Mac mini 2018 and 5K Monitors question...

Ledgem

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Hi everyone,

I have a question about the latest Mac mini and 5K monitors. I suppose this could apply to most Macs with Thunderbolt 3.

Apple's page lists the Mac mini as being able to only support one 5K monitor at its full resolution. But I have a question about that:

1) If you use the monitor at its scaled resolution (so half the resolution, but "pixel doubled" so everything looks very crisp - Apple's default on the retina iMacs), can you use two monitors, as the usable resolution is technically less than 5K?

2) If you use an external GPU, does this get around the limitation such that you can then use two 5K displays? (In other words, is the limitation due to the GPU, or due to the number of Thunderbolt buses)?

Thanks for your thoughts!
 

Stephen.R

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It's a GPU issue that limits how many 5K displays the Mini will run, but it's still sending a 5K image to the display - it's just that everything's been rendered @2x.

For a TB3 5K display your only real choice for an eGPU is the Blackmagic one - the others don't have TB3 output ports, and sending multiple 5K streams back over the TB3 cable to go out via the Mini's TB3 ports to the displays would give terrible performance.
 

frou

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Mar 14, 2009
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Per Intel's specs, the integrated graphics in the MM (UHD 630) supports a maximum resolution of 4096x2304 on any output, so it's already somehow at a stretch to work with one 5K monitor.
 

Ledgem

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It's a GPU issue that limits how many 5K displays the Mini will run, but it's still sending a 5K image to the display - it's just that everything's been rendered @2x.
Now I'm wondering about this: I bought a 27" UltraFine in anticipation of moving to a Mac mini, and intended to use it with my iMac in a dual-monitor configuration until I make the switch. I'm using a Late 2015 27" retina 5K iMac, and according to Apple this system is limited to using the Ultrafine at a 4K resolution. Fine by me for now. Yet to my surprise, it looks identical to my iMac's display. When I look in the preferences panel the resolution is set to Scaled, same as my iMac's internal display, and the operating system claims "Looks like 2560 x 1440" (pixel doubling)... which is also what my iMac says for its own 27" display.

To be totally fair, I bought a cable that was too short to use and so I couldn't have both displays side-by-side; they were next to each other but facing slightly away from one another. It's possible that when I have them both oriented properly then I'll see a difference, yet the stated scaled resolutions make me think that they truly are the same. This leads me to believe one of two things:

1) The late 2015 retina iMac with the upgraded graphics card (which is my model) can actually drive a second 5K display, but Apple's documentation assumes the base graphics card. This seems unlikely, though, as I believe Thunderbolt 2 is the limiting factor preventing a second 5K monitor from being used.
2) Scaling the resolution really outputs the lower resolution, and when Apple talks about being limited to 4K, they're talking about the unscaled resolution. I'll have to test this one by trying to take the display to unscaled, 1:1 mode; if this theory is right, then the iMac's internal display will be able to go to 5120 x 2880 while the Ultrafine will be stuck at 3840 x 2160.

If #2 is truly the case then someone could buy two 5K monitors and use them in scaled mode with the Mac mini without needing an eGPU, and the limitation only applies to people who do not want to scale their displays. I'd imagine the majority are sticking with the default setting of scaling, though. Even if that's right, I don't blame Apple for talking about the limitations, as saying it's supported only in scaled mode would probably lead to a lot of confusion.

For what it's worth, people aren't kidding when they say that the Ultrafine probably uses the same panels as the retina iMacs - the image really looks identical.
 

Stephen.R

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If #2 is truly the case then someone could buy two 5K monitors and use them in scaled mode with the Mac mini without needing an eGPU
That isn't really how it works. I mean, you can do that, but you're not getting the benefit of the 5K panel - but now you have another level of scaling going on: macOS renders everything in 2x on a 5120x2880 (5K) framebuffer, then scales it in software down to 3840x2160 (4k), then sends that signal to the display, which then scales it back up to the physical pixels of the display (5120x2880).

In that scenario a 4K 27" panel would be better (not great still but better) because it'd skip the final scale-up in the display.
 

theluggage

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Jul 29, 2011
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When I look in the preferences panel the resolution is set to Scaled, same as my iMac's internal display, and the operating system claims "Looks like 2560 x 1440" (pixel doubling)... which is also what my iMac says for its own 27" display.
Yeah... you can't actually deduce much from that.

(a) You can run a 4k display in 'looks like 2560x1440' too - as @Stephen.R said, what you get is a 5k screen rendered internally downsampled to 4k before it is sent to the display. That's probably what's happening when you hook the 5k up to your 2015 iMac.

