I have a MP7,1 with a Vega II duo and a couple of XDRs hooked up to it: (up to date on software and firmware)
Youtube:
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I used this as a test:
+ In chrome plays as HDR both windowed and full screen
- In safari no HDR
- In Firefox: no HDR
- In Opera: no HDR
VLC:
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I've never been able to get VLC to output HDR either.
tv.app:
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Apple TV+ content in the tv.app plays HDR (I think) I'm not 100% sure to be honest. Maybe just need more obvious source material.
I do have an LG OLED so I know how it reacts to HDR and Dolby Vision inputs, and the effect is a bit less dramatic on the XDR, still in things like the youtube video it's quite visible without any transition or message on screen that it started to receive HDR content or so.
Settings of the displays:
View attachment 939416
On a battery powered laptops and internal displays etc. you need to do more settings:
Support for high dynamic range (HDR) video, such as HDR movies and TV shows from Apple or other providers, requires the latest version of macOS, a compatible Mac model, and an HDR10-compatible display, TV, or projector.
support.apple.com