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Skipping I-FRMs?

Started by tomthumb, November 23, 2016, 06:22:05 AM

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tomthumb

Version 2.6.15 64-bit
Windows 10

I have some H264 encoded mp4 files, opened with Avidemux. When seeking keyframes, it seems to "skip" some I-frames.

For example:
The first frame has a timestamp of 00:00:00.041 and is noted as an I-FRM.
https://drive.google.com/open?id=0B5DI5ie1Gb9qVkVVb19qVmZtREk

I press the double arrow button to go to the next keyframe, which takes me to another I-FRM with a timestamp of 00:00:05.380
https://drive.google.com/open?id=0B5DI5ie1Gb9qd0tqTnp0R1VFWmM

BUT,
If I press the single arrow backwards until 00:00:03.378, there is another I-FRM.
https://drive.google.com/open?id=0B5DI5ie1Gb9qb3NmbUpWWFM4OEU

There are many of these skipped I-FRMs throughout my videos.

I could be misunderstanding the difference between keyframes and intra-frames, but my quick research on Google indicates they are synonymous.

This happens whether I use the gui button, the keyboard keys, or the Go menu->Next Intra Frame

If I cut a section ending on one of these skipped I-FRMs and save, Avidemux gives me a warning about corruption of the resulting file. However, that file seems to play just fine in VLC.

I have not been able to find anyone else with this issue through using Google or the forum search.

Here is a sample video: https://drive.google.com/open?id=0B5DI5ie1Gb9qN0x6SmpQWHpEMVU

Here is an admlog.txt: https://drive.google.com/open?id=0B5DI5ie1Gb9qNVg2N2FsNWFmbUU

Has anyone else noticed this behaviour? Or am I doing something dumb?


eumagga0x2a

Thank you for the sample, confirming the behaviour on Linux both with hwaccel enabled and with the software decoder.

Jan Gruuthuse

I can confirm the behaviour as described by you.
I-FRM @ 00:00:03.378
Selecting keyframe is done with up/down arrow, so many do not come in that situation.
If you do want to cut on other frames not presented as keyframe by avidemux (up/down arrow key), you need to re-encode and not use the Video Output [Copy] mode.

Time permitting developer(s) could have a look at this. (see reply just above)

mean

I frames are not necessarily IDR
i.e. navigation point

tomthumb

Ah. I'm now reading up on IDR frames. That probably explains it.

Thanks for your help

eumagga0x2a

Thank you for the explanation on my part too. Wonder if the displayed frame type could reflect this difference.