All the commentary seems to be accepting framing of the situation as a "over max...

alin23 · on Feb 5, 2022

They’re not dimming the SDR content. It is an optical illusion.

The HDR white is actually a lot brighter than the SDR white (which means the LEDs behind those pixels are allowed to go to full brightness).

You can see that effect if you watch this video in Safari on a 2021 MacBook Pro: https://files.alinpanaitiu.com/hdr-test-pattern.webm

Here's an article where the nits value is measured using a calibration device: https://www.notebookcheck.net/The-new-MacBook-Pro-14-only-ma...

The measurement shows that SDR content goes up to 500 nits (which was the peak value of previous MacBook displays as well) while HDR content goes up to 1607 nits.

fulafel · on Feb 5, 2022

If we have a monitor that has a brightness of 1600 nits, and whites of some content ("sdr") shows up much dimmer, the pixels the monitor is asked to display by the systmes are dimmed relative to max brightness - no?

alin23 · on Feb 5, 2022

Yes, that’s another way of putting it.

But people usually look at this, relative to previous displays, not relative to peak brightness (since the peak brightness is not achievable for prolonged periods of time because of overheating)

fulafel · on Feb 5, 2022

Overheating sounds interesting! So what happens if you display a max brightness hdr image on the screen for a long time, does the monitor shut off?

Edit: apparently at least on some laptops your screen may automatically adjust brightness downwards if you are in a hot climate and the cooling system fails to keep up: https://9to5mac.com/2022/02/04/macbook-pro-limited-brightnes...

alin23 · on Feb 5, 2022

Yes, the system monitors the display temperature and caps the brightness accordingly. Here are some interesting functions I found related to this inside the system frameworks:

    SkyLight.framework:
        SLSDisplayIsThermallyLimited
    
    CoreBrightness.framework
        -[CBDisplayModuleSKL hasThermalMitigation]
        -[CBThermalBrightnessCap getCurrentCap]
        -[CBThermalBrightnessCap setJetMode:]