Can a Mini actually display 5k of content on a 5k display?

[ispcolohost]

Hey all, I've got an LG 5k monitor hooked to my Mini 2018 running Catalina 10.15.6. "About this Mac" reports it as LG UltraFine Display 27-inch (5120 x 2880) with the built-in Intel 630. I've always had it run with the 'More Space' scaled display setting. I recently tried to display 4k video on the screen and it would not fit, bringing me to the realization that I'm not actually getting 5k of content on my 5k display. In MacOS System Report, it oddly shows my "Resolution" at an impossible 6400 x 3600, and "UI Looks like" of 3200 x 1800.

If I set Displays back to 'default for display' everything of course becomes massive in size, and the same System Report shows my display at 5k resolution, with a looks like of half that, 2560 x 1440.

Is there any way to get an actual 5120 × 2880 of content on a screen?

 

[joevt]

"UI Looks like 2560x1440" is 5K. Everything is drawn twice as tall and twice as wide in Retina modes (also known as HiDPI modes).

You get all the pixels of a 4K video when you are using 1920x1080 HiDPI mode. This mode (which is actually 3840x2160) is scaled up to 5120x2880 by the GPU for output to the display.

Larger retina modes (the native 2560x1440 HiDPI mode or the larger 3200x1800 HiDPI scaled mode) require the video player to scale up 4K video to fill the frame buffer (for example, 4K video is played at 6400x3600 for the 3200x1800 HiDPI mode). Modes larger than 2560x1440 HiDPI are scaled down to 5120x2880 by the GPU for output to the display (for example, the 3200x1800 HiDPI mode, which is actually 6400x3600, is scaled down to 5120x2880 by the GPU for output to the display).

In the Displays preferences panel, if you hold the option key down and click "Scaled", does the 5120x2880 mode appear? Otherwise you can use SwitchResX. 5120x2880 is not a HiDPI mode (text and objects are not drawn twice as tall and twice as wide). There's a "Show Low Resolutions" checkbox which will show other non-HiDPI modes.

 

[kohlson]

Just for fun take a full screen shot (Cmd-Shft-3) and then open in Preview. The entire display buffer is captured, not just the screen "resolution." Should be 6400x3600.

 

[ispcolohost]

Ah awesome; I had no idea about the option key trick on the display prefs screen. That worked; thanks!

I'd used SwitchResX many years ago to get 2560 x 1600 out of my previous 2012-era MacBook Pro, but hadn't tried it on a desktop, and ultimately had to abandon it on the laptop when the system integrity protection issue cropped up. I see that's been resolved in later versions of it + MacOS, so I'll check it out again too.

 

[joevt]

Definitely do as kohlson suggests and compare screenshots of HiDPI and non HiDPI modes. All the pixels are captured from the frame buffer. Compare text and icons in the menu bar to see how they are drawn. Check the info to see the image size.