How do I fade in and out in FFmpeg?

Fading in is no problem, because you start from the timestamp 0 (or from the other known timestamp or frame number), e.g. use:

ffmpeg -i input.mp4 -vf fade=in:0:d=5 -af afade=in:0:d=5 output.mp4 

for 5-second fading in from the start of your clip.

Ignore the error

Only '-vf fade=in:0:d=5' read, ignoring remaining -vf options: Use ',' to separate filters 

— see the answer of the question FFmpeg read, ignoring remaining -vf options: Use ',' to separate filters.

To fade out, you have to provide either

• the start position (the default parameter), i.e.

  • the start frame (s=) for video, or
  • the start sample (ss=) for audio,

  or

• the start time (st=) (this is the same parameter name for both audio and video)

of the fade out.

Let's use the second option — the start time of fading out.

First, figure out the total duration of video or audio (you may find it with FFprobe), then subtract the desired fade-out length from the length of the video.

For example, if you have a 10-minute video (= 600 seconds) and desire a 5-second fade out, you obtain 600 - 5 = 595.

Add filters for fading out to the previous command:

ffmpeg -i input.mp4 -vf fade=in:0:d=5,fade=out:st=595:d=5 -af afade=in:0:d=5,afade=out:st=595:d=5 output.mp4 

Notes:

1. Filtering always require re-encoding.

2. As in and 0 are defaults, you may shorten the command to

    ffmpeg -i input.mp4 -vf fade=d=5,fade=out:st=595:d=5 -af afade=d=5,afade=out:st=595:d=5 output.mp4 

As the topic asks for ffmpeg in general, an additional info to the answer of MarianD if you have only audio; there are no frames, so you have to use the start time option and for that you have to use key/value pairs (at least for fading out):

ffmpeg -i input.m4a -af afade=in:st=0:d=5,afade=out:st=595:d=5 output.m4a 

Re-encoding is required in these cases because filters are being used.

Fading in video (optionally from a start time other than 0 seconds; in this example, 10 seconds) requires specifying the start time - if you want to start somewhere other than the very beginning of the input - as well as the fade duration in the filters:

ffmpeg -ss 00:00:10.0 -i input.mp4 -vf "fade=type=in:start_time=0:duration=5" -af "afade=type=in:start_time=0:duration=5" fadein.mp4 

Fading out video (unless you actually want to fade out at the existing end of the video) requires both applying the audio/video fade filters at the right time, and trimming the video, or the end result will fade out but remain at the original length. Let's say you want to fade out starting at 20 seconds, for a duration of 5 seconds. The video will end up being 25 seconds, so it needs trimming to that, as well as the filters being applied:

ffmpeg -t 00:00:25 -i input.mp4 -vf "fade=type=out:start_time=20:duration=5" -af "afade=type=out:start_time=20:duration=5" fadeout.mp4 

You can not do fade out/in without reencoding.

I might be a bit later here, but to apply seamless fade-in / fade-out, you need just to apply that ordered filters in the end of filters chain:

• fade-in audio/video
• reverse audio/video
• fade-in audio/video
• reverse audio/video

Using audio processing as example:

ffmpeg ... -af '...,afade=t=in:st=0:d=5,areverse,afade=t=in:st=0:d=5,areverse' ... 

That thing applies 5s fade effects on both start and end of audio. It should be noted that on large input files, the reverse filter can yield higher memory consumption, so in such scenarios it would be better to inspect/calculate the duration of audio/video file (and embed that on command).