Filter vignette
  Make or reverse a vignette effect.
    Inputs:
       #0: default (video)
    Outputs:
       #0: default (video)
vignette AVOptions:
  angle             <string>     ..FV..... set lens angle (default "PI/5")
  a                 <string>     ..FV..... set lens angle (default "PI/5")
  x0                <string>     ..FV..... set circle center position on x-axis (default "w/2")
  y0                <string>     ..FV..... set circle center position on y-axis (default "h/2")
  mode              <int>        ..FV..... set forward/backward mode (from 0 to 1) (default forward)
     forward         0            ..FV.....
     backward        1            ..FV.....
  eval              <int>        ..FV..... specify when to evaluate expressions (from 0 to 1) (default init)
     init            0            ..FV..... eval expressions once during initialization
     frame           1            ..FV..... eval expressions for each frame
  dither            <boolean>    ..FV..... set dithering (default true)
  aspect            <rational>   ..FV..... set aspect ratio (from 0 to DBL_MAX) (default 1/1)

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

