October 18, 2026

The mac mini -- pixel doubling

I copy a favorite background image to the Mac. This is avra4_bg.jpg and is 3840x2160 pixels.

I launch the Mac "photos" app and use File--import to import it. At the upper right is a blue circular things that says "get info about this photo". It tells me that the image is 3840x2160. But it won't show me the image full size, it will either show it to me reduced, or expanded to full screen.

The Preview app is recommended to get full scale (actual dimensions, 100 percent). I use Finder to locate the photo, the right click to get a menu. It offers me "display with" and I take the default, which is "Preview". This starts up showing it to me in some reduced size.

At the screen top, I select "View, actual size" and it clearly does something stupid, showing it to me at some size that will never fit on my screen (which is a 3840x2160 monitor).

I click on the upper left "apple" to get system settings and then go to Displays. This is supposed to show me my screen resolution, but I don't see it anywhere.

What does work is to click the upper left "apple", then select "about this mac" Then select "more info" and look for "General" in the left column, scroll down to where it shows "Displays". I see 3840x2160 there.

Try a 1920 pixel wide image

I generate one of these, copy it to the Mac, and use preview to display it. It takes up about 3/4 of the screen (it should take up half). I use "View" -- "Actual Size" and it displays 19 inches wide on my 23 inch screen. It is suggested that I can control this with Settings in Preview. Preview claims that it has configured 100 percent as 1 pixel in the image is 1 point on the screen

Here is a discussion of this. I have things set 1:1 and things still don't make sense.

Image Magick

brew install imagemagick
I try the text only "identify" command. It works and yields the expected results. 
identify zzz.jpg
zzz.jpg JPEG 1920x1080 1920x1080+0+0 8-bit sRGB 524273B 0.010u 0:00.005
However the "display" command does not work. Apparently it expects X11 which is no longer how Mac does things.

Pixel Doubling

Apple plays some games. It certainly looked like it was expanding my 1920 pixel wide image, but it wasn't doubling it. When I search on this, I find that Apple aims to do whatever they need to do to get an approximately 218 ppi (pixels per inch).

I have a 27 inch 4K monitor (3840x2160). What Mac does is to render at 5K and then down scale to the monitor. So what I get is what looks like 2560x1440, and this matches my experience. My 1920 image was 19 inches wide on a 23 inch wide screen (23/19)*1920 = 2324 -- which is believably close to the claimed 2560. It uses fractional scaling.

I read that if I had a 27 inch 5K apple monitor, it would play exactly the same game, rendering at 5K, then scaling by an exact 2x to give what looks like a 2560x1440 workspace.

I read that with their 24 inch 4.5K monitor (actual 4480x2520) they use exact 2x pixel doubling, but give you a logical 2240x1260 workspace.

In summary

My monitor is a 27 inch "4K" monitor 3840x2160
Apple 24 inch 4.5K has 4480x2520 pixels
Apple 27 inch 5K has (had) 5120x2880

What apple does is something they call "pixel doubling", but it ain't quite that simple. They give you "logical" pixels they call your workspace. In the case of a 5K monitor, it actually is doubling.

On an Apple 5K you get a logical 2560x1440 workspace (2x pixel doubling).

With a 24 inch 4.5K monitor, they give you a 2240x1260 pixel workspace, and once again they are exactly doubling pixels.

In my case, I have a 27 inch 4K monitor. Apple gives me a 2560x1440 workspace, but they do fractional pixel manipulation. 3840/2560 = 1.5

Their goal is to get 218 pixels per inch on the screen to make things "look right and nice"

Have any comments? Questions? Drop me a line!