HDR is not brighter. It is a different curve.

A pixel value is not a quantity of light. It is an index into a curve, and the curve decides what the number is worth: the same 0.5807 is a light grey under gamma and 203 nits under PQ. Nothing in the number says which. The field that does is the transfer characteristic, and it is the field nobody reads.

The engine is free and needs no account. And reference white is 203 nits, not the peak — BT.2408 puts graphics white, subtitle white and the level an SDR picture sits at inside an HDR container all at 203, and the 1,000 nits in the header is headroom above it for speculars and skies. Which is why the obvious tone map makes HDR look dark: scale everything by the ratio of the peaks and a 1,000-nit master on a 100-nit screen puts white at 20.3 nits, 79.7% below where a viewer expects it. The highlights keep their relationship to each other and the whole picture goes down with them. One more thing the curve gives for free — 50.8% of the code range is spent below 100 nits, the range SDR already had, because the curve is perceptual and the eye has far more to say about one nit against two than four thousand against five. HDR is mostly about the shadows. Every lane ships a worked example that costs nothing. Writing the report needs credits.