How can I insert the missing keyframe in the video using FFmpeg?

Tech QnA
1

I converted a video using FFmpeg and now want to add the missing keyframe at the beginning of the newly generated video. How can I do that? Are there any flags to use?

2

Hi Lvan!

In order to insert the missing keyframe in the video, FFmpeg provides various flags/options to perform different tasks, and to add a keyframe at the beginning of a video using FFmpeg, you can use the -force_key_frames option and specify the position of the keyframe you want to add.

For example:

ffmpeg -i input.mp4 -c copy -force_key_frames "expr:gte(t,n_forced*2)" output.mp4
Here,

  • -i input.mp4 is the input video file.
  • -c copy is used to copy the audio and video streams without re-encoding and making sure to fast process.
  • -force_key_frames "expr:gte(t,n_forced*2)" is the main part of this command. At regular intervals, in this case, every two seconds, it forces a keyframe. The interval can be changed to suit your needs.
  • The expression expr:gte(t,n_forced*2) It determines whether the current time (t) is greater than or equal to the number of forced keyframes times the interval, in this case, two seconds.
  • output.mp4 is the output video file.
3

For example, to add a keyframe at the very beginning of the video, you can use a command like this:
ffmpeg -i input.mp4 -force_key_frames “expr:gte(t,0)” output.mp4
This will tell FFmpeg to generate a keyframe at the first frame of the video (which is at time t=0). You can also use this option to specify the interval at which keyframes should be generated, by using a value like expr:gte(t,n), where n is the number of seconds between keyframes.

4

Will this increase the size of the final video?

5

Yes, adding keyframes can increase the size of the output video file, as keyframes contain more information than normal frames and are therefore larger.

You can use the -g option to specify the maximum number of frames between keyframes, which can help to balance the trade-off between file size and seeking performance.
For more information about the -force_key_frames option and other options related to keyframes, you can refer to the FFmpeg documentation.