Asset Catalogs, Image Sets, and App Icons

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:


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

Some idioms denote their purpose is for App Store:

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"},...

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.