.ICC File Format – Profile classes and required tags

Tags every profile needs

Every profile except a DeviceLink has:

• desc profileDescriptionTag,
• cprt copyrightTag,
• wtpt mediaWhitePointTag,
• chad chromaticAdaptationTag when the measurements were made under a white other than the PCS’s D50.

A profile whose colorimetric intents do not produce picture-referred colorimetry should also have the ciis tag. Any profile may use the same data for every intent, and the media-relative colorimetric tag for ICC-absolute colorimetric too.

Input profiles (scnr)

For scanners and cameras, in one of three forms:

• N-component LUT-based: A2B0. It may add A2B1, A2B2, B2A0 to B2A2, the D2B and B2D tags, and gamt.
• Three-component matrix-based: rXYZ, gXYZ, bXYZ, rTRC, gTRC and bTRC; the PCS must be PCSXYZ. The AToB and BToA tags and gamt may be added.
• Monochrome: kTRC, optionally with AToB and BToA tags.

Display profiles (mntr)

• N-component LUT-based: A2B0 and B2A0, optionally the other intents, the D2B and B2D tags and gamt.
• Three-component matrix-based: the three matrix columns and three tone curves, PCSXYZ only, optionally AToB, BToA and gamt.
• Monochrome: kTRC.

Output profiles (prtr)

• N-component LUT-based: A2B0, A2B1, A2B2, B2A0, B2A1, B2A2 and gamt, and clrt when the data colour space is an xCLR space. The D2B and B2D tags may be added.
• Monochrome: kTRC, optionally with AToB and BToA tags.

DeviceLink profiles (link)

A conversion from one device straight to another, made by combining profiles. It cannot be undone, does not describe a device and cannot be embedded in an image. It needs desc, cprt, pseq and A2B0; clrt when the data colour space is xCLR, and clot when the PCS field is. It may have a D2B0. Its single A2B0 holds whichever intent the header’s rendering intent field names. The data colour space is that of the first profile in the sequence, and the PCS field that of the last.

ColorSpace profiles (spac)

Conversions between a colour encoding and the PCS, made from a model rather than measurements; they may be embedded in images. They need B2A0 and A2B0, and may have the other intents, the D2B and B2D tags and gamt. Header fields about devices are zero.

Abstract profiles (abst)

An effect applied in the PCS, from PCS to PCS, such as a sepia tone. It needs A2B0 and may have D2B0; it cannot be embedded in an image.

NamedColor profiles (nmcl)

A list of named colours, such as a set of spot inks, each with a PCS value and optionally device values. It needs ncl2. A device may have named colour profiles beside its ordinary ones, one for each set of named colours.

Required tags in version 2

ICC.1:2001-04 (clause 6.3) lists the required tags of version 2 profiles. Every class but DeviceLink needs desc (a textDescriptionType), wtpt and cprt (a textType), and then:

Version 2 transforms are lut8Type or lut16Type, tone curves are curveType, and the intents and models are as in the tables below. Only PCSXYZ can be used with the matrix model; an A2B0 is needed for PCSLAB. The explorer checks version 2 profiles against these tables and version 4 profiles against those above.

Which tag holds which intent

“Colorimetric” covers both the media-relative and the ICC-absolute colorimetric intents. “Undefined” means the ICC does not say how the tag is used there and recommends leaving it out. D2B0 to D2B2 and B2D0 to B2D2 follow the same pattern for floating-point data, and D2B3 and B2D3 hold ICC-absolute colorimetric directly.

Which tag a CMM uses

When a profile holds more than one way to convert, a CMM chooses in this order for input, display, output and colour space profiles, for the intent asked for:

1. The D2Bx or B2Dx tag for that intent, if present and every processing element in it is supported.
2. Otherwise the A2Bx or B2Ax tag for that intent.
3. Otherwise A2B0 or B2A0.
4. Otherwise the tone curves and matrix columns, or kTRC.

For DeviceLink and abstract profiles: D2B0 if present and supported, otherwise A2B0.

Required tags in iccMAX

iccMAX profiles (version 5) follow ICC.2:2023 clause 8 instead. Every class but ColorEncodingSpace, DeviceLink, MultiplexIdentification and MultiplexLink needs desc and cprt, wtpt when the PCS field is non-zero, and swpt when the spectral PCS field is non-zero. Then:

A colorantInfoTag (clin) is recommended for xCLR and ncXXXX spaces, in place of ICC.1’s colorantTableTag. The explorer checks version 5 profiles against these rules.

The intents (ICC.2 Table 25) are as in ICC.1 with a fourth: AToB0 perceptual, AToB1 media-relative colorimetric, AToB2 saturation and AToB3 ICC-absolute colorimetric for input, display, output and colour space profiles (and the BToA, DToB and BToD tags likewise); an abstract profile’s AToB0 is PCS to PCS and a DeviceLink’s is device to device, the others undefined.

Which tag an iccMAX CMM uses

For input, display, output and colour space profiles and a colorimetric PCS: the AToB or BToA tag of the intent asked for; otherwise intent 0; otherwise intent 1; otherwise intent 3. With a spectral PCS, the DToB and BToD tags in the same order. With an MCS connection, A2M0. When converting an extended-range (HDR) PCS to a standard one, the H2S tag of the intent, then 0, 1 and 3. Abstract profiles use A2B0 (colorimetric PCS) or D2B0 (spectral PCS); a DeviceLink uses A2B0; MultiplexIdentification A2M0; MultiplexLink M2A0; MultiplexVisualization the M2B or M2S tags of the intent, then 0, 1 and 3. The BRDF (bAB, bBA, bDB, bBD, bcp, bsp, bMB, bMS) and directional (dAB, dBA, dDB, dBD) tags follow the same order when they are wanted.

The simple models

Monochrome: the grey value goes through kTRC to give the achromatic value of the PCS, from 0 for black to 1 for white, and the PCS value is the PCS white point’s PCSXYZ or PCSLAB value multiplied by it. The curve is usually made from the PCS’s luminance channel, Y or L*. The way back uses the inverse curve.

Three-component matrix: each channel goes through its tone curve to give a linear value, and the linear values are multiplied by the matrix whose columns are rXYZ, gXYZ and bXYZ:

X = rX·R + gX·G + bX·B
Y = rY·R + gY·G + bY·B
Z = rZ·R + gZ·G + bZ·B

where R, G and B are the outputs of rTRC, gTRC and bTRC. The way back, from PCSXYZ to the device, uses the inverse matrix, clips the linear values to 0 to 1, and applies the inverse curves. A curve that is not monotonic has no defined inverse. The same model can be written as a lutAToBType with the tone curves as “M” curves and the matrix scaled by 32768/65535, the PCSXYZ encoding factor.