Filter alphamerge
  Copy the luma value of the second input into the alpha channel of the first input.
    Inputs:
       #0: main (video)
       #1: alpha (video)
    Outputs:
       #0: default (video)
alphamerge AVOptions:

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.

