The retinal display on the new iPhone 4 doubles the pixel resolution of the display. However when it comes to laying out the UI elements you can still think about a full screen display with dimensions of 320 x 480 in portrait mode. The big difference is that these dimensions are now expressed in points NOT pixels. What has changed is the scaling factor of the screen which determines how a point relates to a pixel. For the iPhone 4 the display has a scale of 2 so that 1 point = 2 pixels.
The number of pixels on the display has doubled to 640 x 960 but if you are using UIKit this added complexity is mostly hidden from the application. The UI layout is configured using the point dimensions so thankfully you do not have to worry about redoing the layout just for iPhone 4 devices. Also things like text and standard Apple UI components such as navigation bars, system buttons, etc are automatically displayed at the higher pixel resolution. Note that if you are using core graphics or core animation you will have some additional work to do as those frameworks only work with pixels not points.
There are some additional steps that are worth doing if you want your app to look its best on the iPhone 4.
2x Image Files
Apple has introduced a simple naming convention to allow image files at double the resolution to be loaded when the display will support it. To make use of this feature you need to go back to your original image files and create versions at twice the existing size. So if you have an image that is currently 100×100 pixels named myImage.png you need to create a new version which is 200×200 pixels named myImage@2x.png.
You should probably avoid just taking the existing image file and scaling it up to double the size. Ideally you want to create your images at the higher resolution and then scale down to the standard resolution so that both sizes look their best.
Once you have the high-resolution image files you just need to add them to your project and they will be selected automatically anytime you use UIImage class methods such as +imageNamed: or +imageWithContentsOfFile: So for example:
UIImage *myImage = [UIImage imageNamed:@”myImage.png”];
will load myImage.png on iPhone 3 and iPad devices but will load myImage@2x.png on the iPhone 4. It is possible to omit the filename extension from the name of the image file with iOS 4 but you will need to include it if you want to maintain compatibility with 3.x releases.
As well as updating image files you should also update the application icon files to include high-resolution versions of each file. So since the original iPhone application icon file is 57×57 pixels the 2x version needs to be 114×114 pixels. In addition the settings/search icon which is normally 29×29 pixels needs a high-resolution version that is 58×58 pixels.
With iPhone OS 3.2 for the IPad a new key (CFBundleIconFiles) was introduced to Info.plist to specify the ever growing list of icon files. If you are producing an App that will only run only on iOS 4.0 and 3.2 you can use CFBundleIconFiles (you can delete the old CFBundleIconFile key if you want since it will no longer be used).
If you do not specify the filename extensions you can avoid listing all of the 2x variations of the file. So if we have an iPhone icon, an IPad icon and the corresponding settings/search icons the entry in Info.plist would be as follows:
If we want to maintain compatibility with pre-3.2 devices we cannot rely on the CFBundleIconFiles key. Instead we need to specify the icon files using the Apple determined naming convention. The files we would need to support all devices would be as follows:
- Icon.png – 57×57 pixels application icon for standard resolution iPhone
- Icon@2x.png – 114×114 pixels application icon for high resolution iPhone 4
- Icon-72.png – 72×72 pixels application icon for iPad
- Icon-Small.png – 29×29 pixels settings icon for standard resolution iPhone
- Icon-Small@2x.png – 58×58 pixels settings icon for high resolution iPhone
- Icon-Small-50.png – 50×50 pixels settings icon for iPad
Then in the Info.plist file we need to use the original CFBundleIconFile key to specify just the basename of the icon file (“Icon.png”) as follows:
As with the application icons you can also specify high-resolution versions of your launch images. So a standard resolution portrait launch image named Launch.png is 320×480 pixels. The high resolution version would be 640×960 pixels and named Launch@2x.png.
Likewise a landscape version of the file named Default-Landscape.png would be 480×320 in standard resolution (assuming a status bar) and named Default-Landscape@2x.png with 960×640 pixels in high resolution.
To avoid validation errors when uploading to the App Store I find that the old CFBundleIconFile needs to also specify the extension (Icon.png). I have created an example app to illustrate the different configurations that I will try to post in the next few days to clear up the confusion.
See this post for more details on the problem with the version of Application Loader shipped with Xcode 3.2.3: Application Icon Troubles.