Function ncplane_fadein 

pub unsafe extern "C" fn ncplane_fadein(
    n: *mut ncplane,
    ts: *const timespec,
    fader: fadecb,
    curry: *mut c_void,
) -> c_int

Fade the ncplane in over the specified time. Load the ncplane with the target cells without rendering, then call this function. When it’s done, the ncplane will have reached the target levels, starting from zeroes.