How to find PPI programmatically with precision

Question

I am trying to find out PPI(Pixels Per Inch) in iOS.

I couldn't find any direct way to query this like we do for display size

UIScreen.mainScreen().bounds

There is a way to do it by multiplying scale with standard generic PPI for iPhone(163) or iPad(132) but it's not accurate.

If the formula is right then PPI of iPhone 6 plus is 489 but in reality the PPI is 401 Here is the reference

For now it seems like hardcoding is the way to go.

But I'd like to do it programmatically using a formula.

score 8 · Answer 1 · answered Oct 03 '17 at 06:29

8

I have just ported and updated one of my old ObjC libraries to Swift. You can use it or take parts of the code you need. Get it here: https://github.com/marchv/UIScreenExtension.

The library uses UIScreen.main.nativeScale to convert from Pixels Per Inch (PPI) to Points Per Inch.

Install the library using Cocoapods and then import it:

import UIScreenExtension

And then make use of it:

if let pointsPerCentimeter = UIScreen.pointsPerCentimeter {
   // code
}

answered Oct 03 '17 at 06:29

Jens Schwarzer

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It is worth mentioning that UIScreenExtension misreports PPI for iPad 2 and iPad Mini 1st generation. These should be 132 and 163 respectively as those two devices were not @2x retina. – step_jac Nov 23 '17 at 20:37
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@step_jac I first misunderstood you comment - better get some coffee before reading early in the morning :D OK I can see the problem now - will fix it ASAP - thanks for pointing it out – Jens Schwarzer Nov 24 '17 at 07:21

score 3 · Answer 2 · edited May 23 '17 at 12:24

3

I believe there is no public API to get either PPI or physical size of a screen.

The only way is to hardcode list of devices with their physical sizes and/or PPI's (and you can get a device type out of UIDevice class).

BTW. Here is the question which is pretty much ask the same thing (different way): How do ruler apps stay accurate on all devices?

edited May 23 '17 at 12:24

Community

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answered Apr 19 '16 at 14:41

Victor Ronin

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I guess we can the physical size of screen in points `[UIScreen mainScreen].bounds.size` but how many pixels per point can be found out for all the devices except iPhone 6 plus because it can't be found out like normal PPI * scale because iPhone 6 plus & iPhone 6s plus has introduced new Screen called Retina HD...so I think the best way would be check if the device is iPhone 6 plus then return 401 and for the rest of the devices we can go ahead with the formula I guess... – Durai Amuthan.H Apr 24 '16 at 14:07
A way to detect iPhone 6 & iPhone 6s programatically (http://stackoverflow.com/a/26400686/730807) – Durai Amuthan.H Apr 24 '16 at 14:08

score 1 · Answer 3 · answered Jul 12 '18 at 23:43

iPhone Plus has scale 3, but nativeScale is 2.6.

UIKit samples this content down to fit the actual screen dimensions. Metal or OpenGL ES contents should be rendered at the precise dimensions.

int screenPPI() {
    return [[UIScreen mainScreen] nativeScale] * ((UI_USER_INTERFACE_IDIOM() == UIUserInterfaceIdiomPad) ? 132 : 163);
}

How to find PPI programmatically with precision

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