screencast-splicer

Cuts several recorded videos into pieces and joins them together again.

A tool created to scratch my own itch: This intended for use in screencast post-processing.

Usage

Ensure that you have ffmpeg version 4.2.2 or later installed on your system.

npx screencast-splicer instructions.json screencast.mp4

Input

An example JSON file for instructions.json:

{
  "events": [
    {
      "sourceClip": "one.mp4",
      "sourceStart": "00:00:00.0000",
      "sourceDuration": "00:00:07.5000"
    },
    {
      "sourceClip": "one.mp4",
      "sourceStart": "00:00:05.0000",
      "sourceDuration": "00:00:07.5000"
    },
    {
      "sourceClip": "two.mp4",
      "sourceStart": "00:00:18.0000",
      "sourceDuration": "00:00:12.5000"
    },
    {
      "sourceClip": "one.mp4",
      "sourceStart": "00:00:10.0000",
      "sourceDuration": "00:00:07.5000"
    },
    {
      "sourceClip": "one.mp4",
      "sourceStart": "00:00:15.0000",
      "sourceDuration": "00:00:12.5000"
    }
  ]
}

This assumes that you have one.mp4 and two.mp4 in your current directory.

Output

This will generate a file named screencast-output-ffmpeg.tmp.sh in your current directory, which should contain something similar to:

#!/bin/sh

# slice pieces
ffmpeg -i "one.mp4" -ss "00:00:00.0000" -t "00:00:07.5000" -c copy "screencast-output.01.tmp.mp4"
ffmpeg -i "one.mp4" -ss "00:00:05.0000" -t "00:00:07.5000" -c copy "screencast-output.02.tmp.mp4"
ffmpeg -i "two.mp4" -ss "00:00:18.0000" -t "00:00:12.5000" -c copy "screencast-output.03.tmp.mp4"
ffmpeg -i "one.mp4" -ss "00:00:10.0000" -t "00:00:07.5000" -c copy "screencast-output.04.tmp.mp4"
ffmpeg -i "one.mp4" -ss "00:00:15.0000" -t "00:00:12.5000" -c copy "screencast-output.05.tmp.mp4"

# concatenate pieces
ffmpeg -f concat -safe 0 \
 -i screencast-output-ffmpeg-concat.tmp.txt \
 -c copy ./screencast-output.mp4

# clean up
rm "screencast-output.01.tmp.mp4"
rm "screencast-output.02.tmp.mp4"
rm "screencast-output.03.tmp.mp4"
rm "screencast-output.04.tmp.mp4"
rm "screencast-output.05.tmp.mp4"
rm "screencast-output-ffmpeg-concat.tmp.txt"
rm "screencast-output-ffmpeg.tmp.sh"

Run this file:

./screencast-output-ffmpeg.tmp.sh

Which will run various ffmpeg commands, and then clean up after itself including deleting all the intermediate video clip files.

Caveats

• There is an issue with ffmpeg in which the seek start and stop times are not exact, so clips can start slightly later than they are meant to, or end slightly earlier than they are supposed to. Sometimes this affects audio and video channels separately. This is more of an issue when joining multiple short clips, and not so much with longer clips.
  • Suggestions for improving this are very welcome!
• The all of the input files and the output files should have the same audio and video codecs, resolution, and sample rates. This is inherent from the use of the -c copy option on ffmpeg and is intended avoid transcoding to deliver speedier results.

Author

Brendan Graetz

Licence

GPL-3.0