Constant EMPTY_SLICE_BYTES 

pub(crate) const EMPTY_SLICE_BYTES: usize = 32;

Bytes a slice that refines nothing comes to at most: a frame of x264’s four slices skipping every block takes 61 to 66 at 720p and 1080p with the wire header, a third of the budget at 0.1 Mbit/s, so never a small frame.