Am 30.11.2016 um 09:15 schrieb scott:
>> sRGB does /not/ explicitly specify what full red, green and blue are.
>>
>> What sRGB does define is the _xy coordinates_ - i.e. the absolute hue
>> and saturation - of red, green and blue (aka the "primaries"). This can
>> be visualized as defining the direction (but not the "length") of the
>> red, green and blue axes in XYZ space.
>>
>> sRGB also /does/ explicitly define the _xy coordinates_ of the so-called
>> "illuminant whitepoint" - i.e. which defines what colours are nominally
>> "neutral", i.e. entirely desaturated - and also defines that such
>> neutral colours are to be represented by the red, green and blue
>> channels all set to the same value (which is typical for RGB colour
>> models). This can be visualized as defining the direction (but again not
>> the "length") of the RGB "cube"'s diagonal in XYZ space.
>>
>> sRGB also /does/ explicitly define the _Y coordinate_ - i.e. the
>> luminance - of the brightest representable colour: 80 cd/m^2; however,
>> not everyone adheres to this.
> 
> The exact definition may not be as I simplified to, but the point is the
> same. The "output" of the spec is that XYZ for each of R,G,B are numbers
> that are always the same, there is no dependence on some externally
> provided white point information.

You're right in that respect. The above information was mainly for the
records.

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Am 29.11.2016 um 18:08 schrieb Mike Horvath:
> On 11/29/2016 11:42 AM, clipka wrote:
>> It should be noted however that such a plot is non-trivial: It does
>> /not/ suffice to specify the xy coordinates of the illuminant; the
>> entire colour spectrum needs to be considered instead. For example with
>> illuminants from the F series (fluorescent lighting) we can probably
>> expect the 3D shape to show a distinctive "fingerprint" of the
>> illuminant's spectral emission lines (though I'm not sure how exactly
>> this fingerprint would look like).
>>
> 
> Okay, you've convinced me to give up!
> 
> I like to code but suck at math.
> 
> ;)

You've made me curious though, so see povray.binaries.animations for a
series of animated CIE xyY gamuts of various illuminants.

Using meshes instead of isosurfaces, because the latter would have been
virtually impossible to manage.

Even then it took me several attempts before I knew how the problem
could be tackled.


I was rather surprised at first to see that the shape isn't convex,
until I realized that it would only be convex in CIE XYZ space. But then
it becomes rather boring, and its relation to the xy horseshoe less obvious.

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