YJL

1   Speeding up or slowing down

Using setpts video filter (and asetpts for audio or atempo, see wiki) to manipulate the timestamp of each frame:

ffmpeg -i input.mkv -r FPS -filter:v "setpts=FACTOR*PTS" output.mkv

$\text{FACTOR}$

If we speed each frame’s timestamp (PTS) up by $factor$, $\text{FACTOR}=$

$$\begin{array}{ccc}\frac{1}{factor}& & \text{if speeding up}\end{array}$$

$$\begin{array}{ccc}factor& & \text{if slowing down}\end{array}$$

Say 4x, then the original frame at 0:04 shall become 0:01 in the output video, following the equation, $setpts=\frac{1}{4}*4.0=1.0$ is the expected timestamp for the frame.

$\text{FPS}$

The output frame rate.

For speeding up, the FPS will need a boost in order to prevent frame losses, from FFmpeg wiki:

Note that this method will drop frames to achieve the desired speed. You can avoid dropped frames by specifying a higher output frame rate than the input.

If the original frame rate is $fps$ and speeding factor is $factor$, then it should be

$$\begin{array}{c}\text{FPS}=factor\cdot fps\end{array}$$

Say you have a 30-second video at 15 frames per second, that is 450 frames. When you speed it up by $factor$ of 2, the video length is reduced to $30/factor=15$ seconds, if keeping the same frame rate, that only is $30/factor\cdot 15=225$ frames. Therefore, 225 frames are dropped. By boosting up by the same factor of speeding, you can keep all $30/factor\cdot 15\cdot factor=450$ frames.

For audio, combination of video and audio, and more details, see How to speed up/slow down a video.

2   Trimming

Using -t or -to, from ffmpeg(1):

-to and -t are mutually exclusive and -t has priority.

-t duration (output)

Stop writing the output after its duration reaches duration. […]

-to position (output)

Stop writing the output at position. […]

duration is relative time since the encoding starts, position is absolute to a specified timestamp. The argument of both “may be a number in seconds, or in hh:mm:ss[.xxx] form.”

They can be used in conjunction of -ss:

-ss position (input/output)

When used as an input option (before -i), seeks in this input file to position. Note the in most formats it is not possible to seek exactly, so ffmpeg will seek to the closest seek point before position. […]

When used as an output option (before an output filename), decodes but discards input until the timestamps reach position.

[…]

It uses the same time format as above.

An example of usage, cut from 0:03, for 125-second duration and at 3:14” mark, using the output option:

ffmpeg -i input.mkv -t 125      -ss 3 output.mkv
ffmpeg -i input.mkv -t 00:03:14 -ss 3 output.mkv

3   Concatenating

file 'video1.mkv'
file 'video2.mkv'

ffmpeg -f concat -i videos.lst -c copy output.mkv

ffmpeg \
  -i video1.mkv \
  -i video2.mkv \
  -filter_complex "[0:v:0] [0:a:0] [1:v:0] [1:a:0] concat=n=2:v=1:a=1 [v] [a]" \
  -map "[v]" -map "[a]" \
  output.mkv