Apple Icon Image format explained

Apple Icon Image
Uniform Type:	com.apple.icns
Owner:	Apple Inc.
Genre:	Icon file format

The Apple Icon Image format (.icns) is an icon format used in Apple Inc.'s macOS. It supports icons of 16 × 16, 32 × 32, 48 × 48, 128 × 128, 256 × 256, 512 × 512 points at 1x and 2x scale, with both 1- and 8-bit alpha channels and multiple image states (example: open and closed folders). The fixed-size icons can be scaled by the operating system and displayed at any intermediate size.

As of macOS 11, asset catalogs are the preferred file format for macOS custom icons instead.^[1]

File structure

The file format consists of an 8 byte header, followed by any number of icons.

Header

Offset	Size	Purpose
0	4	Magic literal, must be "icns" (0x69, 0x63, 0x6e, 0x73)
4	4	Length of file, in bytes, msb first

Icon data

Offset	Size	Purpose
0	4	Icon type, see OSType below.
4	4	Length of data, in bytes (including type and length), msb first
8	Variable	Icon data

Icon types

Length (bytes)	Size (pixels)	Supported OS Version	Description
128	32×32	1.0	1-bit mono icon
256	32×32	6.0	1-bit mono icon with 1-bit mask
48	16×12	6.0	1 bit mono icon with 1-bit mask
96	16×12	7.0	4 bit icon
192	16×12	7.0	8 bit icon
64	16×16	6.0	1-bit mono icon with 1-bit mask
128	16×16	7.0	4-bit icon
256	16×16	7.0	8 bit icon
varies (768)	16×16	8.5	24-bit RGB icon
256	16×16	8.5	8-bit mask
512	32×32	7.0	4-bit icon
1024	32×32	7.0	8-bit icon
varies (3072)	32×32	8.5	24-bit RGB icon
1024	32×32	8.5	8-bit mask
576	48×48	8.5	1-bit mono icon with 1-bit mask
1152	48×48	8.5	4-bit icon
2304	48×48	8.5	8-bit icon
varies (6912)	48×48	8.5	24-bit RGB icon
2304	48×48	8.5	8-bit mask
varies (49152 + 4)	128×128	10.0	24-bit RGB icon
16384	128×128	10.0	8-bit mask
varies	16x16	10.7	JPEG 2000 or PNG format or 24-bit RGB icon
varies	32x32	10.7	JPEG 2000 or PNG format or 24-bit RGB icon
varies	48x48	10.7	JPEG 2000 or PNG format
varies	128x128	10.7	JPEG 2000 or PNG format
varies	256x256	10.5	JPEG 2000 or PNG format
varies	512x512	10.5	JPEG 2000 or PNG format
varies	1024x1024	10.7	JPEG 2000 or PNG format (512x512@2x "retina" in 10.8)
varies	32x32	10.8	JPEG 2000 or PNG format (16x16@2x "retina")
varies	64x64	10.8	JPEG 2000 or PNG format (32x32@2x "retina")
varies	256x256	10.8	JPEG 2000 or PNG format (128x128@2x "retina")
varies	512x512	10.8	JPEG 2000 or PNG format (256x256@2x "retina")
varies (1024)	16x16		ARGB or JPEG 2000 or PNG format
varies (4096)	32x32		ARGB or JPEG 2000 or PNG format (16x16@2x "retina")
varies (1296)	18x18		ARGB or JPEG 2000 or PNG format
varies	36x36		JPEG 2000 or PNG format (18x18@2x "retina")
varies	24x24		JPEG 2000 or PNG format
varies	48x48		JPEG 2000 or PNG format (24x24@2x "retina")

The value inside the parenthesis is the uncompressed length for ARGB and 24-bit RGB icons.
data always starts with a header of four zero-bytes (tested all icns files in macOS 10.15.7 and macOS 11). Usage unknown, the four zero-bytes can be any value and are quietly ignored.
These formats are supported in standalone icns files but do not display properly if used as application icon inside a package.

Image data format

Mono icons with alpha mask can display three colors: white, black, and transparent.
The 4-bit an 8-bit icons use a fixed color palette with 16 colors and 256 colors, respectively.
The 24-bit RGB format consists of the three compressed channels tightly packed (see Compression). The icon must start with a four-byte header, see footnote above.
The ARGB format consists of the ascii values for 'ARGB' and the four compressed channels tightly packed (see Compression).

Compatibility

the ARGB fields also accept files in PNG format – but not vice versa, you can not put ARGB images in any of the PNG-only fields (tested on macOS 11).
ARGB images are only supported in macOS 11 and newer – macOS 10.15.7 does not display ARGB images. Yet, even the ARGB keys can be displayed on macOS 10.15 if you set a JPEG 2000 or PNG image (see footnote on usage in app packages above).
The 24-bit RGB icons also allow images in JPEG 2000 and PNG format (tested on macOS 10.15.7 and macOS 11).
The support for newer image types seems to be introduced later than the key field (see previous two points). Therefore, the supported OS version may not be accurate or adjusted based on file format.

