Pixels per inch is wrong on 7/8 Plus phones and minis

[fancyham]

PPI number seems to be wrong for iPhone 6, 7, 8 Plus devices, leading to incorrect dimensionInInches, dimensionInCentimeters values, and pixelsPerInch, that don't match up with physical device.

Library returns screen dimensions of:
5.5 x 3.09 inches for the screen
But the actual physical dimensions are:
4.79 × 2.7

There is a 1.15x error.

This seems to be related to the 1/1.15 downsampling unique to these models from the rendered screen buffer to the device's physical screen.

This helps explain the downsampling: https://www.paintcodeapp.com/news/ultimate-guide-to-iphone-resolutions
(Edit: this link shows the incorrect 401 PPI for drawing to the screen)

So, I think the PPI in this module is calculated using the rendered screen buffer's dimensions (main.nativeBounds.height = 2208, width = 1242), but it should be calculated using device's physical pixels (1920, 1080).

This would be necessary, to do, say a ruler app.

[marchv]

Hi @fancyham and thanks for reaching out...

Hmm, that is a surprise me. When this was made I used an iPhone 7 Plus myself and as you can see in the example in README.md I got 5.5" in out in diagonal screen dimension 👀. Unfortunately, I have updated my phone to iOS 14 beta so I couldn't test it today.

PS. I am familiar with www.paintcodeapp.com/news/ultimate-guide-to-iphone-resolutions already, but thanks anyway :)

What iOS version did you get this on?

Cheers! :)

[fancyham]

Yes, it's pretty strange!

This is on iOS 13.3 on a simulator and the diagonal ends up 6.31". Perhaps it's a bug in the simulator as I don't have a physical device to test on?

I believe the issue is that physically, the screen is indeed 401 ppi, but due to that weird downsampling, when drawing to the screen, it's (401*1.15 ppi).

Here's what I'm displaying in SwiftUI

struct ContentView: View {
    var body: some View {
        VStack(alignment: .leading){
            Text("Length of screen in inches =\n \(Float(UIScreen.lengthOfScreenInInches!)) inches  (\(Float(UIScreen.lengthOfScreenInInches!) * 2.54)cm)\n")
            Text("Width of screen in inches =\n \(Float(UIScreen.widthOfScreenInInches!)) inches  (\(Float(UIScreen.widthOfScreenInInches!) * 2.54)cm)\n")
            Text("")
            Text("Size = \(UIScreen.dimensionInInches!.debugDescription) inches")
            Text("Diagonal = \(sqrt(pow((Float(UIScreen.dimensionInInches!.width)), 2) + pow(Float(UIScreen.dimensionInInches!.height), 2)))")
            Text("Pixels per inch = \(Float(UIScreen.pixelsPerInch!))\n")
            Text("")
            Text("UIScreen.main.nativeBounds.width = \(UIScreen.main.nativeBounds.width)")
        }
    }
}

And getting this for an iPhone 7 Plus (simulator)

I found that I get correct numbers for those devices if I change the 'return 401' to 'return 401*1.15

        case "iPhone7,1":                                fallthrough // iPhone 6 Plus
        case "iPhone8,2":                                fallthrough // iPhone 6S Plus
        case "iPhone9,2", "iPhone9,4":                   fallthrough // iPhone 7 Plus
        case "iPhone10,2", "iPhone10,5":                             // iPhone 8 Plus
            return 401*1.15

[fancyham]

I think this explains what's going on:
https://medium.com/we-are-appcepted/the-curious-case-of-iphone-6-1080p-display-b33dac5bbcb6

The screen is physically ~401 PPI, but internally, it's ~462 PPI.

So if you're able to replicate the same issue on your physical device, one solution could be to change the 'return 401' to 'return 462'

I can post my code if that's helpful.

I suspect Apple was expecting to use a higher-resolution screen but then their display provider couldn't deliver, so they resorted to the downsampling.

[fancyham]

BTW, I wrote to www.paintcodeapp.com/news/ultimate-guide-to-iphone-resolutions and they replied that they realize the 401 PPI number on their page is misleading but that they don't want to imply that the Plus phones have a higher resolution than the X so currently they'll continue listing the screen's physical PPI rather than what might be called "462 virtual PPI".

[fancyham]

Similar issue is happening with the iPhone 12 and 13 mini. Their screens are actually rendered as if on a larger device, then scaled down by a percentage. In this case, 96%.

Drawing a ruler without doing this scaling will yield incorrect measurements.