From kde-devel  Thu Jun 26 22:49:23 2008
From: Matthew Woehlke <mw_triad () users ! sourceforge ! net>
Date: Thu, 26 Jun 2008 22:49:23 +0000
To: kde-devel
Subject: Re: Color Palette Nits
Message-Id: <g416dj$hkp$1 () ger ! gmane ! org>
X-MARC-Message: https://marc.info/?l=kde-devel&m=121452065111527

Percy Camilo Triveņo Aucahuasi wrote:
> Matthew Woehlke wrote:
>> Aside from David's comment (which is right), what is "luminance"? :-)
>> KColorUtils::luma returns the 'apparent brightness' of a color (white is
>> 1.0, black is 0.0, anything else is somewhere in-between). More
>> technically, it returns the Y' component of the color's H'C'Y'
>> colorspace representation.
> 
> As far I know, luminance is apply in colors to obtain a value without gamma
> correction, instead luma that apply gamma correction to every -RGB-
> component of the color.

What I meant was, despite there being a technically correct definition, 
the term is sufficiently abused as to be somewhat confusing. Hence the :-).

>> At the very least, the above is redundant with qGray - KCU::luma uses gamma
> correction as well, which matches with how KCS handles >light/dark stuff.
> 
> In short:
> Luma:
> Y' = 0.2126 R' + 0.7152 G' + 0.0722 B'
> Luminance:
> Y = 0.2126 R + 0.7152 G + 0.0722 B
> 
> KColorUtils only have a luma fuction ... luminance maybe could be this:
> KHCY::lumag in KColorSpaces.

I think non-gamma-corrected is close enough to qGray that we don't 
really need it. Besides, the point of using H'C'Y' in the first place 
was that it was the color space being called for from the Usability 
side, i.e. we would prefer for people to use it instead of whatever else 
:-). (As you may recall, there was some rather violent disagreement with 
that at the time...)

(lumag is intentionally private, it's used as a helper in conversion, 
but it's not meant to be used outside of KHCY. And KColorSpaces itself 
is private outside of kdeui; currently it's only used as the back-end 
for KColorUtils, although at some point I want the standard color picker 
to use HCY as well.)

-- 
Matthew
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