Cutting with ffmpeg. Bad results.

[tomnjerry74 94]

I have an mkv I made directly from MakeMKV. I'm trying to just cut off the first minute or so with ffmpeg and keep getting unusable results.

This is the source video:

ID                                       : 1
ID in the original source medium         : 4113 (0x1011)
Format                                   : VC-1
Format profile                           : Advanced@L3
Codec ID                                 : V_MS/VFW/FOURCC / WVC1
Codec ID/Hint                            : Microsoft
Duration                                 : 52 min 22 s
Bit rate                                 : 30.0 Mb/s
Width                                    : 1 920 pixels
Height                                   : 1 080 pixels
Display aspect ratio                     : 16:9
Frame rate mode                          : Constant
Frame rate                               : 23.976 (24000/1001) FPS
Color space                              : YUV
Chroma subsampling                       : 4:2:0
Bit depth                                : 8 bits
Scan type                                : Progressive
Compression mode                         : Lossy
Bits/(Pixel*Frame)                       : 0.604
Stream size                              : 11.0 GiB (90%)
Language                                 : English
Default                                  : No
Forced                                   : No
Original source medium                   : Blu-ray

And this is the command I'm using:

ffmpeg -fflags +genpts -i st.mkv -ss 00:01:05 -c copy -map 0 output.mkv

And here's the output:

ID                                       : 1
ID in the original source medium         : 4113 (0x1011)
Format                                   : VC-1
Format profile                           : Advanced@L3
Codec ID                                 : V_MS/VFW/FOURCC / WVC1
Codec ID/Hint                            : Microsoft
Duration                                 : 51 min 17 s
Bit rate                                 : 30.0 Mb/s
Width                                    : 1 920 pixels
Height                                   : 1 080 pixels
Display aspect ratio                     : 16:9
Frame rate mode                          : Variable
Frame rate                               : 24.482 FPS
Original frame rate                      : 23.976 (24000/1001) FPS
Color space                              : YUV
Chroma subsampling                       : 4:2:0
Bit depth                                : 8 bits
Scan type                                : Progressive
Compression mode                         : Lossy
Bits/(Pixel*Frame)                       : 0.591
Stream size                              : 11.0 GiB (91%)
Language                                 : English
Default                                  : Yes
Forced                                   : No
Original source medium                   : Blu-ray

 

As you can see, it increases the framerate to variable 24.482, a seemingly random value... (If I stop the conversion mid-way through, the framerate is even larger in the hundreds.)

I've tried forcing it with -r and other commands to keep it constant and they don't help.

Anyway, I think this is messing up the output because if I seek through the video in VLC or MPV, it looks like this for about a second before fixing itself:

When I perform the same cut with MKVToolNix the output is perfectly fine.

This should probably be asked on a different forum, but maybe someone here has had a similar experience due to the nature of the files I'm working with?

Edited March 11, 2022 by tomnjerry74

[tomnjerry74 94]

I think it's because it's not cutting on a keyframe maybe?

Edited January 5, 2022 by tomnjerry74

[Luke 37065]

I would suggest doing this with mkvtoolnix instead.

[tomnjerry74 94]

More commands I've now tried (with same results):

-start_at_zero -copyts
-noaccurate_seek
-avoid_negative_ts make_zero

and moving around the -ss and -to

[tomnjerry74 94]

42 minutes ago, Luke said:

I would suggest doing this with mkvtoolnix instead.

Yeah I'm realizing this is my best bet. Looks like ffmpeg isn't that great at automatically cutting at the nearest keyframe without reencoding.

Just ended up doing:

mkvmerge -o done.mkv --split timestamps:00:01:05.000 st.mkv

Nice, simple command that works perfectly.

Thanks

[rbjtech 4265]

always do -ss FIRST for ffmpeg but ffmpeg is not a great authoring tool tbh - If mkvmerge works, then I would stick to that.

ie - using your example.

Quote

ffmpeg -ss 00:01:05 -fflags +genpts -i st.mkv -c copy -map 0 output.mkv

 

Edited January 7, 2022 by rbjtech