Function held_key_start 

pub(crate) fn held_key_start(
    crf: u32,
    last: Option<(usize, u32)>,
    cap: usize,
) -> u32

The quality index a held key frame of a constant-rate session is first coded at, from crf, where the session’s last held key frame, last = (bytes, quality index), says one at crf would come out past a budget of cap bytes: as much coarser as held_key_retry would take it, so the frame is coded once rather than twice, since a rate control that runs through the held frame (libvpx’s) charges its buffer with both. A key frame coded more than twelve steps from crf says too little about one at it, and the frame is tried at crf.