Other types

OSType	Description
	"Table of Contents" a list of all image types in the file, and their sizes (added in Mac OS X 10.7).A TOC is written out as an identifier (4 bytes) and size (4 bytes). Each subsequent record (8 bytes each) mapsto the icon formats found in the file. The data isn't included in this phase.
	4-byte big endian float - equal to the bundle version number of Icon Composer.app that created the icon
	Usage unknown (all tested files use either "icon" or "template").
	Info binary plist. Usage unknown (only name field seems to be used).
	Nested "template" icns file. Usage unknown.
	Nested "selected" icns file. Usage unknown.
	Nested "dark" icns file. Allows automatic icon switching in Dark mode. (added in macOS 10.14).Note: The contents of this record is a full .icns file with multiple formats. If the record bytes are written outto disk, the icns file header and file size are still required to see the full dark mode icon.

The table of contents is a list of all contained types (4 byte type-name + 4 byte length).
The data for all nested icns files does not contain the icns file-header. So, if you want to save the data to a file you have to prepend the icns header.

Non-PNG / JPEG2000 Element Types

Element types that deal with ARGB (32-bit) or RGB (24-bit) image formats require different types of headers before the binary data. It is important to note that this header is part of the image data and is not the 4-byte big endian icon element type value (e.g. ic04 or ic05).^[2]

ARGB ElementsARGB images must have their binary portion of the image data preceded by the four byte 'ARGB' header. After that, instead of each pixel with each of its four channels stored together (e.g. ARGBARGBARGB), an image with three pixels would be stored in individual channels of pixel data (e.g. AAARRRGGGBBB). In addition, each channel of pixel data needs to be encoded as mentioned below.

RGB ElementsRGB images have their binary portion of the image data preceded by four zero byte characters only when the element type is 'it32'. In all other cases, no header is needed. Channel data is separated as with the ARGB binary data (e.g. RRRGGGBBB instead of RGBRGBRGB). Each channel must also be encoded as mentioned below.

Mask ElementsMask elements are not encoded like ARGB and RGB image color channel data. The data is the same as that of an ARGB image except only the alpha channel data is provided. So for an image that has two pixels, ARGBARGB, the mask data is AA.

Compression

lead	tail	result
		bytes
	byte	copies

Over time the format has been improved and there is support for compression of some parts of the pixel data. The 24-bit RGB and ARGB pixel data are compressed (per channel) with a format similar to PackBits.^[3] Some sources mention that the OS supports both compressed or uncompressed data chunks. However, manually crafting icns files with uncompressed 24-bit RGB or ARGB images will not display properly – at least on newer macOS releases (tested on macOS 11).

Here is a GitHub repo with some swift code that appears to pass the test for both encoding and decoding as described here: ByteRunLengthCoder

The following pseudocode decompresses the data:Example: should decompress to

The following pseudocode compresses the data:Example: should compress to

Known issues

As of macOS 11, there are certain issues / bugs with the file format:

Setting or will display a proper icon. But setting ignores the transparency mask and displays an icon without transparency.
Compressed ARGB data is not interpreted correctly. The last value of the blue channel (aka. the very last value) is ignored and treated as if it were all zero-bytes. Usually this is no issue since most icons will have transparency at the bottom right corner anyway. However, it can become an issue if the last value is a repeating byte (see Compression). Potentially, up to 130 pixels can lack the blue channel value.
A workaround is to append an additional byte at the end which is interpreted as a control character without following data. You can compare the difference with these two examples:
- 1. - 1. macOS 10.15.7 (likely earlier) and later versions have an issue displaying PNG and JPEG 2000 icons for the keys (16x16), (32x32), and (64x64). The keys work fine in a standalone icns file but if used in an application, the icons are displayed completely scrambled. Either use the new ARGB format and (macOS 11+) or the old 24-bit RGB + alpha mask format. Use the latter with the old keys and, or with the newer keys and (writing RGB data into PNG fields). If using ARGB image data, make sure to provide alternative formats for macOS 10.15 and earlier. This issue is especially tricky to detect if you provide both, 16x16 and 16x16@2x icons, because if you connect your Mac to a non-retina monitor, the non-retina 16x16 icon will be used and thus the icon will be displayed scrambled. The field does not seem to be used in application icons and can safely be ignored. Additionally, if you don't provide the smaller icon sizes at all the bug will also manifest when the OS scales down your larger PNG/JPEG 2000 icons, so make sure to render smaller sizes and include them.

Support

Various image viewers can load *.icns files, and free and open source converters from or to PNG also exist.^[4] ^[5] GTK+ can load *.icns resources since 2007.^[6] Other tools supporting the format include the Apple Icon Composer and icns Browser, The Iconfactory, and IconBuilder.
MacOS offers the built-in iconutil command line tool to pack and unpack *.icns files.

References

Web site: Human Interface Guidelines. live. April 10, 2021. Apple Inc.. https://web.archive.org/web/20180618155438/https://developer.apple.com/design/human-interface-guidelines/macos/icons-and-images/app-icon/ . June 18, 2018 .
https://github.com/fiahfy/icns.git Fiahfy ICNS Repo
http://www.macdisk.com/maciconen.php#RLE Macintosh Icons
Web site: libicns. SourceForge project icns. 2009. 2016-08-18.
Web site: png2icns. Moin Uddin. 2016. 2017-05-25.
Web site: Mac OS X icons for GTK+. Lyonel Vincent. 2007. 2016-08-18.

External links

IconFamily (last update 2013) – Open source Objective C class to read and write Apple icns files
osxiconutils (not maintained) – Command line tools to work with Apple icns files
icnsutil – Python library to read and write icns files
icns - Rust crate to read and write icns files
createicns - C library to read and write icns files
icon_records_extractor - C library to extract all icns records as their own icons include dark mode icns.