[Openicc] meta data in test chart

[Graeme Gill]

Chris Murphy wrote:
> But we don't have a v4 ProPhoto RGB, let alone one that describes how ProPhoto RGB
> images should be mapped to the PRMG. However I've been told this would be almost
> trivial because ProPhoto RGB is an output medium metric (hence its former name), but
> the work simply hasn't been done. It's probably Kodak's job to do it but AFAIK there's
> no interest.

I don't think it would be useful in itself. If you construct a V4 ProPhoto
profile that maps the ProPhoto gamut to the PRMG in the A2B, you'll end up with
rather dull images. If you map some other smaller source gamut to the PRMG (say
an sRGB gamut ??:-), then the advantage of a wide gamut space is thrown away.

I don't think a wide gamut space can realistically be used as an output medium
that you render to. Super-wide gamut spaces are often oddly shaped (L*a*b* cube ?),
and many have impossible (non-real world) colors in their gamut. Inflating an
images gamut to match the gamut of a large gamut space and then shrinking it
again when it is rendered to the actual output medium is not going to be a
hi-fidelity operation. There is no "right way" to map to and from impossible
color values, since you can't perform the psycho-visual experiments needed to
figure out what they should map to.

Conclusion - as soon as you move outside reasonably real world colorspace gamuts,
relying on the encoding space to signal the gamut of an image is not a viable
approach. In the future, something like an embedded tag in the image or document
format that signals the intended or important gamut to render, will become the
appropriate technical approach. Some sort of dynamic color management
at the point of rendering into an output media is then desirable.

> Yes, if you use the existing v2 ProPhoto profile, which is why most people end up using
> RelCol+BPC, rather than bump up saturation in the perceptual intent at the time they
> build the profile.

Unless the image has been blown up to fill the ProPhoto colorspace space, a v4 profile
that maps to/from the PRMG won't change this.

Graeme Gill.