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• Fastest seek speed and decoding with ffmpeg and x265 ProRes

  17 mai 2018, par Christopher Jarvis

  I’m trying to optimize seek speed with x265. No matter what encoding settings I try, ProRes still seeks more quickly/gracefully. This makes sense since it was built for editing, but I’m sure there’s got to be something I’m missing to better improve x265.

  So far, -tune fastdecode, keyint=1, maxrate and -b (to remove B Frame calculations) yield the best results, but they’re still unsatisfactory. I’ve been pouring over the docs but there’s so much jargon I just don’t understand. Perhaps another codec like VP9 / WebM would be better for this purpose ?

  From what I can tell, there’s no bottleneck with CPU, read speed or RAM... or GPU for that matter. Monitoring these processes show minimal drain. Is there just an amount of decoding in a highly compressed format like x265 that can’t be circumvented ?

  Thank you in advance for your help.

• How can I seek to frame No. X with ffmpeg ?

  27 août 2018, par Giumo

  I’m writing a video editor, and I need to seek to exact frame, knowing the frame number.

  Other posts on stackoverflow told me that ffmpeg may give me a few broken frames after seeking, which is not a problem for playback but a big problem for video editors.

  And I need to seek by frame number, not by time, which will become inaccurate when converted to frame number.

  I’ve read dranger’s tuts (which is outdated now), and end up with :

  av_seek_frame(fmt_ctx, video_stream_id, frame, AVSEEK_FLAG_ANY);

  It always seek to frame No. 0, and always return 0 which means success.
  Then I tried to read Blender’s source code and found it really complex(maybe I should implement an image buffer ?).

  So, is there any simple way to seek to a frame with just a simple call like seek(context, frame_number)(while getting a full frame, not a broken one) ? Or, is there any lightweight library that simplifies this ?

  EDIT :
  Thanks to praks411，I found the solution :

  void AV_seek(AV * av, size_t frame)
  
  {
  
      int frame_delta = frame - av->frame_id;
  
      if (frame_delta < 0 || frame_delta > 5)
  
          av_seek_frame(av->fmt_ctx, av->video_stream_id,
  
                  frame, AVSEEK_FLAG_BACKWARD);
  
      while (av->frame_id != frame)
  
          AV_read_frame(av);
  
  }
  
  
  
  void AV_read_frame(AV * av)
  
  {
  
      AVPacket packet;
  
      int frame_done;
  
  
  
      while (av_read_frame(av->fmt_ctx, &packet) >= 0) {
  
          if (packet.stream_index == av->video_stream_id) {
  
              avcodec_decode_video2(av->codec_ctx, av->frame, &frame_done, &packet);
  
              if (frame_done) {
  
                  ...
  
                  av->frame_id = packet.dts;
  
                  av_free_packet(&packet);
  
                  return;
  
              }
  
          }
  
          av_free_packet(&packet);
  
      }
  
  }

  EDIT2 :
  Turns out there is a library for this : FFMS2.
  It is "an FFmpeg based source library [...] for easy frame accurate access", and is portable (at least across Windows and Linux).

• ffmpeg - how can I capture the timestamp of a screenshot using a seek and finding the nearest iframe ?

  26 août 2018, par Charles Kirk

  I’ve tried various different methods. ffprobe has always shown sensical results, with the pts_time increasing as the frames go on, using this command :

  ffprobe -show_frames video.mp4 > probestats.txt

  I thought about using that data to find the video frame nearest to the seconds I’m seeking to in ffmpeg, but I am trying to keep this performant, as I’m generating thumbnails on uploads that could exceed 5GB, which means I would end up with 50MB+ probe files to parse.

  So, currently, I’m looping over this ffmpeg command, so as to end up with 10 thumbnails total per file, regardless of length :

  ffmpeg -i video.mp4 -ss 00:00:05 -vf select="eq(pict_type\\,I)[s1];[s1]showinfo[out]" -vframes 1 -q:v 1 thumbs/thumb-1.jpg 2>&1

  The aim is to get the nearest iframe of the seconds I’m seeking to, e.g. find the nearest iframe near the 5 second mark and take a thumbnail — which gives some increased performance.

  The issue is, ffmpeg is outputting seemingly random pts_time’s, definitely not correct compared to the pkt_pts_time shown in ffprobe.

  So as the title states, I’m looking for an ffmpeg command to generate a thumbnail at the nearest iframe to a specified seek, and get the timestamp.