Inserting custom chapters into mkvs - sometimes chapter skips "weirdly"

[hayeled]

Total Noob!

I'm using mkvToolNix to create multiple chapters in mkv files. Some chapters turn out "good" - click next chapter button & it jumps immediately to that point and resumes playing video instantly. Other chapters are a "problem" - clicking next chapter button causes the video to very briefly pause on the "departure frame" (quarter of a second maybe?) then it appears to play a few frames in very rapid succession at the next-chapter-start point before resuming "normal" playback.

As an example, had an mkv with a mix of "good" and "problem" chapters. I'd resigned myself to this being something I just had to tolerate. Messed about readjusting the exact start point of a particular "problem" chapter just cos it didn't start where I wanted it to. Went from an original attempt at, say, chapter start at 25.9 seconds, then re-edited to change it to 25.8 seconds, then 25.75 seconds till it hit the sweet spot I wanted. And at that point, by pure chance, it stopped being a "problem" chapter and was now "good". Just made those numbers up for illustration purposes.

Is this something to do with needing to calculate specific times for chapter start points according to framerate to get an exact "good" start time for a chapter, or... yeah, I really have no idea. Just checking to see if this issue has an obvious cause to people that know about this stuff.

Cheers.

[jagabo]

Chapter points need to be on keyframes. Keyframes may be hundreds of frames apart with long GOP codecs like h.264 or h.265. If you can't live with that you'll need to reencode the videos with keyframes were you want them.

[sneaker]

Difficult to tell. Reason could be because of what jagabo said. Sometimes it can be due to lack of precision in mkv container. E.g. chapter points to 00:30:00.000 but keyframe it at 00:30:001 and the player isn't smart enough to skip to 00:30:001.

[hayeled]

Oh! Righto, cool, new info. Is there a way I can identify where existing keyframes are, time-wise? I'm using h.265 if that's relevant.

Can also reencode, probably. They were h.264 originally, and I used handbrake to make the change. Could I establish my own keyframes at that point, when I do the conversion, or are keyframes already locked in the original h.264 source?

Cheers.

[sneaker]

You can make your own keyframes. Usually you do it via the --qpfile option that x264 and x265 have but I don't know whether it is possibly to use that via HandBrake.
But: AFAIK HandBrake automatically places keyframes at chapter points.

You can use the "Info Tool" from MKVToolNix GUI to see where exactly the keyframes are.

[hayeled]

Excellent. Project for tomorrow, see if I can work it out then. Thanks guys.