How to add a afade effect to a complex filter (In this case volume)?

Question

Here is my current command :

ffmpeg -i [videoFilepath] -i [musicFile] -stream_loop -1 -shortest -filter_complex "[1]volume=0.15:enable=between(t\,4\,10)+between(t\,14\,20)[a1];[0:a][a1]amix=inputs=2" -map 0:v:0 [outputFilepath]

What it does :

• Takes a video file as an input
• Takes a music file as an input
• Loops the music if it is shorter than the video duration
• If the music is longer, outputs a file the duration of the video
• Reduce volume by 85% between 4 and 10 seconds as well as between 14 and 20 seconds
• Mixes the 2 audio streams so we have both the original video audio non-filtered and the music with the volume filter
• Outputs a video file

I want the volume filter to ease-in and ease-out for a duration of 1 second, instead of abruptly changing. Is this possible ?

Answer 1

A quick, maybe imperfect solution – use an expression in the volume filter, e.g. for linearly decreasing volume from 1 to 0.15 in the time interval between 4^th and 5^th second:

volume=1-0.85/(5-4)*(t-4):eval=frame

For time t=4, the volume will evaluate to 1, for t=5 it will be 0.15:

for t=4:    1 - 0.85 / (5-4) * (4-4) = 1 - 0.85 / (10-4) * 0 = 1 - 0 = 1
for t=5:    1 - 0.85 / (5-4) * (5-4) = 1 - 0.85 = 0.15 

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You may use (nested) if functions for evaluating the volume value in different time intervals of your audio stream.

For example, to keep the value 1 for the first 4 seconds you may extend the previous formula

              1-0.85/(5-4)*(t-4)

with the help of the lt (less than) function into

if(lt(t\,4),1,1-0.85/(5-4)*(t-4))

and then – for keeping the value 0.15 after 5^th second – into

if(lt(t\,4),1,if(lt(t\,5),1-0.85/(5-4)*(t-4),0.15))

And so on, always rewriting the last item with a new if function.

Of course, the resulting expression is not the optimal one, but the procedure is pretty straightforward.