[Openicc] meta data in test chart

[Graeme Gill]

Chris Murphy wrote:
> 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.

But what way does it work then ? :-) If you're going to pick some gamut other
than the colorspace gamut, how do you pick it ? What source gamut do you
assume ? You can guess, or pick something arbitrary, but that guess will be
wrong most of the time.

ie. You're just confirming that as soon as you move to encoding
colorspaces that are significantly larger than real devices,
gamuts need to be part of the information flow, just like
colorspaces themselves.

> 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.

Which is great if you want that expansion, but not so good if you actually
want to choose what intent you get.

> 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.

That may be so, but the blue primary in particular is way out there. I think
it is beyond the spectrum locus (it certainly breaks CIECAM02).

> 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'd be fascinated to know how they manage that. The few occasions people
have brought it to my notice, they had problems with dull looking
results until they moved to an image gamut workflow.

> 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.

I'm afraid that the PRMG is way down my list, primarily because I see its main
use case being "office" and un-demanding users. Anybody after precision, quality
and control won't want to use it.

> 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.

"input/output" referred are pretty blunt descriptions. ProPhoto cannot actually
be output referred, since it has an impossible to reproduce primary (blue) :-)

It's not actually the encoding colorspace choice that makes it input/output referred,
it's the intent of the rendering done (if any) in converting something to that
space (although some colorspaces do have viewing condition expectations
attached to them, a connected but independent aspect.) ie. the way I see it,
as soon as you start treating a colorspace as an actual device, and re-render the
image contents to take advantage of that colorspace and work within its
limitations, it's "output" referred.

Graeme Gill.