ios - How to handle image scale on all the available iPhone resolutions?

Question

What sizes would be the best to use for images: background.png, background@2x.png and background@3x.png if we want to use this image for example to cover the full width and half height of the screen on all resolutions for iPhone portrait app?

This is what we have now:

Device          Points    Pixels     Scale  Physical Pixels   PPI   Ratio   Size
iPhone XS Max   896x414   2688x1242  3x     2688x1242         458   19.5:9  6.5"
iPhone XR       896x414   1792x828   2x     1792x828          326   19.5:9  6.1"
iPhone X        812x375   2436x1125  3x     2436x1125         458   19.5:9  5.8"
iPhone 6 Plus   736x414   2208x1242  3x     1920x1080         401   16:9    5.5"
iPhone 6        667x375   1334x750   2x     1334x750          326   16:9    4.7"
iPhone 5        568x320   1136x640   2x     1136x640          326   16:9    4.0"
iPhone 4        480x320   960x640    2x     960x640           326   3:2     3.5"
iPhone 3GS      480x320   480x320    1x     480x320           163   3:2     3.5"

Some people say that for edge to edge image (like a banner on the bottom from left to right edge of the screen) for iPhone 6 Plus they would prepare back@3x.png with width 1242 and for iPhone 6 back@2x.png with width 750 to match the iPhone 6 screen size however I do not think that this is a good idea because 1242 / 3 = 414 and 750 / 2 = 375 so naming them as @2x and @3x does not have sense. And then what width should have back.png - 375 or 414?

Graphics names are using @2x and @3x suffixes so if for example image@3x.png has 30x30 resolution then logically thinking image@2x.png should have 20x20 resolution and image.png should be 10x10. This means that if we want to have sharp full width image for each screen then we probably should create back@3x.png with width 4143=1242px, back@2x.png with width 4142=828px and back.png with width 414px. This however means that on every iPhone except for iPhone 6 Plus you will need to setup your uiimages to use for example aspect fit content mode and they will be scalled down so this again is not a perferct solution and probably would really slow down the application if we use a lot of scalling on older devices.

So what do you think would be the best solution to solve this problem?

Answer 1

You don't have to have each image in all scales if it won't be used. Make only the sizes you need and name them according to their width. For portrait full-device-width images, you need 320px wide at 1x and 2x, 375px wide at 2x and 414px wide at 3x.

4" devices used "-568h" suffix for naming their launch images, so I'd recommend a similar naming scheme:

• ImageName-320w (@1x & @2x)
• ImageName-375w (@2x)
• ImageName-414w (@3x)

Then figure out what image you need at runtime:

NSNumber *screenWidth = @([UIScreen mainScreen].bounds.size.width);
NSString *imageName = [NSString stringWithFormat:@"name-%@w", screenWidth];
UIImage *image = [UIImage imageNamed:imageName];

This might break if other widths are added in future, but so far Apple has always required rebuilding the app to support new displays so I guess it's somewhat safe to assume they will continue doing that.