An ICNS file or image is a file resource that allows macOS applications to display icons in the macOS Dock and Finder. It is a container file format that holds multiple icon images at varying resolution levels and sizes.
An ICNS file is used to store icons for folders, applications, and other items on macOS. It can be identified by the .icns file extension.
The acronym ‘ICNS’ does not officially stand for 'macOS Icon Container Resource.' It is simply known as the ICNS format, used for macOS icons. While it is sometimes informally referred to as 'Mac OS ICNS Resource,' 'Mac OS X Icon Resource,' 'Mac OS Icon,' 'Macintosh Icon,' or 'Apple Icon,' these terms are not standard.
ICNS images may include icons encoded in formats like JPEG2000 or PNG, which allows Finder to use these files as application icons, folder icons, or file icons.
The ICNS file format is unique due to its support for multi-resolution and high-resolution icons for different device displays. The format can contain a variety of icon sizes and color depths, enabling dynamic scaling and display of icons on macOS.
History of ICNS Files
Apple Inc. created the ICNS format in the early 1980s. Since its invention, the file format has evolved to include two image types: the PNG file format version and Mac OS X icon files. The ICNS format was created to help customize how icons appear in macOS applications’ Finder and the OS X dock.
ICNS files are mainly used as file type icons and app icons, appearing on the MacOS dock, desktop, and Finder. Yet, having existed for over four decades, they have only attracted 8,612,345 users globally. ICNS is not a popular format on Windows platforms, and users often convert ICNS files to standard graphic formats.
Properties of ICNS Files
ICNS files are used to represent folders, files, applications, and other resources within the macOS GUI. The file format contains multiple images, each with specific characteristics such as color depth, resolution, and size. These characteristics can be better understood by exploring the structure, storage, compression, animation support, and transparency support of ICNS files.
Structure
ICNS is a raster graphics file type comprising several image variations. Its structure includes a header and binary icon data. The header is 8 bytes long, with the first 4 bytes being the magic number ('icns') and the next 4 bytes representing the file length.
The size and type of every icon image are stored in the icon data section, followed by the binary image data. The image size determines the size of the binary section. ICNS allows the fixed-size icons to be scaled by the Operating Systems and displayed at any intermediate size.
It supports ARGB and RGB color profiles. The file format’s components include one or many images in the PNG format, which is the foundation for the ICNS’s transparency. The ICNS file supports 1-bit and 8-bit alpha channels but differs from Windows' ICO files since ICNS files hold separate icon resources.
ICNS files can include images encoded in formats like PNG, JPEG2000, and others. These files are designed for machine interpretation by macOS and related software, rather than for direct human readability. They contain encoded image data and metadata that require software capable of interpreting their structure.
The ICNS files contain high-resolution images displayed at varying sizes and resolutions within the macOS graphical user interface. The icons range from small to large and are optimized for specific display sizes. The icon files also include metadata defining which image should be displayed at a given resolution.
Storage
As a container file format used by macOS, ICNS stores multiple image files of diverse qualities, resolutions, and sizes, representing a single icon. It stores icons for documents, applications, and other elements in a compressed form. Thus, the stored files are smaller and require less bandwidth or minimal storage space.
Depending on their conditions, handling process, and storage mediums used, ICNS images or files can be stored indefinitely. They can be stored in file systems, folders, content delivery networks, hard drives, the cloud, and the package's Contents or ​Resources directory.
Compression
The ICNS file format has improved over time to support lossless compression of certain sections of the pixel data. The ARGB (ic04, ic05, and icsb) and 24-bit RGB (is32, il32, ih32, it32, icp4, and icp5) pixel data can be compressed per channel using a format analogous to PackBits.
Each ICNS image file consists of a series of pixels, consuming memory space. The ICNS's PackBits algorithm, one of the oldest and simplest types of compression (run-length encoding or RLE), is used to shrink file sizes. This algorithm encodes redundant ICNS data by storing series (runs) of identical pixel colors concisely.
A control byte N marks the beginning of each run. If N falls between -1 and -127, the symbol following the control byte is repeated (-N + 1) times. If N falls between 0 and 127, the following (N + 1) symbols are left uncompressed. This compression technique reduces file size and improves storage space consumption.
While ICNS primarily uses PackBits for compression, which does not offer variable compression, the resulting smaller file sizes help reduce image loading times and enhance user experience, all while preserving the image details.
Editing
The .ICNS extension category or ICNS file is perfectly supported by accustomed graphical editors, such as Apple Preview, Inkscape, XnViewMP, IrfanView, Snap Converter, Iconographer, Icon Machine, Folder Icon X, Axialis IconWorkshop, and Adobe Photoshop with IconBuilder for Mac Plugin (Mac).
