Filter overlay
  Overlay a video source on top of the input.
    slice threading supported
    Inputs:
       #0: main (video)
       #1: overlay (video)
    Outputs:
       #0: default (video)
overlay AVOptions:
  x                 <string>     ..FV..... set the x expression (default "0")
  y                 <string>     ..FV..... set the y expression (default "0")
  eof_action        <int>        ..FV..... Action to take when encountering EOF from secondary input  (from 0 to 2) (default repeat)
     repeat          0            ..FV..... Repeat the previous frame.
     endall          1            ..FV..... End both streams.
     pass            2            ..FV..... Pass through the main input.
  eval              <int>        ..FV..... specify when to evaluate expressions (from 0 to 1) (default frame)
     init            0            ..FV..... eval expressions once during initialization
     frame           1            ..FV..... eval expressions per-frame
  shortest          <boolean>    ..FV..... force termination when the shortest input terminates (default false)
  format            <int>        ..FV..... set output format (from 0 to 5) (default yuv420)
     yuv420          0            ..FV..... 
     yuv422          1            ..FV..... 
     yuv444          2            ..FV..... 
     rgb             3            ..FV..... 
     gbrp            4            ..FV..... 
     auto            5            ..FV..... 
  repeatlast        <boolean>    ..FV..... repeat overlay of the last overlay frame (default true)
  alpha             <int>        ..FV..... alpha format (from 0 to 1) (default straight)
     straight        0            ..FV..... 
     premultiplied   1            ..FV..... 

framesync AVOptions:
  eof_action        <int>        ..FV..... Action to take when encountering EOF from secondary input  (from 0 to 2) (default repeat)
     repeat          0            ..FV..... Repeat the previous frame.
     endall          1            ..FV..... End both streams.
     pass            2            ..FV..... Pass through the main input.
  shortest          <boolean>    ..FV..... force termination when the shortest input terminates (default false)
  repeatlast        <boolean>    ..FV..... extend last frame of secondary streams beyond EOF (default true)

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

