Apple Icon Image 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 and 512×512 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.
| Offset |
Size |
Purpose |
| 0 |
4 |
Magic literal, must be "icns" (0x69, 0x63, 0x6e, 0x73) |
| 4 |
4 |
Length of file, in bytes |
Icon data
| Offset |
Size |
Purpose |
| 0 |
4 |
Icon type, see OSType below. |
| 4 |
4 |
Length of data, in bytes (including type and length) |
| 8 |
Variable |
Icon data |
Compression
Over the time the format have 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 the PackBits algorithm.
Some sources mentioned that the OS supports both compressed or uncompressed data chunks.
Icon types
| OSType |
Length (bytes) |
Size (pixels) |
Description |
| ICON |
128 |
32 |
32×32 1-bit mono icon |
| ICN# |
256 |
32 |
32×32 1-bit mono icon with 1-bit mask |
| icm# |
24 |
16 |
16×12 1 bit mask |
| icm4 |
96 |
16 |
16×12 4 bit icon |
| icm8 |
192 |
16 |
16×12 8 bit icon |
| ics# |
32 |
16 |
16×16 1-bit mask |
| ics4 |
128 |
16 |
16×16 4-bit icon |
| ics8 |
256 |
16 |
16x16 8 bit icon |
| is32 |
varies (768) |
16 |
16×16 24-bit icon |
| s8mk |
256 |
16 |
16x16 8-bit mask |
| icl4 |
512 |
32 |
32×32 4-bit icon |
| icl8 |
1,024 |
32 |
32×32 8-bit icon |
| il32 |
varies (3,072) |
32 |
32x32 24-bit icon |
| l8mk |
1,024 |
32 |
32×32 8-bit mask |
| ich# |
288 |
48 |
48×48 1-bit mask |
| ich4 |
1,152 |
48 |
48×48 4-bit icon |
| ich8 |
2,304 |
48 |
48×48 8-bit icon |
| ih32 |
varies (6,912) |
48 |
48×48 24-bit icon |
| h8mk |
2,304 |
48 |
48×48 8-bit mask |
| it32 |
varies (49,152) |
128 |
128×128 24-bit icon |
| t8mk |
16,384 |
128 |
128×128 8-bit mask |
| ic08 |
varies |
256 |
256×256 icon in JPEG 2000 or PNG format |
| ic09 |
varies |
512 |
512×512 icon in JPEG 2000 or PNG format |
| ic10 |
varies |
1024 |
1024×1024 icon in PNG format (added in Mac OS X 10.7) |
- the values inside the brackets () in the length column is the uncompressed length.
Icons created with 10.7's Icon Composer contain a 'TOC ' entry at the beginning that lists all of the image types contained within the icon and their size.
Some newer icons contain a 'icnV' entry at the end that is 12 bytes long (4 bytes data length). The last 2 bytes appear to always be 0, however the meaning of the other two bytes is unknown.
See also
External links
|
|
|
| Versions |
|
|
| Applications |
|
|
| Utilities |
|
|
Technology and
user interface |
|
|