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Random Color Generator

, 5 Oct 2011
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Eric,Thank...
Eric,
 
Thanks for implementing my color distance code. Since you are using CIE Lab color space from Guillaume's library, I had another look at the RGB <> CIEXYZ <> CIELab formulas. They are not quite correct. In your project, you will not see a big difference. However, if you will use the lib for other purposes, you'll see a BIG difference.
 
Further, Guillaume's library uses sRGB with D50 reference white, like all other copy/paste solutions. If you would compare the conversion with PhotoShop, you'll be surprised that the results do not match. Adobe uses RGB Bradford adapted with D65 reference white.
 
If you want to use RGB Bradford adapted CIELab conversion, do the following:
 
In CIEXYZ class, add:
public static readonly CIEXYZ D50 = new CIEXYZ(0.96422, 1.00000, 0.82521);
 
In RGBtoXYZ method change matrix constants to:
0.4360747  0.3850649  0.1430804
0.2225045  0.7168786  0.0606169
0.0139322  0.0971045  0.7141733
 
In XYZtoRGB method, change matrix constants to (watchout for +/- signs):
3.1338561 -1.6168667 -0.4906146
-0.9787684  1.9161415  0.0334540
 0.0719453 -0.2289914  1.4052427
 
In XYZtoLab method:
change D65 to D50
 
In LabtoXYZ method:
change D65 to D50
 
Here are the corrections for sRGB with D65 reference white:
 
public static CIEXYZ RGBtoXYZ(int red, int green, int blue)
{		
	// normalize red, green, blue values
	double rLinear = (double)red/255.0;
	double gLinear = (double)green/255.0;
	double bLinear = (double)blue/255.0;
 
	// convert to a sRGB form
	// corrected 2.2 to 2.4 (Rob)
	double r = (rLinear > 0.04045)? Math.Pow((rLinear + 0.055)/(1 + 0.055), 2.4) : (rLinear/12.92) ;
	double g = (gLinear > 0.04045)? Math.Pow((gLinear + 0.055)/(1 + 0.055), 2.4) : (gLinear/12.92) ;
	double b = (bLinear > 0.04045)? Math.Pow((bLinear + 0.055)/(1 + 0.055), 2.4) : (bLinear/12.92) ;
 
	// converts using sRGB Working Space Matrix and D65 Reference White
	// expanded constants (Rob)
	return new CIEXYZ(
		(r*0.4124564 + g*0.3575761 + b*0.1804375),
		(r*0.2126729 + g*0.7151522 + b*0.0721750),
		(r*0.0193339 + g*0.1191920 + b*0.9503041)
		);
}
 
public static RGB XYZtoRGB(double x, double y, double z)
{
	// using sRGB Working Space Matrix and D65 Reference White
	// expanded constants (Rob)
	double[] Clinear = new double[3];
	Clinear[0] = x*3.2404542 - y*1.5371385 - z*0.4985314; // red
	Clinear[1] = -x*0.9692660 + y*1.8760108 - z*0.0415560; // green
	Clinear[2] = x*0.0556434 - y*0.2040259 + z*1.0572252; // blue

	for(int i=0; i<3; i++)
	{
		Clinear[i] = (Clinear[i]<=0.0031308)? 12.92*Clinear[i] : (1+0.055)* Math.Pow(Clinear[i], (1.0/2.4)) - 0.055;
	}
 
	return new RGB(
		Convert.ToInt32( Double.Parse(String.Format("{0:0.00}", Clinear[0]*255.0)) ), 
		Convert.ToInt32( