Scene-linear level below which HDR output reproduces the SDR operator
exactly (by construction, whatever the knee). Neutral only starts
compressing once its peak, less a black-level offset of at most 0.04, passes
0.76 — so up to ~0.8 on grey — hence its higher knee: a lower one would roll
off a range SDR still shows linearly (verified never darker than SDR).
Scene-linear level below which HDR output reproduces the SDR operator exactly (by construction, whatever the knee). Neutral only starts compressing once its peak, less a black-level offset of at most 0.04, passes 0.76 — so up to ~0.8 on grey — hence its higher knee: a lower one would roll off a range SDR still shows linearly (verified never darker than SDR).