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Is there any tool that can manipulate / transcode h264 or h265 video streams throwing away approximately half of the frames, without performing a full re-encode?

I'm thinking it would be theoretically possible to preserve motion vectors and such computed data, preserve all I-frames, and just throw away approximately half of the B- and P-frames (possibly adjusting the remaining ones.) The result does not need to be CFR.

Is there a way to do this with ffmpeg? (or any other tool)

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The solution at question " Lower framerate without re-encode " looks promising:

ffmpeg -discard nokey -i input.mp4 -c copy -vsync 0 I-only.mp4

By manpage there should be 5 levels of frame discarding: none, noref, bidir, nokey, all.

I tested this with older ffmpeg 4.2.4 and h265 and it had no effect. Probably not implemented yet.

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  • Comment for the OP question (can't comment yet): I came looking for the same thing. I need to play 1920x1080@60 files on a hardware player that can't effectively drop frames. I don't want to buy a new box just because people started sending me unnecessarily high quality files.
    – user185953
    Jan 4, 2021 at 8:48
  • It is just a matter of time when people start sending me 4k. Do you think it is technically possible to do something like this for halving the resolution?
    – user185953
    Jan 4, 2021 at 8:52
  • I personally think it's technically possible (and feasible) to halve the framerate and/or the resolution without a full re-encode, by analyzing and transforming the already encoded data, in much the same way as JPEG files can be rotated by 90 degrees and/or mirrored without re-encoding, or MP3 files could have their volume normalized without a full re-encode. Whether such a tool already exist is another question. I personally bought new hardware, so this question is not relevant to me anymore, but I'll keep it open because if there is a solution, it can be useful to others.
    – Tobia
    Jan 5, 2021 at 11:09
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I think the term for this effort is statistical multiplexing where your proposed framework would be an efficient implementation thereof.

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