DisplaysShouldNotBeTVs Apple has remapped “white” to something less than 255-255-255, leaving headroom for HDR values, should they be called for. The operating system is complicit in this trickery, so the Digital Color Meter eyedropper shows “white” as 255, as do screenshots.
With Catalina, Apple quietly changed what “white” means for millions of Macs, and none of us noticed.
By the way, another unsolved issue with e.g. the M1 Air is that there is currently no way to disable HDR on the internal display and prevent the sudden shifting in brightness and color profile/tone mapping whenever there's something resembling HDR content on screen.
In some Intel Macs like the 2018 MacBook Air you could Option-click display preferences to uncheck Use extended dynamic range, but no such option appears on the M1 Air at least in Ventura.
When I Quick Look an HDR image, the brightness still increases and shows extremely bright HDR areas on that image much brighter than the white of the desktop (in cases where I'm not at maximum hardware backlight). And if I open some apps such as Blender, they very slightly activate HDR mode as well and shift around the entire color profile. With dithering disabled, this is super obvious as if I have a banding test image open at the same time the banding will shift a ton during the HDR transition.
I've messed with about every possible IOMFB numeric/boolean property that can easily be modified and cannot get internal display HDR to disable.
Unlike iOS, the "double invert" trick does not work — if you combine Invert Colors with the Option-click "Inverted Framebuffer" option in BetterDisplay, HDR will still be present. (This is different to iOS where in that case if you combine two Invert Colors, Zoom and Classic Invert, HDR can be disabled)
You can also search the Console app's log for "SkyLight" or "headroom" and see messages relating to the HDR changes taking place.
It's also really deeply embedded into the color management system as well — so even if you use max brightness (to "prevent" there from being any headroom for HDR) but then apply color profile dimming or even overlay dimming, the HDR image will still "shine through it".
One way to semi-bypass HDR is to activate full screen "Ctrl-Scroll" accessibility zoom, but you can still notice the hardware backlight behind the LCD changing whenever e.g. the Quick Look window is hidden and shown even if the colors are now "clamped"
It also points to a flaw of the "quantization" feature of BetterDisplay @waydabber, as even if quantization is set to 16, and I have a gradient displaying 16 shades on screen, there can still be an HDR white showing on screen that is brighter than that which is implying that the video card is not actually being limited to 16 shades. (I understand this may be a limitation of achieving the quantization directly through the video card LUT, as the video card is probably not taking the values given to it exactly as specified. There's also another issue with quantization I noticed where macOS will sometimes display in-between "smoothing" shades that are more precise than the set quantization level, the video card is definitely doing some kind of smoothing out of the LUT and not using it directly.)
This "fake HDR" is something that is not present in say, Asahi Linux, at all, so it's another piece of the puzzle to investigate in regards to macOS
@aiaf