Why can't my video play from where I drag the playback bar?

Question

I am stuck and really do not know how to explain it in correct manner. Thus I put it in the layman details.

I have two 30~ second length mp4 videos saved in cloud storage.

• video1 is generated by somebody else and
• video2 is generated by me on ffmpeg.

They both have the same video content.

My issue is

If I open video2 in Safari browser, I cannot locate the accurate time (by sec) in the video player progress bar.

What I mean is, for example I take steps:

1. I pause the video,
2. drag the progress bar to the first second,
3. I play the video.

the video player will jump to 0 and play, instead of playing from 1 second.

There are multiple time-slots in video2 that have this issue.

However, video1 does not have this issue.

video1 info from ffmpeg

Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'origin.mp4':
  Metadata:
    major_brand     : isom
    minor_version   : 512
    compatible_brands: isomiso2avc1mp41
    encoder         : Lavf56.4.101
  Duration: 00:00:33.78, start: 0.000000, bitrate: 584 kb/s
    Stream #0:0(und): Video: h264 (High) (avc1 / 0x31637661), yuv420p, 640x360 [SAR 1:1 DAR 16:9], 545 kb/s, 23.98 fps, 23.98 tbr, 11988 tbn, 47.95 tbc (default)
    Metadata:
      handler_name    : VideoHandler
    Stream #0:1(und): Audio: aac (LC) (mp4a / 0x6134706D), 44100 Hz, stereo, fltp, 32 kb/s (default)
    Metadata:
      handler_name    : SoundHandler

video2 info from ffmpeg

Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'my.mp4':
  Metadata:
    major_brand     : isom
    minor_version   : 512
    compatible_brands: isomiso2avc1mp41
    encoder         : Lavf57.71.100
  Duration: 00:00:35.04, start: 0.000000, bitrate: 734 kb/s
    Stream #0:0(und): Video: h264 (High) (avc1 / 0x31637661), yuv420p, 640x360 [SAR 90:67 DAR 160:67], 599 kb/s, 23.98 fps, 23.98 tbr, 24k tbn, 47.95 tbc (default)
    Metadata:
      handler_name    : VideoHandler
    Stream #0:1(eng): Audio: aac (LC) (mp4a / 0x6134706D), 48000 Hz, stereo, fltp, 129 kb/s (default)
    Metadata:
      handler_name    : SoundHandler

Answer 1

video1 has a very short keyframe interval - it has a keyframe every few frames. video2 doesn't. Some players have the ability to decode upto non-KF silently and start playing from seek point. If yours doesn't, then it will start from the nearest keyframe preceding the seek point.

ffprobe origin.mp4 -show_entries frame=key_frame,pkt_pts_time -select_streams v -of compact=p=0 | grep ame=1

Result:

key_frame=1|pkt_pts_time=0.000000
key_frame=1|pkt_pts_time=0.333667
key_frame=1|pkt_pts_time=0.667334
key_frame=1|pkt_pts_time=1.001001
key_frame=1|pkt_pts_time=1.334668
key_frame=1|pkt_pts_time=1.668335
key_frame=1|pkt_pts_time=2.002002
...
key_frame=1|pkt_pts_time=32.699366
key_frame=1|pkt_pts_time=33.033033
key_frame=1|pkt_pts_time=33.366700
key_frame=1|pkt_pts_time=33.700367

Video2:

ffprobe my.mp4 -show_entries frame=key_frame,pkt_pts_time -select_streams v -of compact=p=0 | grep ame=1

Result:

key_frame=1|pkt_pts_time=0.000000
key_frame=1|pkt_pts_time=2.293958
key_frame=1|pkt_pts_time=5.171833
key_frame=1|pkt_pts_time=7.257250
key_frame=1|pkt_pts_time=9.718042
key_frame=1|pkt_pts_time=13.221542
key_frame=1|pkt_pts_time=15.473792
key_frame=1|pkt_pts_time=21.271250
key_frame=1|pkt_pts_time=24.357667
key_frame=1|pkt_pts_time=29.195833
key_frame=1|pkt_pts_time=32.407375

---

Add -g 8 to ffmpeg transcode command to place a keyframe every 8 frames.