Re: A discussion on accurate color ...
Re: A discussion on accurate color ...
- Subject: Re: A discussion on accurate color ...
- From: Tom Lianza <email@hidden>
- Date: Mon, 23 May 2011 11:45:01 -0400
- Thread-topic: A discussion on accurate color ...
Hi Barry,
I read your interesting article and I have some comments that might be
applicable:
When speaking with conservationists, I always ask the question: "What is the
intended use of the data after acquisition?". The concept of "accurate"
color reproduction cannot be considered outside of the illuminant that is
used to view the color. In my opinion, the selection of D50 as a standard
illuminant ranks as one of the worst decisions in the history of color
analysis and the application of d50 to LAB normalization(i.e. Wrong Von
Kreiss) just compounds the issue. d50 is physically unrealizable and it
does not represent any viewing conditions that one would expect in a
conservation exercise. As a matter of fact, the UV content of D50 far
exceeds the allowable exposure for most conservation considerations.
Naturally, to examine the colors as rendered to sRGB, you would need to do
some chromatic adaptation to "discount" the illuminant. This is also
problematic. Take a look at www.brucelindbloom.com -> info -> chromatic
adaptation evaluation. The XYZ scaling is the one used in LAB. Really,
Really bad.... Note also, that chromatic adaptation scaling tends to fail on
the low side of the actual color shift. (i.e. It minimizes the shift in
colorfulness (C*) in the LAB system). It is highly probable, that more
colors actually fall out of the sRGB gamut than you have calculated, if you
treat the data spectrally and then do the conversions.
My advice to conservationists is to aquire the ground truth data spectrally.
Avoid LAB at all costs. Apply the physical illuminant to the spectral data
and aquire the camera data under that illuminant. Build the camera profile
using XYZ as calculated under the illuminant in use. Note that the ICC
profile will insist that the data in XYZ space be chromatically adapted
using Bradford or some other specified adaptation. Great care was taken to
insure that the chromatic adaptation be invertible so this will not do harm
to the original data capture. The invertablity of Bradford insures that
nothing is lost on output.
So my advice:
1. train the camera under the illuminant used to view the art.
2. Measure the art or target spectrally.
3. Convert to XYZ space using the viewing illuminant applied to the spectral
data.
4. build the appropriate profile or calibration table using this data.
In this instance, you should avoid LAB like the plague. It is an indelicate
obscuration of the intent of the rendering.
Regards,
Tom Lianza.
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