Apple Icon Image format

Apple Icon Image

Filename extension	.icns
Type code	icns
Uniform Type Identifier (UTI)	com.apple.icns
Developed by	Apple Inc.
Type of format	icon file format

The Apple Icon Image format is the icon format used in Apple Inc.'s Mac OS X. It supports icons of 16 × 16, 32 × 32, 48 × 48, 128 × 128, 256 × 256, 512 × 512, and 1024 × 1024 pixels, 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.

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

Compression

Over time the format has been improved and there is support for compression of some parts of the pixel data. The 32-bit ("is32", "il32", "ih32","it32") pixel data is often compressed (per channel) with a format similar to PackBits.
Some sources mentioned that the OS supports both compressed or uncompressed data chunks.

Icon types

OSType	Length (bytes)	Size (pixels)	Supported OS Version	Description
ICON	128	32	1.0	32×32 1-bit mono icon
ICN#	256	32	6.0	32×32 1-bit mono icon with 1-bit mask
icm#	48	16	6.0	16×12 1 bit mono icon with 1-bit mask
icm4	96	16	7.0	16×12 4 bit icon
icm8	192	16	7.0	16×12 8 bit icon
ics#	64 (32 img + 32 mask)	16	6.0	16×16 1-bit mask
ics4	128	16	7.0	16×16 4-bit icon
ics8	256	16	7.0	16x16 8 bit icon
is32	varies (768)	16	8.5	16×16 24-bit icon
s8mk	256	16	8.5	16x16 8-bit mask
icl4	512	32	7.0	32×32 4-bit icon
icl8	1,024	32	7.0	32×32 8-bit icon
il32	varies (3,072)	32	8.5	32x32 24-bit icon
l8mk	1,024	32	8.5	32×32 8-bit mask
ich#	288	48	8.5	48×48 1-bit mask
ich4	1,152	48	8.5	48×48 4-bit icon
ich8	2,304	48	8.5	48×48 8-bit icon
ih32	varies (6,912)	48	8.5	48×48 24-bit icon
h8mk	2,304	48	8.5	48×48 8-bit mask
it32	varies (49,152)	128	10.0	128×128 24-bit icon
t8mk	16,384	128	10.0	128×128 8-bit mask
icp4	varies	16	10.7	16x16 icon in JPEG 2000 or PNG format
icp5	varies	32	10.7	32x32 icon in JPEG 2000 or PNG format
icp6	varies	64	10.7	64x64 icon in JPEG 2000 or PNG format
ic07	varies	128	10.7	128x128 icon in JPEG 2000 or PNG format
ic08	varies	256	10.5	256×256 icon in JPEG 2000 or PNG format
ic09	varies	512	10.5	512×512 icon in JPEG 2000 or PNG format
ic10	varies	1024	10.7	1024×1024 in 10.7 (or 512x512@2x "retina" in 10.8) icon in JPEG 2000 or PNG format
ic11	varies	32	10.8	16x16@2x "retina" icon in JPEG 2000 or PNG format
ic12	varies	64	10.8	32x32@2x "retina" icon in JPEG 2000 or PNG format
ic13	varies	256	10.8	128x128@2x "retina" icon in JPEG 2000 or PNG format
ic14	varies	512	10.8	256x256@2x "retina" icon in JPEG 2000 or PNG format

• the values inside the brackets () in the length column is the uncompressed length.

Other types

OSType	Length (bytes)	Description
'TOC '	varies	"Table of Contents" a list of all image types in the file, and their sizes (added in Mac OS X 10.7)
'icnV'	4	4-byte big endian float - equal to the bundle version number of Icon Composer.app that created to icon

See also

• Icon Composer
• icns Browser
• libicns - Open-source C library for working with icns format files
• Computer icon
• The Iconfactory
• IconBuilder
• Wikipedia:Icons
• PackBits
• compare: ICO (icon image file format)

External links

• [1]