Remove all non key frames from video without re-encoding

Question

I have a HEVC mkv video from which I need to remove all but key frames without re-encoding whole thing.

I found out that I can extra key frames using this

ffmpeg -i full.mkv -c:v copy -vf "select=eq(pict_type\,PICT_TYPE_I)" key.mkv

but I get:

Filtergraph 'select=eq(pict_type\,PICT_TYPE_I)' was defined for video output stream 0:0 but codec copy was selected.
Filtering and streamcopy cannot be used together.

What do I do?

Answer 1

You'll have to use the bitstream filter filter_units for this:

ffmpeg -i test.mkv -c copy -map v -bsf:v "filter_units=pass_types=16-23|32-34" key.mkv

This will only pass through random access pictures from the HEVC stream. This should cover most of them but HEVC has more keyframes types than H.264 so I'll update the types above if there are some I've missed.