(b) You can't trust the info the system gives you: About This Mac -> System Report -> Displays is currently assuring me that the external display connected to my iMac has a resolution 6016x3384... If you believe that my $400 Dell S2817Q has magically morphed into an Apple Pro Display XDR, I'll sell you the stand for $900! Its reporting the resolution of the internal buffer - its set to 'looks like 3008x1692' so its being rendered internally at 6016x3384 and then downsampled to 4k. In "looks like 2560 x 1440" mode, System Report shows a 4k display as 5120x2880. It isn't.

4k in 'Looks like 2560x1440' mode is actually pretty good, though, and you need to look closely to distinguish it from 5k.

AFAIK the primary reason that the Mini can't use two 5k displays is that each 5k display needs two DisplayPort 1.2 streams*, so it would be like driving four displays from a GPU that only supports three...

However, as far as the Mini is concerned, even if it will run two 4k displays in 'looks like 1440p', the GPU is doing the work of rendering two 'virtual' 5k displays and downsampling them to 4k - and that's probably going to be a bit much for the Mini's integrated GPU. When you get your mini, be prepared to buy an eGPU just to run two 4k displays smoothly (especially in scaled mode). Certainly, don't get the 8G RAM version, because the iGPU uses system ram for video...

* Your 2015 iMac only has Thunderbolt 2 and although Thunderbolt 2 supports two DisplayPort 1.2 streams, it doesn't allow a single peripheral to extract both (which is why you couldn't get a dual-display TB2 dock, or hang a DisplayPort adapter off the back of the old Thunderbolt display). Now, its vaguely plausible that there's an un-documented 'feature' that allows a TB3 peripheral connected via the TB2-TB3 adapter to extract both streams from a TB2 computer (I don't have enough tech knowledge about Thunderbolt to debunk that) but its more likely that you're just mistaking 'looks like 1440p @ 4k' for true 5k.
 

Stephen.R

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However, as far as the Mini is concerned, even if it will run two 4k displays in 'looks like 1440p', the GPU is doing the work of rendering two 'virtual' 5k displays and downsampling them to 4k - and that's probably going to be a bit much for the Mini's integrated GPU.
Can confirm. A mini will drive one 4K in a non-“efficient” scaling mode (ie looks like 1440p) or at least two in “efficient” scaling mode (looks like 1080p) without issues. Two in non-efficient scaling is too much, even with 64GB of ram.
[doublepost=1563649062][/doublepost]I should clarify: it’ll run the displays at “looks like 1440p” but it is laggy and can be glitchy.
 

frou

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(b) You can't trust the info the system gives you: About This Mac -> System Report -> Displays is currently assuring me that the external display connected to my iMac has a resolution 6016x3384... If you believe that my $400 Dell S2817Q has magically morphed into an Apple Pro Display XDR, I'll sell you the stand for $900! Its reporting the resolution of the internal buffer - its set to 'looks like 3008x1692' so its being rendered internally at 6016x3384 and then downsampled to 4k. In "looks like 2560 x 1440" mode, System Report shows a 4k display as 5120x2880. It isn't.
There's yet another source of screwiness provided by the Display tab of About This Mac (distinct from System Report).

Here that tab talks about 4K but in actuality there's nothing relating to 4K in this setup (confirmed by looking at the active signal information in the monitor's OSD menu)

Screenshot 2019-07-21 at 00.00.08.png
 
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Ledgem

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Well, I guess my observations shouldn't guide others' decisions, because it seems that my Mac is running the UltraFine at native 5K:

Mac:
Display Type: Built-In Retina LCD
Resolution: 5120 x 2880 Retina
Framebuffer Depth: 30-Bit Color (ARGB2101010)
Main Display: Yes
Mirror: Off
Online: Yes
Rotation: Supported
Automatically Adjust Brightness: No
LG UltraFine:
Resolution: 5120 x 2880 (5K/UHD+ - Ultra High Definition Plus)
UI Looks like: 2560 x 1440
Framebuffer Depth: 30-Bit Color (ARGB2101010)
Display Serial Number: (I've deleted this)
Mirror: Off
Online: Yes
Rotation: Supported
Automatically Adjust Brightness: No
Connection Type: DisplayPort

My iMac is a late 2015 iMac, which, according to Apple, isn't supposed to be able to support the Ultrafine at 5K. The only reason I can guess that I'm getting full 5K with 60 Hz refresh is because I ordered this system with the upgraded graphics card (Radeon R9 M395X with 4 GB); maybe Apple's guidance was for the late 2015 iMacs with the standard graphics, and they just didn't make the distinction.

Either way, I'll defer to all of you for the answers to the original question.