[argyllcms] Re: negative primaries

  • From: Elle Stone <l.elle.stone@xxxxxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Tue, 8 Jun 2010 10:08:09 -0400

On 6/6/10, Graeme Gill <graeme@xxxxxxxxxxxxx> wrote:

> If the colors are really outside the spectrum locus, then I guess
> it's acceptable to clip them, since they must be erroneous.
> But on the other hand if they are a result of a chromatic adaptation, or of
> the metameric errors introduced by the different spectral
> sensitivities of a sensor and the standard observer (non Luther
> condition sensors), then these apparently imaginary colors may
> actually correspond to real world colors, and shouldn't be clipped,
> since they can be brought into the real world gamut during gamut mapping.
> [Another possibly source of such "imaginary" colors may be
>   the choice of the white point. If there is a highly saturated
>   source in the scene that is brighter than the chosen white point,
>   then it may well end up outside the spectrum locus if its Y value
>   gets clipped to that of the white.]

I think that saturation and chosen white point does play a role,
especially as checking with profcheck shows my matrix camera profile
oversaturates already saturated yellows. As a test, desaturating by
50% with Cinepaint hue-saturation before converting to ProPhoto does
retain blue channel detail.

It looks to me like the detail in the blue channel is real even if the
color isn't. It is different from detail in other channels, very
evidently so in some images. For color prints, perhaps having a solid
black blob for the blue channel of a flower makes no visual
difference. For black and white, the blue channel often has a lot of
interesting detail and tonality.

Also, in some images, not necessarily saturated yellow images, after
assigning my camera profile, the blue channel is smooth and detailed.
After converting to ProPhoto, the blue channel has black holes and
pits that look like "blue channel noise". Again, not visible in a
color print or screen display.

> It would be interesting to see if non-Luther condition cameras
> routinely produce such imaginary XYZ values for highly saturated
> source colors. If so, then the ICC's advice to clip such colors
> is probably not so good...
> Graeme Gill.

Are there any cameras that do meet the Luther-Ives condition?

I would love to know if anyone has checked with their own cameras and
raw files, if there are cases where the blue channel detail disappears
or gets "noisy" after converting to ProPhoto, when the detail was
there in a colprof matrix camera space. You can easily check with
dcraw using "-o 0" and then assigning your camera profile, or with
UFRaw using the camera profile as both input and output profile.

Elle Stone

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