Am 20.11.2016 um 04:47 schrieb Mike Horvath:
> I'm having a problem converting between units. Here is the code I'm
> trying to adapt to POV-Ray:
> 
>    
https://github.com/THEjoezack/ColorMine/blob/master/ColorMine/ColorSpaces/Conversions/LchConverter.cs
> 
> 
>         internal static IRgb ToColor(ILch item)
>         {
>             var hRadians = item.H * Math.PI / 180.0;
>             var lab = new Lab
>                 {
>                     L = item.L,
>                     A = Math.Cos(hRadians) * item.C,
>                     B = Math.Sin(hRadians) * item.C
>                 };
>             return lab.To<Rgb>();
>         }
...

> 
> Here is the POV-Ray version I created:
> 
>     // input L = between 0 and 100
>     // input C = between 0 and 100
>     // input H = between 0 and 360
>     // output L = between 0 and 100
>     // output A = between -128 and +128
>     // output B = between -128 and +128
>     #macro CLCH2LAB(Color)
>         #local LCHFT = color Color;
>         #local L = LCHFT.red;
>         #local C = LCHFT.green;
>         #local H = LCHFT.blue;
>         #local hRadians = radians(H);
>         #local A = cos(hRadians) * C;
>         #local B = sin(hRadians) * C;
>         <L,A,B>
>     #end
> 
> When I plug the vector <50,50,180> into this function I get
> <50.00000,-50.00000,0.00000> as a result. However, the Web converter
> (http://colormine.org/convert/lch-to-lab) says it should be
> <51.622535970468874,-37.18943330724933,2.558221480506817>
> 
> It's a simple function. I don't see anything obvious that is wrong with
> it. Do you?

I see something obviously wrong with the web converter: the "CIE-L*ab"
(whatever colour space that even is supposed to be; L*a*b, maybe?)
result it computes cannot possibly be the same as the "lab" interim
result computed by the `ToColor` function you've posted, as that leaves
the "L" component entirely untouched, so it should evaluate to 50.

Judging from the results the web converter computes for Lch to XYZ,
which gives out-of-range values for an obviously within-range Lch value
of <50,50,180>, that web converter can't be trusted.

The first other web converter I've tested does convert "Lch" <50,50,180>
to "Lab" <50,-50,0>, matching your implementation.

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