Re: Camera Gamut (was Re: basiccolor INPUT)
Re: Camera Gamut (was Re: basiccolor INPUT)
- Subject: Re: Camera Gamut (was Re: basiccolor INPUT)
- From: dpascale <email@hidden>
- Date: Mon, 07 Nov 2011 20:45:04 -0500
Richard,
Your post made me look into an old email I sent to Mr. Berns in 2007
following the article you refer too.
Since these are my words and there is no confidential or personal content,
here is the letter:
-------------
"Dear Mr. Berns,
I read with curiosity your comment titled “Let’s Call It Color-Gamut
Rendering,”ref-1 which
caught my attention because it conveyed various concepts in my mind.
While I agree with the overall content of your comment, uneasiness remained
with the
expression. I found it was because of the word “rendering,” which triggers
my neurons
corresponding to “output”, especially when positioned as the last word.
I understand that a camera output is rendered, but, in this case, it could
be argued that the
rendering is effectively done before the gamut is generated. It then became
clear that the
end result of the measuring instrument is a “Color-Rendering Gamut,” i.e.
the gamut of the
rendered color.
This simple word inversion better conveys, at least in my neurons, the
underlying process.
It is also structured in the same way as other acronyms, such a CRI
(Color-Rendering
Index), CMF (Color-Matching Function), which are structured as
Noun-Verb-Noun.
Furthermore, this makes for a catchy acronym, CRG, with a similarity to CRI
(but likely
more useful), and a word sequence that reads like a quantity, while
“Color-Gamut
Rendering” reads like a method. As a test, I simply swapped the two
expressions in your
text, and saw no obvious disagreement, except that the revised expression
seemed to better
fit the concept. I also made a quick Web search and found no direct hit with
my revised
wording, although these three words are often used close together in the
same sentence; my
suggestion should thus not contradict another usage.
I concur with your suggestion to formalize the descriptive words for the
range of colors
measured by an instrument, and with your choice of words, but I would say
“Let’s Call It
Color-Rendering Gamut”.
(...)
1- COLOR research and application, Volume 32, Number 4, August 2007, p. 334"
-------------
In essence the same concept but looked with a different eye.
Danny
----- Original Message -----
From: "Richard Wagner" <email@hidden>
To: "ColorSync Forum" <email@hidden>
Sent: Monday, November 07, 2011 5:03 PM
Subject: Camera Gamut (was Re: basiccolor INPUT)
On Nov 6, 2011, at : Iliah Borg <email@hidden> wrote:
I think we have a little language problem here. To quote, "there is no
such thing as a camera, or scanner, gamut"
http://www.cis.rit.edu/research/mcsl2/outreach/faq.php?catnum=3#255
Thanks for the link, Iliah.
"there is no such thing as a camera, or scanner, gamut"
There are several interesting discussions of this postulate in the
literature. Unfortunately, the articles are expensive unless you have
access to a research library, so I've taken the liberty of summarizing two
of them for those without such access.
Berns (1) takes the above position and argues that the term ‘‘color gamut’’
historically has been associated with color output such as optimal color
stimuli and additive and subtractive imaging systems. He notes that from
both historical and current contexts, ‘‘color gamut’’ is associated with
producing color, not measuring color; that is, color gamuts are defined by
output devices, not input devices. If a camera is considered a measurement
device - not unlike a colorimeter or spectrophotometer - it cannot have a
"color gamut" as it is able to respond to all colors presented to it
(i.e., there is no such thing as an "out of gamut" color for the device).
Berns takes issue with the use of the expression "color gamut" in
scientific comparisons of digital cameras, and argues that "if a
measurement device has a color gamut, ... [then] a miscalibrated
spectrophotometer (calibrating to black instead of white) or camera with
lens cap attached have small color gamuts." Rather than use the term
"color gamut," he proposes the use of the expression, "color gamut
rendering."
"That is, under a set of stated conditions a target of colors has been
rendered to a set of colorimetric coordinates owing to various parameters.
The stated conditions might include an ensemble of spectral reflectance
factors obtained using a spectrophotometer with stated measurement
geometry (the target), the camera-taking light source, the camera’s
optical properties and methodology of obtaining colorimetric coordinates
if known, and the standard illuminant and observer for the colorimetric
data. Differences in input device characteristics would result in the
target’s color gamut being rendered differently."
Brill (2) urges the explicit adoption of a definition for the camera
color gamut. In his view, a viable definition seems to be the set of CIE
XYZ triplets such that, for any such XYZ, there exists a spectral power
distribution in a predefined set whose camera values map through a
predefined algorithm to that XYZ.
Brill notes that the term "color gamut" is problematic when applied to a
scanner or digital camera. However, he acknowledges that it has been used
in this context in various venues, including referred articles and
conferences. In such discussions, "the definition is always assumed and
never stated." Brill believes it is time to "make explicit what is
implicitly understood, or metaphorically, to spray-paint the invisible
definition."
Brill discusses alternative definitions: he finds it "hard to grasp what
could be a camera color gamut, because any light (visible or not) maps to
some point in the camera color space—even if it is to the origin of the
space. One could define a sensor (camera) gamut as the set of spectral
radiances to which the sensor responds, but that would equate ‘‘spectrum’’
to ‘‘color’’ and the gamut could not be characterized in a space of
reduced dimensionality such as a color space. Alternatively, a camera
gamut could be ‘‘the largest set of scene colors that produce unique
outputs,’’ but this definition also has difficulties." Brill notes that
the term ‘‘camera color gamut’’ is a misnomer, because what is being
discussed is not a property of just a camera. Rather, it is a property of
a set of spectral power distributions (A), a camera (B), and an algorithm
that maps camera values to CIE XYZs (C).
Brill ultimately proposes the following definition:
"Camera color analysis gamut:
Attribute of a set of spectral power distributions (SPDs) (A), a camera
(B), and an algorithm (C) that maps camera values to CIE XYZ triplets,
that attribute being the set of CIE XYZ triplets for each of which there
is an SPD in (A) that, when acquired by camera (B), produces camera values
that, when processed by algorithm (C), produces that XYZ triplet. The
gamut can be represented in a color space derived from XYZ triplets (such
as CIELAB), or in a reduced space such as chromaticity (x,y). (The
alternative term camera color gamut is also used). "
Brill closes with the following statement:
A definition of camera gamut is new, and needed. The definition I propose
here is now being discussed by representatives of several standards
bodies. The quality of the algorithm, or of the camera, or of the set of
SPDs, are valid subjects of research papers, but one must start with a
definition such as the one here so people know what is being discussed.
Both of these short papers are excellent and make points worth
understanding and remembering.
--Rich Wagner
===
(1)
Let's call it “color-gamut rendering”
COLOR RESEARCH & APPLICATION
Volume 32, Issue 4, August 2007, Pages: 334–335,
Roy S. Berns
(2)
Camera color gamut: Spray-painting the invisible definition;
COLOR RESEARCH & APPLICATION;
Volume 32; Issue 3; June 2007; Pages: 236–237;
Michael H. Brill
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