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

[Graeme Gill]

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.

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.

> 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.
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 can well imagine that a "perceptual" profile that doesn't know what source
  gamut it's mapping from, does essentially a colorimetric transform with the
  clipping edges softened slightly. It's not a true gamut mapping in my book though.]

Graeme Gill.