.ICC File Format – Versions and iccMAX

Version 2 and version 4

Version 2.4 (ICC.1:2001-04) already added the chromaticAdaptationTag. In version 2 the matrix tags were called redColorantTag, greenColorantTag and blueColorantTag, with the same signatures. Version 4 also lists more technology signatures (LCD, OLED and the motion-picture ones) and two more chromaticity sets (P3 and BT.2020, in 4.4). A version 4 CMM reads version 2 profiles; version 2 CMMs may not read version 4 ones, which is why both are still made.

The dictType and the metadataTag (meta) came from an ICC votable proposal, Dictionary Type and Metadata TAG Definition, submitted on 26 January 2010 by Ann McCarthy, Max Derhak and Lars Borg. Its adoption added the type and the tag “to the next minor revisions of the ICC.1 specifications, both ICC.1:2001-04 (version 2) and ICC.1:2010 (version 4)”, and its introduction proposes tags “that can be incorporated into any ICC profile of any version”, since no CMM computes with them. The published ICC.1:2010 text (version 4.3) does not include them: they first appear in the specification itself in ICC.1:2022 (4.4), whose foreword lists “metadataTag and dictType have been added” among the changes from ICC.1:2010. The explorer accepts both in version 2 and version 4 profiles of any minor version.

iccMAX (ICC.2, version 5)

iccMAX, published as ICC.2 and as ISO 20677, extends the format for cases that a fixed D50 colorimetric PCS cannot describe: colorimetry under other illuminants and observers, spectral reflectance, transmission and emission, fluorescence (bi-spectral data), and multiplex channels whose meaning is not colour. It keeps the header, tag table and typed-data layout of ICC.1, but has its own lists of tags and types (ICC.2 clause 1: some ICC.1 types are removed and others added). An iccMAX reader is expected to read ICC.1 profiles too; an ICC.1 reader is not expected to use iccMAX ones. Its current edition is ICC.2:2023 (profile version 5.0).

Not in iccMAX’s lists: the types lut8Type, lut16Type, chromaticityType, colorantTableType, namedColor2Type, profileSequenceDescType, profileSequenceIdentifierType, responseCurveSet16Type and viewingConditionsType, and the tags that use them or the matrix model (rXYZ, gXYZ, bXYZ, rTRC, gTRC, bTRC, kTRC, chad, chrm, clrt, clot, lumi, meas, ncl2, pseq, psid, resp, view, vued, gamt and the preview tags). The explorer reports an ICC.1-only tag found in a version 5 profile as not in ICC.2’s tag list. meas is a special case: clause 9 does not list it, but 10.2.15 extends measurementType as “a backwards compatible extension of ISO 15076‐1 with the same tag signature”, so the explorer only notes it (measurementType in iccMAX).

Header

A ColorEncodingSpace profile uses only the size, the version, the class, the data colour space (RGB or YCbr) and acsp; its other header bytes are zero or as in ICC.1 (Table 13 calls bytes 4 to 7, 20 to 35 and 40 to 127 reserved, while 7.2.3 allows them “set according to ISO 15076‐1 or optionally zero filled”). Either way bytes 100 to 127, which ICC.1 does not have, are zero. A spectral-only profile has a PCS field of zero and only DToB and BToD tags.

The header fields agree with each other (ICC.2 7.2.6 and 7.2.21 to 7.2.23):

• The channel count of an rs, ts or es spectral PCS equals the steps of the spectral range. For bs, the “N total channels” of a Donaldson matrix are read as the spectral range’s steps (reflected wavelengths) times the bi-spectral range’s steps (incident wavelengths).
• With no spectral PCS both ranges are zero, and the bi-spectral range is zero unless the spectral PCS is bs or sm.
• Bytes 10 and 11 of the version are zero when the sub-class field is zero.

The explorer checks all of these.

iccMAX tags and their permitted types

Every tag of ICC.2:2023 clause 9, with the types it allows there. Where a tag holds a structure or array, the identifier it must carry is given.

Types iccMAX adds

multiProcessElementsType gains the elements calc (a programmable calculator), xclt (extended CLUT), eclt, emtx, eobs and iemx (emission), rclt and robs (reflectance), smet (sparse matrix), tint (tint array), tmap (tone map), and JtoX and XtoJ (colour appearance), and its curve sets gain single-sampled and calculator-sampled curves. All of them, and the structures and arrays, are described byte by byte on iccMAX types and elements, and the explorer decodes them all.