries, like libavformat and whatever other libraries I need.

  


  I am looking for documentation to get me started, or maybe a quick fix to the example program I am using.

  


  I know ffmpeg (the project) provides example programs to help developers get started. I'm using the transcoding example, as it's close to my end goal, but it exits during init with an error about an audio issue.

  


  Here I am using the transcoding example program that come with ffmpeg v4.4, on Ubuntu 18.04. My input source has one video channel (h264) and one audio channel (pcm_mulaw).

  


  $ LD_LIBRARY_PATH=../../dist/lib ./transcoding rtsp://ip-camera/stream out.flv
...
  Stream #0:0: Video: h264, yuv420p, 1280x720, q=2-31, 20 tbn
  Stream #0:1: Audio: pcm_mulaw, 8000 Hz, 0 channels, s16
auto_resampler_0 @ 0x55da787fa140] [SWR @ 0x55da787fa5c0] Rematrix is needed between mono and 0 channels but there is not enough information to do it
[auto_resampler_0 @ 0x55da787fa140] Failed to configure output pad on auto_resampler_0


  


  In libswresample/swresample.c :

  


  320      if ((!s->out_ch_layout || !s->in_ch_layout) && s->used_ch_count != s->out.ch_count && !s->rematrix_custom) {
321          av_log(s, AV_LOG_ERROR, "Rematrix is needed between %s and %s "
322                 "but there is not enough information to do it\n", l1, l2);
323          ret = AVERROR(EINVAL);
324          goto fail;
325      }


  


  I'd really like it if I could make the transcoding example program work (fix it, or maybe use it appropriately if I am misunderstanding something). But short of that, where should I look for documentation about using the ffmpeg libraries ?

  


  I don't even care about the audio. If I can just disable the audio, I would be happy with that solution. I tried tracking the "-an" option to ffmpeg (the program) to see how it does that in source code, but the options handling is a mess and I can't distinguish the parts of the code that I need from all the noise.

  


  ffmpeg has web pages like this that aren't useful at all. There is documentation in the source code that looks like it should be viewed as HTML, but I don't see it exported anywhere. "make doc" generates a very small set of man pages that are insufficient to get me started.

  


• Trying to crop an image in ffmpeg using alphamerge but produces wrong alpha

  12 septembre 2021, par Alex Styl

  I am using alphamerge in order to crop an image within a circle.

  


  What I have so far is :

  


  ffmpeg -f lavfi -i "color=c=white:size=240x240" -i avatar.png -i mask.png -filter_complex \
 "[1][2]alphamerge[img]; \
 [0][img]overlay[out]" -c:v png -map "[out]" -pix_fmt rgba -t 5 -y out.mp4 2>&1


  


  with avatar.png and mask.png respectively being :

  


  

  


  This produces the following output (1 frame of the output video) :

  


  

  


  which is unexpected, given the original input is much darker than this.

  


  How can I crop the 'avatar.png' using the 'mask.png' so that the output is the avatar.png cropped in a circle and keeping the same alpha ?

  


  PS : The important bit here is for me to be able to crop the original image and maintaining the correct colors/apha of the original image. If there is an other way of doing this (other than alphamerge) I am happy to hear it.

  


• Can I reduce buffering and delay using ffmpeg ?

  28 juin 2021, par Trivalio

  I'm currently working (as a total beginner) on a nginx-RTMP streaming server and i've tried to implement ffmpeg filters but as i tested them i saw that there is a lot of delay due to these filters.
I'm having a hard time finding a ffmpeg function to reduce it.
Here is what i've done so far :

  


  worker_processes auto;
rtmp_auto_push on;
events{}
rtmp{
    serveur{
        listen 1935;
        listen [::]:1935 ipv6only=on;
        chunk_size 4096;
        buflen 5s;
        application live{
            live on;
            record off;
        }
        application ffmpeg{
            live on;
            record off;
            exec /usr/bin/ffmpeg -re -i rtmp://localhost/$app/$name -vf mpdecimate,hue=s=0 -vcodec flv -acodec copy -f flv rtmp://localhost/live;
        }
    }
}


  


  i use OBS to stream to the ffmpeg application and i use VLC to see the stream exiting from the live application.

  


  I've tried many functions from rtmp but since the delay doesn't seems to come from rtmp, it doesn't change anything.
If you have any idea how to improve that, you will make me a quit happy person.

  


  Thank you !