geocom:

 

phrozenpenguin:

 

This is more what I was trying to understand. I don't quite understand the "trick" though as there are only a set number of pixels (4k) which halved is 1080p. Is this macOS doing some clever rescaling like it does on a Retina display if you go for the "More Space" option? To get true retina (pixel doubling) of 1440p you need to have a true 5k display.

 

 

It is complex but the very simplified explanation is that Apple has decided on 2x being a standard way to scale.

 

So yes by default a 4k screen is halved to look like 1080 however if you do want to set it to say something looking like 1440p macOS wont pixel double the images so they will just deliver these as 1x which looks bad especially if you just put down the cash for a 4k screen so to trick it you tell it you have a 5k screen instead of 4k and it will deliver you the 2x images your graphics card will just downsample this image to 4k

 

Apple have done this themselves with the iPhone. The plus has a 1080 screen on it the OS will draw a screen at 1242x2208 and downsample it to 1080 x 1920 https://www.paintcodeapp.com/news/iphone-6-screens-demystified