[Openicc] ProPhoto, ICCv4, was: meta data in test chart

[Chris Murphy]

On Jan 27, 2011, at 4:52 PM, Graeme Gill wrote:

> Chris Murphy wrote:
>>> That's what happens if you store an image as ProPhoto, and then gamut map to your
>>> output space, on the assumption that the image has actually been rendered into
>>> ProPhoto. Having it rendered to some printer space, but stored in ProPhoto breaks
>>> conventional perceptual rendering workflows.
>> 
>> I'm not understanding the distinction you're making. Here is an example workflow:
> 
> The distinction I'm making is:
> 
>    "rendering into a space" modifies the contents gamut to best exploit and work
>    within the limitations of the colorspace gamut.
> 
>    "encoded into a space" takes the content colormetric values and converts them
>    into the same colorimetric value in that colorspace.

OK I'm fine with the distinction and agree with it. Lightroom is doing both things, although the default rendering is mimicing a manufacturer's rendering; whereas the "ideal" rendering is done manually be the user (at least that's the point of the app).


> Generally with small gamut spaces you are forced to do the former. With very large
> gamut spaces the former doesn't make a whole lot of sense. With spaces with
> gamuts somewhere in between, chaos reigns.

I guess in the world of sRGB vs Adobe RGB vs ProPhoto RGB and then to floating-point based spaces like scRGB we have a rather large range of what could be considered large gamut. I suppose scRGB could be described as almost gamutless since most encodeable values are imaginary colors. But yes, I'm understanding what you're saying. While I still think it's correct to say one is rendering into ProPhoto RGB using any Raw processing application, what hinders this from being fully utilized is we're making these rendering choices based on a display that is distinctly not capable of showing us everything in ProPhoto RGB (or even much of it).

> 
>> 3. Print all three images, using either perceptual and relative colorimetric.
>> 
>> You will get six prints, and all of them, while not identical, are virtually identical.
> 
> That depends a whole lot on where the profiles came from, and how they were created.

Agreed. No doubt about it.

> I can easily give you an output profile that perceptual gamut maps from the whole
> ProPhoto gamut, or the PRMG gamut, or some smaller gamut. You _will_ notice the
> difference.

I believe you.


Chris Murphy