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Re: [css4-color] Re: CIE color definitions in CSS3 color module

From: Alan Gresley <alan@css-class.com>
Date: Fri, 17 Sep 2010 14:08:43 +1000
Message-ID: <4C92E9CB.8080101@css-class.com>
To: Christoph Päper <christoph.paeper@crissov.de>
CC: "www-style@w3.org list" <www-style@w3.org>
Christoph Päper wrote:
> Alexis Shaw:
>> On 11 September 2010 21:49, Christoph Päper <christoph.paeper@crissov.de>wrote:
>>
>>>> xyz(x, y, z)
>> X, Y and Z colours are normalised as values so that Y is between 0 and 1 in
>> the ICC profile standard, and between 0 and 100 in the CIE standard. this
>> means that X and Z can be reasonably be in [0,2) and [0, 200) respectively.
> 
> Hm, I’d like them all to be positive percentages, like most other color components are or can be (excluding hue which is an <angle> and alpha which is a percentage but written as a float in [0, 1]); or X and Z could be translated to signed percentages (i.e. subtract 100).


Why percentages? Why all positive? Does the XYZ refer to XYZ color 
space or xy chromaticity space? How does one specify chromatic colors 
that exist in the triangle xy=(0,0), xy=(1,0) and xy=(-(1/3), 1(1/3))?


[snip]
>>> Is it important to have the whitepoint specified by the author or could CSS
>>> safely select one?
>>>
>>>> or a custom white point defined by an @whitepoint rule.
>>> What properties / descriptors would this have?
>>>
>> all white points can be defined as a color in the XYZ color space.
> 
> Then ‘@whitepoint’ would essentially be a way to specify a color and ‘white_point’ in ‘luv()’ and ‘lab()’ essentially is just a color?


There is no true white point except in RGB color space with cartesian 
coordinates in three dimensions, ie an x-y-z matrix [1]. The notion 
that there is a white point is as arbitrary as floating x, y, x 
points. One point can only have one true location. It is the where the 
floating x, y, x points converge from. In sRGA space, this is black. 
In XYZ color space, this is black.


1. <http://en.wikipedia.org/wiki/X-Y-Z_matrix>


-- 
Alan http://css-class.com/

Armies Cannot Stop An Idea Whose Time Has Come. - Victor Hugo
Received on Friday, 17 September 2010 04:09:14 GMT

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