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• What are good settings for transcoding videos uploaded to my app ?

  14 mai 2020, par Dmitry Minkovsky

  I am working on an app that allows users to share videos. The problem is that many videos are very high bitrate. For example, A 4-minute H264 video from an old iPhone is encoded at 1080p and runs 17,000 kb/s ( 500 megabytes). Accepting and distributing such videos at this bitrate/resolution is not practical for a social application.

  



  I have been playing with ffmpeg to transcode videos to smaller sizes and higher compression, but have not achieved acceptable results. For example :

  



  ffmpeg \
    -i in.mov \
    -vf scale=w='if(gt(iw\,ih)\,780\,-2)':h='if(gt(iw\,ih)\,-2\,780)' \ 
    -c:v libx264 \
    -crf 28 \
    -preset medium \
    -pix_fmt yuv420p \
    -movflags +faststart \
    out.mp4


  



  This command transcodes the above-mentioned 500MB file down to 70MB. It scales the larger dimension of the video to 780 pixels and compresses the video quite a bit. The results are okay, but the file is still large.

  



  Taking the longer dimension down to 480 pixels, the file is reduced to 40MB. Still quite large, and now significantly degraded. Also, the transcoding still takes quite a long time : about 1-1.5x on my 4 year old i7 Macbook Pro with 16GB RAM.

  



  I'm not sure how to improve on this. H265 is not supported in browsers. I am wondering :

  



  

  • How can I reduce size further ?

  


  • How can I transcode faster than 1x without significantly reducing quality ? Even 2-3x doesn't seem great ?

  


  



  Is this as good as it gets ?

  


• FFMPEG - Overlay WebM over PNG Start Position

  26 avril 2022, par joshuaalvarez

  I am trying to place WebM videos over PNGs layered on top of each other. I have successfully done so, however, the WebM videos are not present in the first frame in the video and are added one frame at a time. For example, if I place 2 WebM videos over a PNG, it will layer one WebM per frame at a time.

  


  It may be important to note that I am using PNGs and WebMs as the inputs (WebM to use alpha channel) and exporting as MP4.

  


  I am using complex filters and the overlay in this way : overlay=format=auto per image/video.

  


  Here is a link to a MP4 that shows my issue. See how there is only the PNG portion in the video at 0:00 and the WebM portions come in shortly after.

  


• What exactly is time_base in ffmpeg ?

  29 juin 2019, par Gilgamesh22

  I have a project in which I receive frames sequentially to encode a mp4-h264 video. I also receive the timestamp in milliseconds along side the frame to indicate where in the video to place the frame. Because of this I decided to have a time_base of 10000 to simplify the placement of the frame since the frame rate is variable with an average fps of 30. I later learned that this causes problems in some video players. I was wondering if there is a proper way to accomplish this. at the moment my current solution is to set the time_base to 30 and simply set the frame->pts to timestamp of the frame divide by (10000/30).

  Original Setup

      AVCodecContext *cctx;
  
      AVStream* stream;
  
  
  
      stream->time_base.num = 1;
  
      stream->time_base.den = 10000;
  
  
  
      stream->r_frame_rate.num = 30;
  
      stream->r_frame_rate.den = 1;
  
  
  
      cctx->time_base.num = 1;
  
      cctx->time_base.den = 10000;
  
  
  
      cctx->framerate.num = 30;
  
      cctx->framerate.den = 1;

  My Questions

  1) what exactly is time_base because I always though time_base was simply a used to extract location of the frame and was abstracted away during the encoding process.

  2) what is the difference between framerate and time_base.

  3) why does a large time_base with large gaps of missing frames cause this problem in only some players.