[OCIO-V2] ICC Display Profile Support with V4 `wtpt` tag

[sobotka]

Given that V4 makes it impossible to deduce the implicit white point of a characterization, how is OCIO V2 dealing with this rather impossibly large obstacle?

As per the more recent meetings and the integration with a virtual display slot for system wide ICCs, I'm wondering how OCIO-V2 will deduce the appropriate white point, or if there will be a way to explicitly flag it as such?

That is, for those unfamiliar, ICC V2 had a "convention" whereby the absolute white point of the given characterization was conventionally communicated via the wtpt tag. Sadly, with V4, the consortium decided to force this tag to refer to the ICC white point for the Profile Connection Space, aka ICC D50. As such, the decision has made it fundamentally impossible to determine the absolute white point for the colourimetry, which is clearly a problem facing OCIO-V2 and the integration of ICC display characterizations.

One viable solution is to provide a white point hint in the virtual display configuration, that will assume all of the matrices and other such common transforms for display colourimetry is provided "as is" relative to the ICC D50 PCS white point, and OpenColorIO V2 would be able to then transform the ICC D50 canonized PCS to the appropriate white point chromaticity given via the hint.

It should be noted that to the best of my knowledge, there are no solutions available currently. Graeme Gill, author of Argyll CMS has registered the arts tag, with a guideline of protocol to deduce absolute colourimetry. ICCMax also has the option of specifying the absolute white point. However, given the near-zero usage of these options still requires a potential workaround short term for the ICC V4 surface of characterizations.

Relevant quote from the ICC V4 specification, page 40:

9.2.34 mediaWhitePointTag Tag signature: ‘wtpt’ (77747074h) Permitted tag type: XYZType

This tag, which is used for generating the ICC-absolute colorimetric intent, specifies the chromatically adapted nCIEXYZ tristimulus values of the media white point. When the measurement data used to create the profile were specified relative to an adopted white with a chromaticity different from that of the PCS adopted white, the media white point nCIEXYZ values shall be adapted to be relative to the PCS adopted white chromaticity using the chromaticAdaptationTag matrix, before recording in the tag. For capture devices, the media white point is the encoding maximum white for the capture encoding. For displays, the values specified shall be those of the PCS illuminant as defined in 7.2.16.

See Clause 6 and Annex A for a more complete description of the use of the media white point.