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Three component (RGB) characterisation registry

The characterisation data which may be defined in this part of the characterization registry are derived from various sources. Some of these are attributable to published National or International Standards, whereas others are based on colour specifications published by individual companies or consortia that do not have the same standard status, but are very widely used and have thereby become de-facto standards. Their provision in this registry serves two purposes. The data can be used by profile builders to ensure that profiles produced for these colour spaces are based on 'standard' data. They also enable standard file formats such as PDF/X and JDF, which provide the facility for characterisation data to be referenced, to do so by name only. Receivers of files that include such a name will know that the full data, if needed, is available from the ICC registry.

In most cases the RGB specifications as published do not use the same reference colour space as that used by ICC (D50). In such cases this means that the specifications provided for these RGB colour spaces have had to be extended in this registry to provide the characterisation data necessary for building ICC profiles. These extensions follow each of the formal specifications in a section headed 'Hints for profile makers'.

Two types of extensions have been provided. The first is a chromatic adaptation correction to convert the data from the specified reference colour space to the D50 colour space required for ICC profiles. In calculating this use has been made of a matrix (derived from the Bradford chromatic adaptation transformation) provided on the ICC web-site for converting D65 data to D50.

The second extension is appropriate when deriving tables for the colorimetric rendering intents in version 4 profiles. In order to understand this some background may be helpful. In earlier versions of the ICC specification a single PCS was assumed that was based on the concept of a virtual reflection print with no defined gamut or black point. In the latest version this has changed. While the PCS for the perceptual rendering intent is still assumed to be that of a reflection print, but with a defined dynamic range, the PCS for the colorimetric rendering intents is no longer assumed to be the colorimetry based on any specific media, but simply the colorimetry of the media as measured. However, for encoding convenience, all of the RGB colour spaces specified produce 1 and 0 in each of RGB when XYZ is set to the white and 0, 0, 0 respectively, whereas in practice some degree of flare will be present if they are intended to represent real devices in real viewing situations. The amount of this flare will vary with the actual viewing conditions used. However, it has been concluded by the committee developing ISO 22028-1 (a standard pertaining to colour image encodings) that for measurement consistency a level of flare should be assumed for display RGB colour encodings that is consistent with the 0:45 measurement condition assumed for ICC PCS measurements. Within ISO 22028-1 suggestions are made as to this level of flare and the corrections recommended in this registry are consistent with those. Compensation for flare can be added when interpreting the encoded colorimetry and that is what has been proposed in this document.

The second extension is not appropriate for making version 2 profiles as common practice in constructing version 2 profiles is to place the RGB encoding zero at zero in the PCS. For ROMM RGB and the sRGB family encodings, this can be accomplished by simply using the encoding equations that place encoding zero at XYZ zero. Then, it is up to the output profile to know that XYZ zero in the version 2 PCS represents the encoding black point.

Information about the details of the respective three component colour encoding is given when the name below is clicked on. For all except the eciRGB data set the equations defining the characterisation data for v4 colorimetric profiles may be obtained as a pdf file by clicking on the appropriate file name within the description. For eciRGB the data given is appropriate for v2 profiles. Further consideration needs to be given to extending this for v4.

In many cases links are given to example profiles. These are for convenience only and do not represent any endorsement by ICC.

Adobe RGB (1998)