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The Perceptual Reference Medium gamut (PRMG)

A perceptual rendering is one where the goal is to produce a pleasing reproduction of an original (the source) on a particular destination output medium.

With v4 of the ICC specification, the source profile perceptual intent transform is intended to color render or re-render to the v4 Perceptual Reference Medium (PRM), and similarly the destination profile transforms from the PRM to the destination medium.

A practical PRM is defined in ICC v4. It has a reference viewing condition based on the ISO 3664 P2 viewing condition for appraisal of hard copy prints, with an illuminance of 500 lux and a neutral surround reflectance of 20%. Whereas the ICC v2 PCS used an ideal print based on a reference medium defined as perfect diffuse reflector with an unlimited colour gamut, the v4 PRM is based on a high quality virtual photo print medium with a 288:1 dynamic range, having a neutral reflectance of 89% and a darkest printable colour having a neutral reflectance of 0.30911%.

The target gamut of the PRM was not specified in the initial publication of ICC.1:2004-10. The gamut was defined in the first amendment to ICC.1:2004-10, and is in the current v4 specification. The Perceptual Reference Medium Gamut (PRMG) is recommended as the target gamut, and numeric values for the gamut boundary of the PRMG in the CIELAB colour space are given.

The specification provides ICC profile optional tags for the perceptual and saturation rendering intents that identify whether the PRMG was used in rendering to or from the PRM.

Since the publication of ICC.1:2004-10, ICC has provided several resources related to the PRMG for developers and users of v4 profiles and workflows.

The iccMAX specification also recommends the PRMG. iccMAX includes a gamut boundary definition based on vertices and associated faces which fits well with the PRMG data. A vertex list and face list for the PRMG are available.

Developers are encouraged to make use of the PRMG in defining transforms between data and PCS encodings for the Perceptual rendering intent, and to add the perceptualRenderingIntentGamutTag to profiles indicate that the PRMG was so used.