Avidemux 2.7.8 - problem with cutting outside keyframes

[phaolo]

Sometimes I need to cut ouside keyframes. To avoid re-encoding the full video, I losslessly cut as much as possible and encode just a small segment (with as similar settings as possible).
The problem is that this trick often fails (I get a garbled image after the cut, till a keyframe) and I don't know why exactly.

For example:

S=video start
E=video end
k=keyframe
|=cut


S------------------------k---|------k-------|---k----E

part1 = lossless
part2 = removed
part3 = encoded

When I rejoin p1 & p3 I'd expect them to be fixed like:
S------------------------k---K---k----E

Doesn't it work like this?

[eumagga0x2a]

with as similar settings as possible

Being close is not enough. The codec extradata from both videos must match perfectly (bit-perfectly).

[phaolo]

with as similar settings as possible

Being close is not enough. The codec extradata from both videos must match perfectly (bit-perfectly).

Err.. what is a codec extradata?
I used the same video codec, encoding method (e.g:CRF), frame size, framerate, audio codec and kHz. What more is needed?

[butterw]

Look for the line Encoding settings in MediaInfo Video properties, most x264/x265 encoded files will have it.

[phaolo]

Look for the line Encoding settings in MediaInfo Video properties, most x264/x265 encoded files will have it.

Oof.. I don't know how to replicate all that.
Could I pass such parameters to Avidemux?

[butterw]

If you didn't encode the original video yourself or used custom settings: it may prove difficult.

[eumagga0x2a]

Err.. what is a codec extradata?

If your source video with a H.264 or HEVC video track is a MP4/MOV or MKV, load it in Avidemux, open Avidemux log and search for the lines following "The video codec has some extradata". This binary structure, represented as hex, must be identical for all merged videos.

This is more difficult to assess in MPEG-TS streams as they by design need to handle codec parameters switching on the fly and extradata is prepended to each keyframe.