An ICNS image can be resized using software such as Inkscape, Snap Converter, Iconographer, Icon Machine, XnView MP, Adobe Photoshop, IconWorkshop, Apple Preview, IrfanView, Deark, Konvertor, Folder Icon X, and Snap Converter.
Animation
The ICNS file supports animation. The file format contains icon features, which enable websites or computer applications to be customized to support multiple color depths and resolutions, allowing for animation or lively content.
Animated ICNS files use 1—and 8-bit alpha channel transparency, making them professional, sharp, and visually presentable compared to other file formats. Moreover, animated ICNS is customized, resulting in increased user experience.
Animated ICNS supports multiple color depths and resolutions compared to other file formats. However, it requires higher technical skills to create, resulting in larger file sizes and more storage space.
Animated ICNS encounters limited web browser support and incompatibility with most non-MacOS platforms and software, hindering their extensive online usage.
Transparency
ICNS supports alpha channel transparency. The content of the ICNS file typically consists of one or more images in PNG format. This format is handy as a base for the ICNS since it allows transparency. The ICNS file supports 1-bit and 8-bit alpha channels.
Best Use Case of ICNS Files
- Suitable for use on Mac computers as it stores icons for files and applications.
- Suitable for visual identification as it supports high-quality and high-resolution images.
- Suitable for keeping multiple sizes of icons for different uses.
- Suitable for automatically selecting the most appropriate icon for the current display settings
- Suitable for scaling images as it enhances user experience by offering visually distinct images
- Suitable for efficient navigation and organization of files on an online platform.
Unrecommended Use Cases of ICNS Files
- Unsuitable for use on non-Mac systems or software as it might not work properly.
- Unsuitable for web graphics as they must be converted to other file formats for use in websites.
- Unsuitable for creating images that require support and high compatibility with most web browsers
- Unsuitable for creating photorealistic or highly detailed icons because of its color depth, forcing web designers to use other higher-color-depth image formats like AVIF.
ICNS Browser Support
ICNS is supported by web browsers such as:
- Chrome
- Firefox
Browsers that do not support ICNS files are:
- Microsoft Edge
- Discord
- Opera
- Safari
- Vivaldi
- Internet Explorer
However, users can convert their ICNS files to highly compatible and well-supported file formats such as PNG for high browser compatibility.
Possible ICNS Browser Errors
Users require ideal software or programs like Apple or MacOS to open an ICNS file. However, without proper software, users are likely to receive the following browser error on Windows, MacOS, or iOS/Android:
- How do you want to open this file?
- Incorrect Windows Registry ICNS file paths
- Windows cannot open this file
- Something has caused the ICNS file to be corrupted.
- Windows has inadequate resources to open the icon file
- File not supported error
Advantages of ICNS
- Scalability: The operating system can scale the fixed-size icons up or down and display them at any intermediate size.
- High-Resolution: The ICNS files contain high-resolution images, which are displayed at varying sizes and resolutions within the macOS graphical user interface.
- Smaller File Sizes: Supports smaller files with cryptic characters whose average size ranges from 26KB to 660 KB.
- Transparency: It supports transparency through its 1- and 8-bit alpha channels.
- Editability: It is a container file format that can hold multiple image sizes and resolutions of the same icon, which are easily edited.
Disadvantages of ICNS
- Limited Browser Support: Most web browsers do not widely support ICNS files or images, limiting their usage online.
- Limited Software Support: ICNS files are incompatible with non-MacOS programs, devices, or software and are only compatible with MacOS, Windows, and Linux.
- Coding and Encoding Challenges: The ICNS format allows some icon types to be encoded as PNG or JPEG 2000 data. However, when encoding icons, its library often uses PNG format, but when decoding icons, it cannot decode JPEG 2000 icons as it returns an error.
How to open ICNS files
Software used to open ICNS file format includes Axialis IconWorkshop, Microsoft File Explorer, Apple Preview (macOS), Snap Converter, XnViewMP (multiplatform), Iconographer, and Inkscape (multi-program). Apple Preview can reveal and display images contained in the ICNS files.
The ICNS file format is compatible with MacOS, Windows, and Linux stationary laptops or computers. However, it is incompatible or unsupported by Android and iOS mobile operating systems, among others.
Converting ICNS
Online ICNS converters such as Converjack can help you effortlessly convert your ICNS file to TIFF, PNG, PDF, EXR, JPX, JPEG, JPG, BMP, GIF, ICO, and HEIC. Take or drop your ICNS file to the opened on Convertjack for effortless conversion to any format you choose.