An Apple spokesperson said the new iMac will use an algorithm that employs both temporal and spatial dithering. The former takes advantage of the human eye's tendency to mix two colors in close proximity to create a blend of the two, while the latter achieves the same effect by having a pixel flash between two colors very rapidly.
This essentially tricks the eye into thinking it sees more colors than the display is capable of producing, Mantiuk said. The trick is already widely used by software like Photoshop and to get 8-bit output on 6-bit panels, he said, adding that the vast majority of users likely won't be able to distinguish the output from that of a true 10-bit display.
"The human eye is very unlikely to spot any difference," he said.
For the small number of pro users for whom it does matter, such as Mantiuk, the Apple spokesperson said it's possible to connect a third-party display to get an end-to-end 10-bit experience."
So the grand new (5000$ base prize!!!) iMac 2017 display which allows "10 Bit color" is not actually a 10 bit display at all, but a standard 8 bit panel with the same dithering trickery used to fake 10 bit, as in the older Macs. But if you still have money left after buying that machine you can go and buy an actual 3rd party 10 bit display to get 10 bits!! Wow, am i underwhelmed again!
Motivated by this i looked if iFixit has a tear-down of the 2015 iMac 27" 5k Retina, and they do:
https://www.ifixit.com/Teardown/iMac+Intel+27-Inch+Retina+5K+Display+Teardown/30260
There you can learn about the chips used in the panel controller board, and the best guess for what timing controller (tcon) they use points to a modified 8 bpc/24bpp DP 663 controller:
https://www.paradetech.com/products/dp663/
The photograph from the panel tells that the actual LCD panel is a LM270QQ1-SDA2 panel from LG. Googling for that gives us this web-site with specs for that panel:
http://www.panelook.com/LM270QQ1-SDA2_LG%20Display_27.0_LCM_parameter_23491.html
Which tells us that it is a 8 bpc panel for 16.7 million colors / 24 bpp max.
So if this info is all correct then none of the existing or upcoming Apple iMac computers has true 10 bit color display capabilities on their built in displays at all. All they do is fake 10 bpc via their current proprietary dithering algorithm while completely trashing display timing precision, or maybe on the 2017 iMac by using the built in dithering of the gpu's display engine - which would only achieve fake 10 bpc instead of the fake 11 bpc you and i measured, but at least with less broken display timing.
That would mean it would make zero sense for them to use a 10 bit framebuffer at all, as they do on the iMac, as it wouldn't be any better than the 8 bit framebuffer on the MacBookPro's. Maybe the whole reason they run with a "permanently on" 10 bit framebuffer on the "10 bit capable" iMacs is to simplify their code in case somebody connects an actual 10 bit panel from a 3rd party vendor.
Seems your HP Linux laptop will remain the only machine in your lab atm. which provides actually trustworthy stimulus reproduction at true 10 bpc and with solid timing without the danger of introducing low-level confounds.
-mario