Apple Icon Image format

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Template:Short description Template:Use mdy dates Script error: No such module "Infobox".Template:Template otherScript error: No such module "Check for unknown parameters". 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.[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

OSType Length (bytes) Size (pixels) Supported OS Version Description
Template:Mono 128 32×32 1.0 1-bit mono icon
Template:Mono 256 32×32 6.0 1-bit mono icon with 1-bit mask
Template:Mono 48 16×12 6.0 1 bit mono icon with 1-bit mask
Template:Mono 96 16×12 7.0 4 bit icon
Template:Mono 192 16×12 7.0 8 bit icon
Template:Mono 64 16×16 6.0 1-bit mono icon with 1-bit mask
Template:Mono 128 16×16 7.0 4-bit icon
Template:Mono 256 16×16 7.0 8 bit icon
Template:Mono varies1 (768) 16×16 8.5 24-bit RGB icon
Template:Mono 256 16×16 8.5 8-bit mask
Template:Mono 512 32×32 7.0 4-bit icon
Template:Mono 1024 32×32 7.0 8-bit icon
Template:Mono varies1 (3072) 32×32 8.5 24-bit RGB icon
Template:Mono 1024 32×32 8.5 8-bit mask
Template:Mono 576 48×48 8.5 1-bit mono icon with 1-bit mask
Template:Mono 1152 48×48 8.5 4-bit icon
Template:Mono 2304 48×48 8.5 8-bit icon
Template:Mono varies1 (6912) 48×48 8.5 24-bit RGB icon
Template:Mono 2304 48×48 8.5 8-bit mask
Template:Mono varies1 (49152 + 4)2 128×128 10.0 24-bit RGB icon
Template:Mono 16384 128×128 10.0 8-bit mask
Template:Mono varies 16x16 10.7 JPEG 2000† or PNG† format or 24-bit RGB icon[2]
Template:Mono varies 32x32 10.7 JPEG 2000† or PNG† format or 24-bit RGB icon[2]
Template:Mono varies 48x48 10.7 JPEG 2000† or PNG† format
Template:Mono varies 128x128 10.7 JPEG 2000 or PNG format
Template:Mono varies 256x256 10.5 JPEG 2000 or PNG format
Template:Mono varies 512x512 10.5 JPEG 2000 or PNG format
Template:Mono varies 1024x1024 10.7 JPEG 2000 or PNG format (512x512@2x "retina" in 10.8)
Template:Mono varies 32x32 10.8 JPEG 2000 or PNG format (16x16@2x "retina")
Template:Mono varies 64x64 10.8 JPEG 2000 or PNG format (32x32@2x "retina")
Template:Mono varies 256x256 10.8 JPEG 2000 or PNG format (128x128@2x "retina")
Template:Mono varies 512x512 10.8 JPEG 2000 or PNG format (256x256@2x "retina")
Template:Mono varies1 (1024) 16x16 ARGB or JPEG 2000† or PNG† format
Template:Mono varies1 (4096) 32x32 ARGB or JPEG 2000† or PNG† format (16x16@2x "retina")
Template:Mono varies1 (1296) 18x18 ARGB or JPEG 2000† or PNG† format
Template:Mono varies 36x36 JPEG 2000 or PNG format (18x18@2x "retina")
Template:Mono varies 24x24 JPEG 2000 or PNG format
Template:Mono varies 48x48 JPEG 2000 or PNG format (24x24@2x "retina")
  • 1. The value inside the parenthesis is the uncompressed length for ARGB and 24-bit RGB icons.
  • 2. Template:Mono 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 Template:Mono 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 Template:Mono 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 (Template:Mono, Template:Mono, Template:Mono, Template:Mono) 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
'TOC ' "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) maps to the icon formats found in the file. The data isn't included in this phase.

'icnV' 4-byte big endian float - equal to the bundle version number of Icon Composer.app that created the icon
'name' Usage unknown (all tested files use either "icon"[3] or "template"[4]).
'info' Info binary plist. Usage unknown (only name field seems to be used).
'sbtp' Nested "template" icns file. Usage unknown.
'slct' Nested "selected" icns file. Usage unknown.
FD D9 2F A8 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 out to 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).[5]

ARGB Elements ARGB 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 Elements RGB 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 Elements Mask 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
valueScript error: No such module "Check for unknown parameters". 		tail
bytesScript error: No such module "Check for unknown parameters". 		result
uncompressedScript error: No such module "Check for unknown parameters".
Template:Samp...Template:SampScript error: No such module "Check for unknown parameters". Template:Samp...Template:SampScript error: No such module "Check for unknown parameters". Template:Samp...Template:SampScript error: No such module "Check for unknown parameters". bytes
Template:Samp...Template:SampScript error: No such module "Check for unknown parameters". Template:SampScript error: No such module "Check for unknown parameters". byte Template:Samp...Template:SampScript error: No such module "Check for unknown parameters". 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 (Template:Mono, Template:Mono, Template:Mono, Template:Mono, Template:Mono, Template:Mono) and ARGB (Template:Mono, Template:Mono, Template:Mono) pixel data are compressed (per channel) with a format similar to PackBits.[6] Some sources mention that the OS supports both compressed or uncompressed data chunks.Script error: No such module "Unsubst". 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:

