[stavros]

Ah, this kind of makes sense, but then why wouldn't they map the old 100% to the new maximum?

I can understand the increased granularity, if you're making monitors that go much brighter, then you get more light levels, but why define the old maximum to be less than the new maximum?

[krackers]

So there is always headroom to show HDR content.

[stavros]

So the innovation of HDR is that it always makes my screen dimmer so it can sometimes drive it to full brightness and say "look, magic"?

[krackers]

Yes that's essentially how I understand it: hardware-wise HDR is basically "a really bright screen" coupled with good black levels. To ensure that you always get the HDR effect at any brightness, you need to reserve some headroom. It should be noted that you probably could not sustain this peak brightness for long anyhow due to thermal issues.

You might be interested in reading https://prolost.com/blog/edr which explains in more detail.