Filter cropdetect
  Auto-detect crop size.
    Inputs:
       #0: default (video)
    Outputs:
       #0: default (video)
cropdetect AVOptions:
  limit             <float>      ..FV..... Threshold below which the pixel is considered black (from 0 to 65535) (default 0.0941176)
  round             <int>        ..FV..... Value by which the width/height should be divisible (from 0 to INT_MAX) (default 16)
  reset             <int>        ..FV..... Recalculate the crop area after this many frames (from 0 to INT_MAX) (default 0)
  reset_count       <int>        ..FV..... Recalculate the crop area after this many frames (from 0 to INT_MAX) (default 0)
  max_outliers      <int>        ..FV..... Threshold count of outliers (from 0 to INT_MAX) (default 0)

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

