How does a 4K Retina display 'double up' the pixels?

Question

I did the following experiment on a 21.5 retina display:

Open Pages, and write some lines with Courier New at point size 4, 3, 2 and even 1.

Use default screen 2K High Resolution and then set view to Actual Size

Use extreme 4K Resolution and again set view to Actual Size.

The typefaces with 3 and 2 points are readable in both modes, but they seem better in 4K. How is this?

I mean, is not "2K Retina" supposed to present a default resolution of 2K but render objects at 4K? Just to be sure, "2K low resolution" seems to have a worse rendering of the 4, 3 and 2 typefaces. But 4K seems the winner. It is just my bias, or is it a fact that it renders better than 2K?

Answer 1

On older displays, the pixels were sufficiently large that you could see each one on the screen.

Newer displays have smaller pixels, which gives a sharper, less 'blocky' image. Eventually, displays' pixels became so small, and so densely packed that the human eye cannot distinguish individual pixels from a standard distance. (That's why the term 'Retina' is used.)

The problem with such high-density displays is that text is also reduced in size, appearing very small, and difficult to read.

Apple solved this with its 'Retina display' screen rendering. What it does is to take a high-density (HiDPI) display and magnify everything by a factor of 2. So an image that would have been displayed using 512 x 512 pixels will actually be displayed using 1024 x 1024 pixels. The image stays sharp because the 2x2 square that represents "1 pixel" can actually have different colors in each pixel.

So by pretending the display is half the size, you get 'normal'-sized images and text, but with twice the fine detail. I would recommend using the Default scaling for most purposes. You can of course zoom in and out in Safari, Pages, Preview, etc.

Answer 2

Apple has used scaled vector shapes to draw things on printers and screens since the original days of Macintosh. Fonts and Typefaces, Adobe Illustrator and other uses are programmed and supported for quite a while.

• https://en.wikipedia.org/wiki/Retina_display

This was renewed when retina displays came out with too many pixels for apps that didn’t contemplate a screen as dense dots. Rather than break these apps, Apple enhanced code and libraries to not preserve 1:1 pixel spacing or elements like icons and text that would render as minuscule at best. The OS has code to load 2x and 3x size images and scale them appropriately so that you get all the details of a 5k / 4k or just Retina display so that as you noticed, Apple is able to get use of all the pixels you throw at your screen while also keeping application spacing and text and lines readable.

• https://developer.apple.com/design/human-interface-guidelines/macos/icons-and-images/image-size-and-resolution/

On other OS, you might have a less nuanced model in many cases and people have to down scale the resolution as the only lever to change text sizes and on screen icons for apps not willing to do that lifting themselves. By drawing paths and correctly substituting in high fidelity icons and images, the result is precise details, better curves and appropriate sizing than you might expect from the “resoltion” you see scaled in Display Preferences on macOS. Elements don’t change on-screen size even though the sharpness does render with 4k to 8k displays like the Pro Display XDR.