[Openicc] meta data in test chart

[Chris Murphy]

On Jan 25, 2011, at 4:49 PM, Graeme Gill wrote:

> 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.

I don't think it works that way.The goal isn't that you take the entire ProPhoto RGB space and smash it into the PRMG.  The effort with mapping sRGB to the PRMG (and back for re-rendering) was not restricted to the idea of taking the entire gamut boundary and mapping it to the PRMG. Some real colors that can be captured, that end up being slightly less saturated in sRGB due to gamut restriction end up printing MORE saturated because the mapping to the PRMG allows for gamut expansion, not just compression, to/from the PRMG. sRGB was probably a harder case because of where its primaries and implied constant hue lines are compared to the PRMG.


> I don't think a wide gamut space can realistically be used as an output medium
> that you render to.

While we call ProPhoto a wide gamut space it isn't that huge. The vast majority of the colors are still real (visible) colors and many of them are capturable which is why Reference Output Medium Metric was concocted in the first place.


> 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.

Ignoring the gamut of ProPhoto RGB, and assuming a one size fits all gamut mapping in only the output device profile working with everything from sRGB to ProPhoto RGB can't be ideal. Yet that's the current workflow. Most photographers who use ProPhoto RGB are relatively satisifed by the results they get with existing profiles. 

I think better is possible but again I think the discussion of v4 + PRMG is a long road possibly to no where conversation because there appears to be no support. If there were, I think we're talking about maybe a fine tuning heuristic. It's not like the effort behind sRGB.

And the challenge of HDR is even greater than that of what we're presently talking about with ProPhoto RGB to a print space, both of which are output-referred. Yet a lot of work is being done in this area and the results are pretty impressive, even if they're not "perfect" even if there's still a long way to go, the idea we can't do any sort of automatic gamut and dynamic range mapping isn't true. Professionals will still need to look at a reference which for now will be an output-referred dynamic range display device, and work prints, and decide their HDR renderings. 

> 
> 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.

I agree we have to work with what we have to work with. And for now that's a v2 paradigm, and a mix of automatic, semi-automatic, and manual means of changing HDR image states to output-referred. But within that output-referred rendering, I think the idea behind the v4 + PRMG workflow does work better than ignoring the issue entirely, but as of yet it's not so much better than ignoring the issue that any commercially produced ICC profiling software builds such profiles. And even if they did, it's pointless without source profiles as well.


> 
>> 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.

Pretty rare to use the entire space. The space is shaped the way it is because it's defined by three primaries. This isn't how print works. The mapping to the PRMG does help normalize some of the gamut mapping into two steps that should improve the perceptual results. It certainly has with sRGB. I know that this strategy wouldn't work with scRGB, that's a different beast.


Chris Murphy