Issues with Mac mini M2 Pro and 8k displays, plus solution

I previously asked a question about a Mac mini M2 Pro with an 8k@60 Hz display connected over HDMI, asking about using chroma 4:4:4. I found a solution, so I‘ll describe it here.

I have a display that supports 8k@60 Hz with chroma 4:4:4. With my Mac mini, 8k@60 Hz uses chroma 4:2:0 subsampling, while 8k@30 Hz uses chroma 4:4:4. The difference is that chroma 4:4:4 is true 8k, while chroma 4:2:0 is 8k for luminance, but only 4k for colour. In other words, each group of four pixels in 8k has independent brightness, but a shared colour. Unsurprisingly, this approach can make text difficult to read with certain colour combinations.

I couldn't get an answer from Apple Support as to whether the M2 Pro is capable of 8k@60 Hz with chroma 4:4:4, but I’ve determined that it is, and that the limitation to chroma 4:2:0 subsampling is an apparent bug in Apple’s software.

The solution is to use BetterDisplay, which can be downloaded from GitHub at waydabber/BetterDisplay. The Pro version is $19, but well worth it.

The following steps work to achieve chroma 4:4:4.

1. Download and install BetterDisplay
2. In BetterDisplay to create and enable a virtual 8k display*
3. In System Settings/Displays, set the virtual display to mirror the main display

Once the virtual display is connected, the main display magically switches to chroma 4:4:4 mode. By default, the virtual display is used as an extended display, which isn’t useful because there’s no actual display, so anything moved to it isn’t visible. Switching to mirroring solves that problem.

There really should be a way to configure the chroma settings from System Settings/Displays. Unless and until Apple adds that option or fixes the bug causing 8k@60 Hz to use chroma 4:2:0, even when the cable and display support chroma 4:4:4, this workaround does the job.

BetterDisplay adds other nice features too, such as allowing scaling factors other than 200%, by creating virtual displays larger than 8k, e.g., a display with 10k/12k native and 5k/6k HiDPI modes, and then setting the main display to mirror the virtual display.

* A different resolution for the virtual display seems to work too, so a lower resolution might reduce the mirroring overhead.