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: Iliah Borg via colorsync-users <email@hidden>
- Date: Wed, 8 Jan 2020 17:20:41 -0500
Dear Wayne,
I'm afraid we are going nowhere with this. Gamut is a certain subset of
colours. No colour output is available from the sensor until conversion. The
sensor output and the colour rendering are in different physical "units".
We even have different understanding what a camera observer is, what I mean
saying camera observer is quite parallel to what a colorimetric observer is.
I'm well past the idea of using sensors with narrow responses in photographic
cameras. Even for 5 / 7-capture monochrome cameras we still use broad responses
of the filters in front of the lens.
Still, we use LUTs, not matrices, to cope with some issues introduced by
matrices.
On Jan 8, 2020, at 5:01 PM, Wayne Bretl wrote:
> " 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
>
>
>
>
--
Best regards,
Iliah Borg
LibRaw, LLC
www.libraw.org
www.rawdigger.com
www.fastrawviewer.com
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