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I need to transcode a video to another one with the following properties:

  • GOP size is always fixed at a given value
  • Only IDR frames are used as key-frames (not I-slices)
  • SPS/PPS/VPS frames are repeated every time before the IDR frame.

Is there a way to do this with ffmpeg?

Also, it would be great if you can also tell me of a simple way of verifying all this (perhaps using ffprobe?).

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  • Which output format?
    – Gyan
    Aug 26, 2019 at 15:36
  • H.265 output format.
    – Elektito
    Aug 26, 2019 at 15:39
  • As in, a raw bitstream?
    – Gyan
    Aug 26, 2019 at 15:42
  • Frankly, doesn't really matter. I can fix the container if I need to, later. But the raw stream is what matters to me.
    – Elektito
    Aug 26, 2019 at 15:43

1 Answer 1

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Minimal command line, for a GOP size of 2 seconds:

ffmpeg -i input -force_key_frames expr:gte(t,n_forced*2) -c:v libx265 -x265-params open-gop=0:scenecut=0:repeat-headers=1 output.hevc

Verification:

ffmpeg -i output.hevc -c copy -bsf:v trace_headers -f null -

After each line containing key frame e.g.,

Packet: 18115 bytes, key frame, no pts, dts 2319977, duration 40000.

You should see the VPS/SPS/PPS. The first Slice Segment Header that follows should have a NAL unit type of 20 which is IDR_N_LP.

ffprobe can't invoke bitstream filters.

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  • The seems to be working. Thanks. The verification command doesn't work for me though ("Unknown bitstream filter trace_headers"). Also, not that it's a problem, but there is a "supplemental enhancement information" NAL after VPS, SPS and PPS which wasn't there before. Could you tell me why this particular command causes that to appear?
    – Elektito
    Aug 27, 2019 at 7:28
  • Oh, and also, the expr:gte(t,n_forced*2) part of the first ffmpeg command needs to be quoted. I'd suggested you edit that in the interest of the future copy-pasters!
    – Elektito
    Aug 27, 2019 at 8:17
  • So maybe I was wrong. I tried this with another video and looks like it does not always work. Keyframes frequently happen before the 250 frames (10 seconds) that I set is up (I'm assuming the n_forced*2 determines GOP size and I changed it to n_forced*10). I tried adding min-keyint=250:keyint=250 to the x2675 params. No effect. I also tried the -g option of ffmpeg. Still the same thing happens.
    – Elektito
    Aug 27, 2019 at 11:51
  • if your source is a container with VFR stream and/or fps < 25, then there may not be 250 frames in a 10 second interval. You can change t to n and the value to a frame distance
    – Gyan
    Aug 27, 2019 at 14:40
  • The FPS I'm sure is 25 (or so ffprobe says). I'm not sure about VFR (that's variable frame rate, right?), but I think it's unlikely. I think the problem is that whatever we do libx265 seems to not honor min-keyint. Your force_key_frames option does work and it fixes my main issue, but x265 right at the beginning says that Keyframe min / max / scenecut : 126 / 250 / 0 even though I've set both min-keyint and keyint to 250.
    – Elektito
    Aug 27, 2019 at 15:04

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