FFMPEG slow / fast motion part a video anywhere

Question

am trying to cut a part of Video

Here is the code i have tried Let's say i have 6 seconds video and i want to slow down 2:5 seconds video that 3 second video

First approach after searching is, Cut the video, then apply slow filter then concat it with original video. But this is a mid part not entire video. How can i modify below code

ffmpeg -i Soon.mp4 -filter_complex [0:v]trim=2:5,setpts=PTS-STARTPTS[v1];[v1]setpts=0.5PTS[v1];[0:a]atempo=2[a] concat=n=1:v=1:a=1 -map "[v]" -map "[a] -preset superfast -profile:v baseline output.mp4

Answer 1

This slows down one part of a video and keeps the rest as is.

ffmpeg -i Soon.mp4
  -filter_complex
     "[0:v]trim=0:2,setpts=PTS-STARTPTS[v1];
      [0:v]trim=2:5,setpts=2*(PTS-STARTPTS)[v2];
      [0:v]trim=5,setpts=PTS-STARTPTS[v3];
      [0:a]atrim=0:2,asetpts=PTS-STARTPTS[a1];
      [0:a]atrim=2:5,asetpts=PTS-STARTPTS,atempo=0.5[a2];
      [0:a]atrim=5,asetpts=PTS-STARTPTS[a3];
      [v1][a1][v2][a2][v3][a3]concat=n=3:v=1:a=1"
  -preset superfast -profile:v baseline output.mp4

How does it work?

• The trim and atrim filters cut the video into different parts, from 0–2 seconds, from 2–5, and from 5 to the end.
• In each part a setpts/asetpts filter is applied, which, when using the option PTS-STARTPTS, "resets" the presentation timestamps of each frame in each part, so that they can later be concatenated easily.
• Each part is given an output label (e.g. [v1] through [v3]).
• For the video parts that need to be slowed down, every presentation timestamp is doubled (2*(…)), which effectively halves the speed of the video. If you wanted to speed up the video, use a multiplier lower than 1.
• For audio parts to be sped up / slowed down, use the atempo filter, whose parameter is the speedup (e.g., 0.5 = half speed).
• These parts are finally concatenated using the concat filter.

Answer 2

Thanks to Gyan's answer! Taking that plus a bit research, I have applied it to speed up a video with audio by 6 times. Be aware that atempo argument has a value ranging between 0.5 and 2. So three atempo were used to make up 6 (2 x 2 x 1.5). So the idea is to make the video from 00:00:00 to 00:17:30, a total of 1050 seconds, to play 6 times faster and the rest stays unchanged. Code example as follows:

ffmpeg -i in_file.mp4 -filter_complex \
   "[0:v]trim=0:1050,setpts=1/6*(PTS-STARTPTS)[v1];\
   [0:v]trim=1050,setpts=PTS-STARTPTS[v2];\
   [0:a]atrim=0:1050,asetpts=PTS-STARTPTS,atempo=2,atempo=2,atempo=1.5[a1];\
   [0:a]atrim=1050,asetpts=PTS-STARTPTS[a2];\
   [v1][a1][v2][a2]concat=n=2:v=1:a=1" \
   output_file.mp4

Answer 3

@ slhck

This slows down one part of a video and keeps the rest as is.

I've never seen PTS-STARTPTS before. I have always used just PTS for this purpose. I have a recent script that hangs with just PTS but it works with PTS-STARTPTS. Can you explain why?

With PTS

ffmpeg -i "input.mkv" \
-filter_complex \
" \
[0:v]trim='00\:08\:23.961':'00\:08\:24.379',setpts=4*PTS,curves=psfile=curves40.acv,unsharp=13:13:1.5:13:13:1.5[v0]; \
[0:a]atrim='00\:08\:23.961':'00\:08\:24.379',asetpts=PTS,atempo=0.5,atempo=0.5[a0]; \
[0:v]trim='00\:08\:23.961':'00\:08\:24.379',reverse,setpts=4*PTS,curves=psfile=curves40.acv,unsharp=13:13:1.5:13:13:1.5[v1]; \
[0:a]atrim='00\:08\:23.961':'00\:08\:24.379',areverse,asetpts=PTS,atempo=0.5,atempo=0.5[a1]; \
[v0][a0][v1][a1]concat=n=2:v=1:a=1[out] \
" \
-map "[out]" \
-r $fps -c:v libx264 -preset slow -crf 10 -map_chapters -1 -bf 2 -flags +cgop -pix_fmt yuv420p -codec:a aac -strict -2 -b:a 384k -r:a 48000 \
-movflags faststart "otput.mp4"

With PTS-STARTPTS

ffmpeg -i "input.mkv" \
-filter_complex \
" \
[0:v]trim='00\:08\:23.961':'00\:08\:24.379',setpts=4*(PTS-STARTPTS),curves=psfile=curves40.acv,unsharp=13:13:1.5:13:13:1.5[v0]; \
[0:a]atrim='00\:08\:23.961':'00\:08\:24.379',asetpts=PTS-STARTPTS,atempo=0.5,atempo=0.5[a0]; \
[0:v]trim='00\:08\:23.961':'00\:08\:24.379',reverse,setpts=4*(PTS-STARTPTS),curves=psfile=curves40.acv,unsharp=13:13:1.5:13:13:1.5[v1]; \
[0:a]atrim='00\:08\:23.961':'00\:08\:24.379',areverse,asetpts=PTS-STARTPTS,atempo=0.5,atempo=0.5[a1]; \
[v0][a0][v1][a1]concat=n=2:v=1:a=1[out] \
" \
-map "[out]" \
-r $fps -c:v libx264 -preset slow -crf 10 -map_chapters -1 -bf 2 -flags +cgop -pix_fmt yuv420p -codec:a aac -strict -2 -b:a 384k -r:a 48000 \
-movflags faststart "otput.mp4"

In both cases I get frame= 0 fps=0.0 q=0.0 size= 0kB time=00:00:00.00 bitrate=N/A speed= 0x but it works in this case.