Talk:Chromaticity

message: This should really be merged into Saturation: Colour theory, this would end a stub, and join it to its "father" article. It would be better if it was a "sub-chapter" to Saturation: Colour theory.
I'm not sure I agree.  I'd like to include a bit about the meaning of chromaticity in accelerator physics (specifically in storage rings), and that wouldn't fit into Saturation: Colour theory at all.  Unless a separate page was created...
SteveM 09:06, 24 July 2006 (UTC)
replies: text: Strongly oppose. Chromaticity encapsulates saturation and hue. It should stay here so we can discuss chromaticity diagrams, and perhaps perceptual uniformity. --Adoniscik (talk) 16:06, 4 February 2008 (UTC)
replies: text: This 2006 is proposal is no longer active (no tags are on the articles). Dicklyon (talk) 16:11, 4 February 2008 (UTC)
thread_title:  merge with saturation? 

message: In the section "In color science," the article currently states, "the chromaticity coordinates are a* and b* in CIELAB, u and v in CIELUV...." Firstly, the CIE has not, to my knowledge, sanctioned or promulgated chromaticity coordinates for the CIELAB color space (if it did, chances are they would be something like a*/L* and b*/L*, which is essentially how it works in CIECAM02). a* and b* do not fit the definition of chromaticity coordinates. Secondly, the chromaticity coordinates in CIELUV are u', v', not u, v, as the article states. Accordingly, I am removing the reference to the CIELAB color space, and correcting the reference to the second. Lovibond (talk) 01:28, 19 January 2013 (UTC)
replies: text: On second thought, more corrections are necessary, as CIELUV needs to be moved to the second paragraph. Lovibond (talk) 01:31, 19 January 2013 (UTC)

text: True, a* and b* are not exactly chromaticity; a*/L* and b*/L* would not be either; you can't get there from here.  On the other hand, a* and b* function sort of like chromaticity, and I wouldn't be surprised if some sources refer to them as such.  If we mention them, it should be in the context of a source and an explanation.  I haven't looked to see what it said, but I probably have objection to taking them out.  Dicklyon (talk) 01:33, 19 January 2013 (UTC)
replies: text: "a* and b* are not exactly chromaticity" is an understatement! They're not exactly chromaticity in the same sense that C* is not exactly excitation purity. As far as a*/L* and b*/L*, they do behave much more like chromaticity than a* and b*. Saturation is computed as a distance in chromaticity space, and is defined as colorfulness relative to brightness. And colorfulness is to chroma (which may be resolved into two components, a* and b*) as brightness is to lightness. If a source refers to a* and b* as chromaticity coordinates, it is contrary to the definitions in the International Lighting Vocabulary, CIE Publication 15, Fairchild, Hunt, and other authorities, and therefore of questionable reliability on at least that point. Lovibond (talk) 02:13, 22 January 2013 (UTC)

text: Good call. I don’t like the section header “in color science” either. It would be nice to first explain that “chromaticity” technically describes a 2-dimensional quantity which is 1–1 related to relative cone responses (or approximations thereof, or linear combinations of those, such as xy, u'v', etc.), under an assumption of linearity (Grassmann’s Law). Then it would be nice to show some chromaticity diagram where relative cone responses are the axes, and a few other chromaticity diagrams, explaining how they arose, explain the advantages (mainly linearity of light combinations?) and down-sides (perceptual non-uniformity [macadam diagrams], hue shift along straight lines, inconsistency between hue purity and perceived chroma/colorfulness, etc.) to using those as color models. It might be a good idea to show how instead of x, y we can write a chromaticity in terms of "dominant wavelength" and "hue purity". It might be nice to show some other diagrams that can be easily drawn on a chromaticity diagram, such as the gamut of diffuse-reflective non-fluorescing surface colors under some specific light source. Then finally it would be nice to contrast with other models which separate luminance/lightness from color in different ways, ideally with some diagrams showing 3-dimensional gamuts, starting with CIELUV, but also maybe CIELAB, Munsell, and e.g. CIECAM02. Then again, writing all that, sourcing it, &c. &c. would be a big bunch of work, and I dunno if anyone has time to do it. :-) –jacobolus (t) 02:20, 19 January 2013 (UTC)
replies: text: I'm not sure what you mean by "1–1 related to relative cone responses (or approximations thereof, or linear combinations of those, such as xy, u'v', etc.)"; that's pretty squishy.  It's not 1–1 with the tristimulus, obviously, but it's 1–1 with some kind of 2D projection; not clear that "relative cone responses (or ...)" captures the essense of that.  Depending on how one defines it, a* b* might or might not be a chromaticity.  What do you call these mappings that are linear in numerator and denominator?  Projections?  Projections through the origin?  Dicklyon (talk) 03:41, 19 January 2013 (UTC)
replies: text:  The standard term for this construct in mathematics is projectivization. Coordinates with include denominator are called "affine coordinates" or "non-homogeneous coordinates" on a projective plane. Incnis Mrsi (talk) 06:42, 19 January 2013 (UTC)

text:  I shouldn’t have said “relative cone responses”; I’m not sure what the clearest phrasing is. What I meant though was that chromaticity describes cone responses for a particular luminosity. Saying that it’s some linear transformation of a projection of cone responses onto a plane would maybe be better. I don’t think a* or b* (or u*, v*) is ever really “chromaticity”. –jacobolus (t) 04:32, 22 January 2013 (UTC)
thread_title:  Chromaticity in color spaces 

message: Substituted at 11:43, 29 April 2016 (UTC)
thread_title: Assessment comment

message: It seems like the wikilink to luminance is not quite right. Would a link to Lightness be more appropriate since that is the article dealing with lightness in color?   The luminance article is mostly about a measure of light intensity as opposed to about color.  Thoughts?
Jazzlw (talk) 20:16, 13 December 2016 (UTC)
thread_title:  Luminance vs Lightness