I'm almost done building my first iPhone app and am trying to add a background image, and am finding it a little confusing, because nowadays there's like 3 or 4 different size screens among the different iPhone versions, with different resolutions to boot.
So while I'm aware of the whole [email protected] thing, I still don't know what I really need. If I want my app to run on iPhone 4/4s, 5s/5c, 6/6+, how many different versions of a background image do I need, and at what sizes and resolutions?
I have googled around and have not found any cohesive answers up to date for 2014.
Also, if the iPhone 6 is 1334x750 @3x, does that mean I'm supposed to include a 4002x2250 background? And then for the 1920x1080 iPhone 6+ @3x, a 5760 x 3240 image? That's MASSIVE! I feel like I must be understanding this incorrectly.
If you want to support native resolution of iPhone 6/Plus, you need to add launch images (prior to iOS 8) or launch screen xib (iOS 8).
iPhone 4/4S: 640 x 960
iPhone 5/5S: 640 x 1136
iPhone 6: 750 x 1334
iPhont 6 Plus: 1242 x 2208
That means you need to prepare 4 launch images with above resolution if you want to support these devices. You can use iOS simulator to capture screenshots with different resolutions. Your app will run in compatibility mode on new resolution devices if it can't find the launch image of the specific resolution. compatibility mode means your view will be scaled to fit the new screen size when still have the same logical size.
EDIT:
I think op misunderstands what do @2x and @3x mean. The resolution of iPhone 6 is 750(pixels) x 1334(pixels), 326 pixels per inch. This is the REAL resolution. And 375(points) x 667(points) is the logical size if the native resolution is supported. iPhone 6 Plus's resolution is 1242(pixels) x 2208(pixels), 401 pixels per inch and the logical size is 414(points) x 736(points).
This is how images with different resolutions work on iOS device:
Let's say you want to run your app on iPhone 4s, iPhone 5/5S, iPhone 6/plus. First thing you should do is to provide 4 launch images to support native resolutions of these devices. When iOS launch your app, it will check if the app provides the right launch image to support the native resolution of current device. If iOS finds it, then use that in launch time and the logical size of screen is right, your app runs as normal. Otherwise, your app will run in compatibility mode in which all of the views will be scaled.
Suppose there is an image named foo.png in your app of which the logical size is 100(points) x 100(points). You want this image looks same in all the above devices. You should provide 2 versions of this image. One is 200(pixels) x 200 (pixels) and should be named foo.png@2x and the other is 300(pixels) x 300(pixels) named foo.png@3x. If you load this image with [UIImage imageNamed:@"foo"], on devices except iPhone 6 plus, the app will load the image named foo.png@2x. Otherwise the app will load foo.png@3x and sample it down to 300 * 84%(pixels) x 300 * 84%(pixels).
And if you load an image from an url and need to render it on the runtime. Let's say the size you get is {width:100, height:100}, the scale is 1.0. This means the REAL size of this image is 100 * 1.0(pixels) x 100 * 1.0(pixels. If you don't want it to be scaled, you need to calculate the logical size yourself. You do it like this:
UIImage *image = ... // you get it from an url
CGFloat scale = [UIScreen mainScreen].scale;
CGFloat width = image.size.width / scale;
CGFloat height = image.size.height / scale;
CGRect frame = CGRectMake(50.0f, 50.0f, width, height)];
UIImageView *imageView = [[UIImageView alloc] initWithFrame:frame];
imageView.contentMode = UIViewContentModeCenter;
imageView.image = image;
[self.view addSubview:imageView];