Asset catalogs are collections of specialized files (such as graphics files) as well as JSON files containing metadata. The metadata from the JSON files are used for organizing the specialized files and denoting their purpose within the set. In the case of graphic sets like Image Sets, they contain several pieces of information.

Image Sets

Here is a sample Image Set:

{ "images" : [ { "idiom" : "universal", "filename" : "apple-watch@1x.png", "scale" : "1x" }, { "idiom" : "universal", "filename" : "apple-watch@2x.png", "scale" : "2x" }, { "idiom" : "universal", "filename" : "apple-watch@3x.png", "scale" : "3x" } ], "info" : { "version" : 1, "author" : "xcode" } }

First, we have a collection of images with the following properties:

filename – the file name for the particular image

– the file name for the particular image scale – the designated scale for the image file, as of now there are four scales used:

– the designated scale for the image file, as of now there are four scales used: default unspecified – vector pdf file for any display

1x – iPhones up to 3GS, iPod Touch up to 3rd Gen

2x – iPhone 4, iPhone 4s, iPhone 5, iPhone 5c, iPhone 5s, iPhone SE, iPhone 6, iPhone 6s, iPhone 7, iPhone 8, iPhone Xr, iPod Touch 4th Generation and Up, MacBook Pro 3rd Generation and up, MacBook (2015 and newer)

3x – iPhone 6+, iPhone 6s+, iPhone 7+ iPhone 8+ iPhone X, iPhone Xs, iPhone Xs Max, iMac 4K and 5K

idiom – the designated device and purpose for the particular image file

Idioms

Idioms designate the device and purpose of the image file. Here is the general device values of the field for the particular property:

universal – The default value, designates the image file works on any device and platform.

– The default value, designates the image file works on any device and platform. iphone – The image is for iPhone devices.

– The image is for iPhone devices. ipad – The image is for iPad devices.

– The image is for iPad devices. mac – The image is for Mac computers.

– The image is for Mac computers. watch – The image is for the Apple Watch devices.

– The image is for the Apple Watch devices. tv – The image is for Apple TV.

Some idioms denote their purpose is for App Store:

ios-marketing – An image for the App Store icon

– An image for the App Store icon watch-marketing – An image for the App Store icon.

App Icons

App Icons contain the same information as an Image Set with a few particular properties for app icons on certain devices:

... { "size" : "24x24", "idiom" : "watch", "scale" : "2x", "role" : "notificationCenter", "subtype" : "38mm" }, ...

size the size of the image file in points, this designates whether the image

the size of the image file in points, this designates whether the image subtype – typically used for Apple Watch icons, they designate what Apple Watch size the image file is for

– typically used for Apple Watch icons, they designate what Apple Watch size the image file is for role – typically used for Apple Watch icons, they designate a specific purpose for the icon. The values could be:

– typically used for Apple Watch icons, they designate a specific purpose for the icon. The values could be: appLauncher – An image shown app launcher.

companionSettings – An image for the Apple Watch Settings app

notificationCenter – An image for the notification center on watchOS.

quickLook – An image used for a short look notification

longLook – An image used for a long look notification



With all these properties an App Icon set can contain as many as 41 individual files. That’s why it is best to use an automated method to render these files. Check out our series of blog posts on automating that process for more details.