.ICC File Format

Introduction

An ICC profile describes the colours of a device or a colour space so that colour management software can convert between them: a camera’s or scanner’s RGB, a display’s RGB, a printer’s CMYK, or a standard space such as sRGB or Adobe RGB. The format is defined by the International Color Consortium (ICC), founded in 1993, and published as ICC.1, which is also the international standard ISO 15076-1.

A profile relates the device’s values to a device-independent profile connection space (PCS), CIE XYZ or CIELAB under the D50 illuminant. Converting from one device to another goes through the PCS: the first profile maps the source values into it, and the second maps them out to the destination. Profiles are stored as files, usually with the extension .icc (and .icm on Windows), and are also embedded in images and documents so that their colours can be shown as intended.

There are two versions in use. Version 2 profiles were defined up to ICC.1:2001-04 and are still the most common; version 4, first published as ICC.1:2001-12 and now ICC.1:2022 (profile version 4.4), clarified the PCS and added new tag types. iccMAX (ICC.2, profile version 5) extends the format with spectral and other connection spaces. The Versions and iccMAX page lists the differences.

Structure

A profile is a 128-byte header, a tag table, and the tags’ data. Every number is big-endian. The header gives the profile’s size, its version, its class (input, display, output, device link, colour space, abstract or named colour), the data colour space and the PCS, and the signature acsp at byte 36. The tag table is a count and one 12-byte entry per tag: a four-character signature, an offset and a size.

Each tag’s data starts with a four-character type signature that says how it is laid out, so a reader can show a tag it does not know the meaning of. The tags hold the description and copyright, the media white point, tone curves and matrix columns for simple RGB and grey profiles, and lookup-table transforms for the rest. Which tags a profile must have depends on its class. Several tags may share one block of data.

Embedded profiles

Most profiles people meet are inside other files rather than on their own. The same bytes are stored whole, or in pieces, in these places:

• PNG: the iCCP chunk, compressed with zlib.
• JPEG: APP2 segments marked ICC_PROFILE, split into numbered pieces when it does not fit one segment.
• TIFF: tag 34675 (InterColorProfile), an UNDEFINED array.
• BMP: a version 5 bitmap header with the colour space type MBED points to the profile after the pixels.
• PDF: an ICCBased colour space stream, and output intents.

Bit 0 of the profile flags says whether a profile is embedded. The explorer opens a profile found in a PNG, JPEG or BMP file as a file of its own.

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