private double Distance(LCH first, LCH second)
{
var lab1 = LCH.LCHToLab(first);
var lab2 = LCH.LCHToLab(second);
var deltaL = second.L - first.L;
var lRoof = (first.L + second.L) / 2;
var sl = 1 + 0.015 * Math.Pow(lRoof - 50, 2) / Math.Sqrt(20 + Math.Pow(lRoof - 50, 2));
var added1 = Math.Pow(deltaL / sl, 2);
var deltaC = second.C - first.C;
var cRoof = (first.C + second.C) / 2;
var sc = 1 + 0.045 * cRoof;
var added2 = Math.Pow(deltaC / sc, 2);
var smallDeltaH = 0.0;
var a1 = lab1.A + lab1.A * (1 - Math.Sqrt(Math.Pow(cRoof, 7) / (Math.Pow(cRoof, 7) + Math.Pow(25, 7))));
var a2 = lab2.A + lab2.A * (1 - Math.Sqrt(Math.Pow(cRoof, 7) / (Math.Pow(cRoof, 7) + Math.Pow(25, 7))));
var c1 = Math.Sqrt(Math.Pow(a1, 2) + Math.Pow(a2, 2));
var h1 = Math.Atan2(lab1.B, a1) % 360;
var h2 = Math.Atan2(lab2.B, a2) % 360;
if (Math.Abs(h1 - h2) <= 180)
{
smallDeltaH = h2 - h1;
}
else if (Math.Abs(h1 - h2) > 180 && h2 <= h1)
{
smallDeltaH = h2 - h1 + 360;
}
else if (Math.Abs(h1 - h2) > 180 && h2 > h1)
{
smallDeltaH = h2 - h1 - 360;
}
var deltaH = 2 * Math.Sqrt(first.C * second.C) * Math.Sin(smallDeltaH / 2);
var hRoof = 0.0;
if (Math.Abs(h1 - h2) <= 180)
{
hRoof = (h2 + h1) / 2;
}
else if (Math.Abs(h1 - h2) > 180 && h2 + h1 < 360)
{
hRoof = (h2 + h1 + 360) / 2;
}
else if (Math.Abs(h1 - h2) > 180 && h2 + h1 >= 360)
{
hRoof = (h2 + h1 - 360) / 2;
}
var t = 1 - 0.17 * Math.Cos(hRoof - 30) + 0.24 * Math.Cos(2 * hRoof)
+ 0.32 * Math.Cos(2 * hRoof + 6) - 0.2 * Math.Cos(4 * hRoof - 63);
var sh = 1 + 0.015 * cRoof * t;
var rt = -2 * Math.Sqrt(Math.Pow(cRoof, 7) / (Math.Pow(cRoof, 7) + Math.Pow(25, 7)))
* Math.Sin(60 * Math.Exp(-Math.Pow((hRoof - 275) / 25, 2)));
var added3 = Math.Pow(deltaH / sh, 2);
var added4 = rt * deltaC / sc * deltaH / sh;
return(Math.Sqrt(added1 + added2 + added3 + added4));
}
