RE: Humans (and cameras and scanners) do not have a color gamut (?)
RE: Humans (and cameras and scanners) do not have a color gamut (?)
- Subject: RE: Humans (and cameras and scanners) do not have a color gamut (?)
- From: Wayne Bretl via colorsync-users <email@hidden>
- Date: Wed, 8 Jan 2020 15:01:55 -0700
" That matrix defines the relation between the camera observer and
colorimetric observer."
That is not quite correct. The relationship between the camera observer and
colorimetric observer is defined by the concatenation of the sensor spectral
response and the matrix. On the input side of the sensor, it is a
multidimensional problem with the number of dimensions determined by the
number of significantly different wavelength response points. I like to
think of it as related to sampling theory. If the spectra of the light
source, the object reflectivity, and the sensor responses are all broad and
smooth, almost any case can be matrixed to give a close approximation to a
colorimetric camera. This also holds true pretty well if only one of the
three items' (illumination, object, sensor) spectra is narrow or spiky. So,
for example, you can get a pretty good color rendering index with a lamp
spectrum that is not really much like daylight or incandescent. But mix that
with a sensor that has narrow responses, and all bets are off - for example,
note the greenish rendition obtained with old-fashioned office fluorescent
lamps and color film, and the still-off rendition of various objects when
the white balance was corrected with a magenta filter.
Another example: have you heard of the selected ordinate method of computing
CIE tri-stimulus values? This was a method in which, instead of uniformly
spaced and variously weighted spectral samples, non-uniformly spaced and
unity-weighted samples were used. Quite a simplification in the days before
computers, and it gave approximate but good enough results when applied to
broad-spectrum objects under broad spectrum illumination. In this case, each
"sensor" channel had a response at only 30 single wavelengths and zero
response elsewhere. Plus, there was an abbreviated version that used only 10
discrete wavelengths!
A combination that does not work well is a sensor with narrow responses with
an object with narrow spectrum as well. Such highly saturated object colors
will experience an error quite different and greater than that of
broader-spectrum objects, as show by the spectrum loci in Holm's paper and
the gamut-interior plots in my paper. In those plots, a matrix was applied
that produced a decent colorimetric correction for test chart patch spectra,
but the narrower spectra show huge errors.
-----Original Message-----
From: Iliah Borg <email@hidden>
Sent: Wednesday, January 08, 2020 2:36 PM
To: Wayne Bretl <email@hidden>
Cc: 'Andrew Rodney via colorsync-users' <email@hidden>
Subject: Re: Humans (and cameras and scanners) do not have a color gamut (?)
> See Jack Holm's paper ... where he derives the spectral locus of a
> combination of a sensor's outputs and a subsequent matrix
I've read that paper when it was first published on the web ;) It doesn't
define sensor gamut, or gamut of sensor outputs.
To quote,
"This paper reports on the observed characteristics of the capture color
analysis gamuts resulting from a number of capture devices/media, and scene
analysis color matrices."
The result depends on the matrix. Strongly depends, as the paper itself
demonstrates. That matrix defines the relation between the camera observer
and colorimetric observer.
On Jan 8, 2020, at 4:21 PM, Wayne Bretl via colorsync-users wrote:
> Arrgh - addressing problems - went only to iliah and not the whole
> list as intended
>
> -----Original Message-----
> From: Wayne Bretl <email@hidden>
> Sent: Wednesday, January 08, 2020 2:13 PM
> To: 'Iliah Borg' <email@hidden>
> Subject: RE: Humans (and cameras and scanners) do not have a color
> gamut (?)
>
> See Jack Holm's paper, which I posted above, where he derives the
> spectral locus of a combination of a sensor's outputs and a subsequent
matrix:
> http://www.color.org/documents/CaptureColorAnalysisGamuts.pdf
>
> Although he does not show the sensor outputs themselves, they have
> reached their extreme possible ratios when the stimulus is a single
> wavelength, and therefore a pseudo chromaticity chart could be plotted
> using the ratios of one channel to the sum of the three on one axis
> and a ratio of a second channel to the sum on the orthogonal axis.
> This chart would have a defined gamut of sensor values (inside the
> spectrum locus) but would not represent colors per se until processed
through a matrix.
>
> Holm shows the resulting chromaticities for multiple sensors and
> several reasonable matrices. He includes cases for motion picture
> negative film that clearly show a triangular gamut limit (spectral locus).
>
> -----Original Message-----
> From: Iliah Borg <email@hidden>
> Sent: Wednesday, January 08, 2020 1:59 PM
> To: Wayne Bretl <email@hidden>
> Cc: Andrew Rodney <email@hidden>; Andrew Rodney via
> colorsync-users <email@hidden>
> Subject: Re: Humans (and cameras and scanners) do not have a color
> gamut (?)
>
>
> On Jan 8, 2020, at 3:52 PM, Wayne Bretl via colorsync-users wrote:
>> .
>> The discussion has moved beyond the true but uninformative statement
>> that cameras (sensors to be precise) do not have a color gamut , to
>> considering how the gamut of sensor outputs
>
> I would very much like a definition here. What is the gamut of sensor
> outputs?
>
> And of course, how to measure it?
>
> --
> Best regards,
> Iliah Borg
> LibRaw, LLC
> www.libraw.org
> www.rawdigger.com
> www.fastrawviewer.com
>
>
>
>
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--
Best regards,
Iliah Borg
LibRaw, LLC
www.libraw.org
www.rawdigger.com
www.fastrawviewer.com
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