<templatestyles src="Pre/styles.css"/>

While there is compressed data:
    Read one byte as an unsigned number N
    If N < 0x80:
        Output the next (N + 1) bytes
    Else:
        Output the next byte (N - 0x80 + 3) times

Example: 02 01 02 02 80 03 81 04 82 05 should decompress to 01 02 02 03 03 03 04 04 04 04 05 05 05 05 05

The following pseudocode compresses the data:

<templatestyles src="Pre/styles.css"/>

function Encode(input data)
  Initialize output as an empty array
  Set index to 0

  While index Template:Dfn the count of data
    Initialize sequence as an empty array
    Set count to 0

    // Unique sequence
    While count Template:Dfn 0x7F and index Template:Dfn count of data
      If index + 2 Template:Dfn count of data and data<templatestyles src="Template:Tooltip/styles.css" />[index]Script error: No such module "Check for unknown parameters". Template:Dfn data<templatestyles src="Template:Tooltip/styles.css" />[index+1]Script error: No such module "Check for unknown parameters". and data<templatestyles src="Template:Tooltip/styles.css" />[index]Script error: No such module "Check for unknown parameters". Template:Dfn data<templatestyles src="Template:Tooltip/styles.css" />[index+2]Script error: No such module "Check for unknown parameters".
        Break the loop  // Start of a repeating sequence
      End If

      Append data<templatestyles src="Template:Tooltip/styles.css" />[index]Script error: No such module "Check for unknown parameters". to sequence
      Increment index
      Increment count
    End While

    If sequence is not empty
      Append (count - 1) to output
      Append all items in sequence to output
    End If

    If index Template:Dfn count of data
      Break the loop
    End If

    // Repeating sequence
    Set repeatedByte to data<templatestyles src="Template:Tooltip/styles.css" />[index]Script error: No such module "Check for unknown parameters".
    Set count to 0
    While count Template:Dfn 0x7F and index Template:Dfn data and data<templatestyles src="Template:Tooltip/styles.css" />[index]Script error: No such module "Check for unknown parameters". Template:Dfn repeatedByte
      Increment index
      Increment count
    End While

    If count Template:Dfn 3
      Append (0x80 + count - 3) to output
      Append repeatedByte to output
    Else  // Less than 3 repeating bytes
      Append (count - 1) to output
      Repeat (count) times
        Append repeatedByte to output
      End Repeat
    End If
  End While

  Return output
End function

Example: 01 02 02 03 03 03 04 04 04 04 05 05 05 05 05 should compress to 02 01 02 02 80 03 81 04 82 05

Known issues

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

  1. Setting Template:Mono or Template:Mono will display a proper icon. But setting Template:Mono ignores the transparency mask and displays an icon without transparency.
  2. 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:
    • 69636E73 00000024 69633034 0000001C 41524742 FFFFFBFF FF00FB00 FF00FB00 FFFFFBFF
    • 69636E73 00000025 69633034 0000001D 41524742 FFFFFBFF FF00FB00 FF00FB00 FFFFFBFF 00
  3. macOS 10.15.7 (likely earlier) and later versions have an issue displaying PNG and JPEG 2000 icons for the keys Template:Mono (16x16), Template:Mono (32x32), and Template:Mono (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 Template:Mono and Template:Mono (macOS 11+) or the old 24-bit RGB + alpha mask format. Use the latter with the old keys Template:Mono and Template:Mono, or with the newer keys Template:Mono and Template:Mono (writing RGB data into PNG fields[2]). 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 Template:Mono 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 apps can open *.icns files including Preview and GTK+, with some offering the ability to convert *.icns to and from PNG as well.[7][8][9] Former tools specializing in icon file support include Icon Composer and icns Browser from Apple, IconBuilder from The Iconfactory, and CandyBar from Panic.

MacOS offers the built-in iconutil command line tool to pack and unpack *.icns files.[10]

See also

References

  1. ↑ Script error: No such module "citation/CS1".
  2. ↑ a b c Script error: No such module "citation/CS1".
  3. ↑ Script error: No such module "citation/CS1".
  4. ↑ Script error: No such module "citation/CS1".
  5. ↑ Fiahfy ICNS Repo
  6. ↑ Macintosh Icons
  7. ↑ Script error: No such module "citation/CS1".
  8. ↑ Script error: No such module "citation/CS1".
  9. ↑ Script error: No such module "citation/CS1".
  10. ↑ Script error: No such module "citation/CS1".

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.

Template:Mac OS X Template:Graphics file formats

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