[R] grDevices::convertColor XYZ space is it really xyY?

[Ken Knoblauch]

If they sum to 1 then they are one and the same.  Look at how
chromaticity coordinates are defined in terms of the tristimulus
values.

Quoting Bryan Hanson <hanson at depauw.edu>:

> Thank you Ken.
>
> 90% of my problem was missing the factor of 100.  I was just   
> inputing xyY as a test, I wasn't sure whether the docs were being   
> clear about nomenclature.  Speaking thereof, the D65 values used in   
> the example: I thought they were chromaticity coordinates, but   
> apparently they are tristimulus values - is that correct?
>
> Thanks again.  This solves several problems in a package I am   
> developing.  Bryan
>
> On Jun 12, 2013, at 4:36 PM, Ken Knoblauch <ken.knoblauch at inserm.fr> wrote:
>
>> You seem to treating the input values as xyY when they should be XYZ
>> (case matters).
>> So, I would do something like this
>>
>> D65 <- c(0.3127, 0.329, 0.3583)
>>
>> X <- 100 * D65[1]
>> Y <- 100 * D65[2]
>> Z <- 100 * D65[3]
>> XYZ <- data.frame(X = X, Y = Y, Z = Z)
>>
>> convertColor(XYZ, from = "XYZ", to = "sRGB")
>>     [,1] [,2] [,3]
>> [1,]    1    1    1
>>
>>
>> Quoting Bryan Hanson <hanson at depauw.edu>:
>>
>>> Ken, I followed your suggestion and perhaps I don't understand   
>>> what  to expect from convertColor or maybe I'm not using it   
>>> correctly.   Consider the following tests:
>>>
>>> D65 <- c(0.3127, 0.329, 0.3583) # D65 chromaticity coordinates
>>> X <- D65[1]*D65[3]/D65[2] # conversion per brucelindbloom.com
>>> Y <- D65[3]
>>> Z <- D65[3]*D65[3]/D65[2]
>>> XYZ <- data.frame(X = X, Y = Y, Z = Z) # D65 in tristimulus values (?)
>>> colnames(XYZ) <- c("X", "Y", "Z")
>>>
>>> tst1 <- convertColor(XYZ, from = "XYZ", to = "sRGB")
>>> tst2 <- convertColor(D65, from = "XYZ", to = "sRGB")
>>> # none of these are 1,1,1, namely white (they are ~ 0.6, 0.6, 0.6)
>>>
>>> So it looks like D65, a white standard, does not come back to    
>>> something near white in the sRGB space.  What am I doing wrong   
>>> here,  or what do I misunderstand?  Please don't say everything!
>>>
>>> Thanks, Bryan
>>>
>>> On Jun 12, 2013, at 2:57 PM, Ken Knonlauch <ken.knoblauch at inserm.fr> wrote:
>>>
>>>> Bryan Hanson <hanson <at> depauw.edu> writes:
>>>>
>>>>>
>>>>> grDevices::convertColor has arguments 'from' and 'to' which can
>>>> take on value 'XYZ'.  Can
>>>> someone confirm
>>>>> that 'XYZ' is the same as the CIE chromaticity coordinates
>>>> are also sometimes refered to
>>>> as 'xyY' in
>>>>> the literature?  Or are these the CIE tristimulus values?
>>>> It looks to me like the first case is
>>>> true, but I
>>>>> would appreciate hearing from one of the people in
>>>> know.  Thanks, Bryan
>>>>>
>>>>>
>>>>
>>>> I.'d look at the code or put in some known data to test it,
>>>> but XYZ are tristimulus values and xyY are chromaticity
>>>> coordinated and the luminance which is the Y tristimulus
>>>> value for the CIE 1931 standard observer.
>>>>
>>>> Ken
>>>>
>>>> ______________________________________________
>>>> R-help at r-project.org mailing list
>>>> https://stat.ethz.ch/mailman/listinfo/r-help
>>>> PLEASE do read the posting guide   
>>>> http://www.R-project.org/posting-guide.html
>>>> and provide commented, minimal, self-contained, reproducible code.
>>>
>>>
>>
>>
>>
>> --
>> Kenneth Knoblauch
>> Inserm U846
>> Stem-cell and Brain Research Institute
>> Department of Integrative Neurosciences
>> 18 avenue du Doyen Lépine
>> 69500 Bron
>> France
>> tel: +33 (0)4 72 91 34 77
>> fax: +33 (0)4 72 91 34 61
>> portable: +33 (0)6 84 10 64 10
>> http://www.sbri.fr/members/kenneth-knoblauch.html
>>
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>>
>
>



-- 
Kenneth Knoblauch
Inserm U846
Stem-cell and Brain Research Institute
Department of Integrative Neurosciences
18 avenue du Doyen Lépine
69500 Bron
France
tel: +33 (0)4 72 91 34 77
fax: +33 (0)4 72 91 34 61
portable: +33 (0)6 84 10 64 10
http://www.sbri.fr/members/kenneth-knoblauch.html