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I have a file

Other - Analogue Clock-JEJqy1Wlovw.mp4 which is just a download from https://www.youtube.com/watch?v=JEJqy1Wlovw

It's 2 minutes 13 seconds long.

I had some issues cutting it to the second.

I was able to use this command

ffmpeg -ss 0 -i infile.mp4 -c copy -t 60 output.mp4

to make a file that was 60 seconds, so no problem so far.

That's the file exactly 1 minute, no issue there so far.

C:\vids\a>dir
blahhclo.mp4   9,590,540 bytes

C:\vids\a>


C:\vids\a>mediainfo blahhclo.mp4
General
Complete name                            : blahhclo.mp4
Format                                   : MPEG-4
Format profile                           : Base Media
Codec ID                                 : isom
File size                                : 9.15 MiB
Duration                                 : 1mn 0s
Overall bit rate                         : 1 278 Kbps
Writing application                      : Lavf58.3.100

Video
ID                                       : 1
Format                                   : AVC
Format/Info                              : Advanced Video Codec
Format profile                           : High@L4.0
Format settings, CABAC                   : Yes
Format settings, ReFrames                : 3 frames
Codec ID                                 : avc1
Codec ID/Info                            : Advanced Video Coding
Duration                                 : 1mn 0s
Bit rate                                 : 1 146 Kbps
Width                                    : 1 920 pixels
Height                                   : 1 080 pixels
Display aspect ratio                     : 16:9
Frame rate mode                          : Constant
Frame rate                               : 25.000 fps
Color space                              : YUV
Chroma subsampling                       : 4:2:0
Bit depth                                : 8 bits
Scan type                                : Progressive
Bits/(Pixel*Frame)                       : 0.022
Stream size                              : 8.21 MiB (90%)

Audio
ID                                       : 2
Format                                   : AAC
Format/Info                              : Advanced Audio Codec
Format profile                           : LC
Codec ID                                 : 40
Duration                                 : 1mn 0s
Bit rate mode                            : Constant
Bit rate                                 : 126 Kbps
Channel count                            : 2 channels
Channel positions                        : Front: L R
Sampling rate                            : 44.1 KHz
Compression mode                         : Lossy
Delay relative to video                  : 40ms
Stream size                              : 920 KiB (10%)



C:\vids\a>

But I do have an issue when I try to cut it at the following times, and it's whether I try to cut the 1 minute one or the whole 2 min 13sec one.

If I cut with -ss 0 -t 1, it produces a file that is 1s 22ms.

So that's not too bad that's fairly close to one second. 1.022 seconds is quite good for an attempt to cut one second.

If I cut with -ss 0 -t 2, it produces a file that is 3s 19ms That is terrible! it's meant to be one second!

If I cut with -ss 0 -t 30 then it is fine it does 30s 1ms.

Or this video for example, also an mp4 https://www.youtube.com/watch?v=pJ3-xf26wDE

C:\blah>ffmpeg -ss 189 -i vid.mp4 -c copy -t 6 someoutput.mp4

C:\blah>mediainfo someoutput.mp4

produces a duration of 8s 383ms

That is so wrong though 'cos it should be 6 seconds! So that is two seconds too long!

C:\blah>ffmpeg -ss 191 -i vid.mp4 -c copy -t 6 someoutput2.mp4

C:\blah>mediainfo output2.mp4

shows that ffmpeg produced a duration of 10s 403ms

which is crazily wrong 'cos the ffmpeg command specified 6 seconds!

So that's not just one second out, that is 4 seconds out!

That's almost 50% longer than it should be!

barlop
  • 25,198

1 Answers1

3

If you need frame accuracy, don't use -c copy, which means the same as -c:a copy -c:v copy -c:s copy. It copies the bitstream, and if the specified start or end time isn't falling on an I-Frame, you get the nearest one, which can be quite far away from the desired location. See also the Seeking wiki.

You can however re-encode the video to get accurate cutting. The audio can be copied,as basically, an audio stream has "only keyframes". The audio should be copied, not re-encoded. For example, in order to encode the video to H.264 and copy the audio, apply the command:

ffmpeg -ss 191 -i vid.mp4 -c:v libx264 -c:a copy -t 6 someoutput3.mp4

That works; it produces a file that is 6s 26ms. So that is very much to the nearest second as opposed to the one with bitstream copy, that was 10.4 seconds.

Re-encoding may reduce the quality of your video stream. To change the quality of the video, read more about it in the H.264 encoding guide.

This command:

ffmpeg -ss 193 -i vid.mp4 -c:v libx264 -c:a copy -t 4 someoutput4.mp4

Produces a 4s 26ms file. Close to 4s, as it should be, whereas if I did it with bitstream copying it'd be just over 5 seconds.

barlop
  • 25,198