Filter afade
  Fade in/out input audio.
    Inputs:
       #0: default (audio)
    Outputs:
       #0: default (audio)
afade AVOptions:
  type              <int>        ..F.A.... set the fade direction (from 0 to 1) (default in)
     in              0            ..F.A.... fade-in
     out             1            ..F.A.... fade-out
  t                 <int>        ..F.A.... set the fade direction (from 0 to 1) (default in)
     in              0            ..F.A.... fade-in
     out             1            ..F.A.... fade-out
  start_sample      <int64>      ..F.A.... set number of first sample to start fading (from 0 to I64_MAX) (default 0)
  ss                <int64>      ..F.A.... set number of first sample to start fading (from 0 to I64_MAX) (default 0)
  nb_samples        <int64>      ..F.A.... set number of samples for fade duration (from 1 to I64_MAX) (default 44100)
  ns                <int64>      ..F.A.... set number of samples for fade duration (from 1 to I64_MAX) (default 44100)
  start_time        <duration>   ..F.A.... set time to start fading (default 0)
  st                <duration>   ..F.A.... set time to start fading (default 0)
  duration          <duration>   ..F.A.... set fade duration (default 0)
  d                 <duration>   ..F.A.... set fade duration (default 0)
  curve             <int>        ..F.A.... set fade curve type (from 0 to 17) (default tri)
     tri             0            ..F.A.... linear slope
     qsin            1            ..F.A.... quarter of sine wave
     esin            2            ..F.A.... exponential sine wave
     hsin            3            ..F.A.... half of sine wave
     log             4            ..F.A.... logarithmic
     ipar            5            ..F.A.... inverted parabola
     qua             6            ..F.A.... quadratic
     cub             7            ..F.A.... cubic
     squ             8            ..F.A.... square root
     cbr             9            ..F.A.... cubic root
     par             10           ..F.A.... parabola
     exp             11           ..F.A.... exponential
     iqsin           12           ..F.A.... inverted quarter of sine wave
     ihsin           13           ..F.A.... inverted half of sine wave
     dese            14           ..F.A.... double-exponential seat
     desi            15           ..F.A.... double-exponential sigmoid
     losi            16           ..F.A.... logistic sigmoid
     nofade          17           ..F.A.... no fade; keep audio as-is
  c                 <int>        ..F.A.... set fade curve type (from 0 to 17) (default tri)
     tri             0            ..F.A.... linear slope
     qsin            1            ..F.A.... quarter of sine wave
     esin            2            ..F.A.... exponential sine wave
     hsin            3            ..F.A.... half of sine wave
     log             4            ..F.A.... logarithmic
     ipar            5            ..F.A.... inverted parabola
     qua             6            ..F.A.... quadratic
     cub             7            ..F.A.... cubic
     squ             8            ..F.A.... square root
     cbr             9            ..F.A.... cubic root
     par             10           ..F.A.... parabola
     exp             11           ..F.A.... exponential
     iqsin           12           ..F.A.... inverted quarter of sine wave
     ihsin           13           ..F.A.... inverted half of sine wave
     dese            14           ..F.A.... double-exponential seat
     desi            15           ..F.A.... double-exponential sigmoid
     losi            16           ..F.A.... logistic sigmoid
     nofade          17           ..F.A.... no fade; keep audio as-is

This filter has support for timeline through the 'enable' option.

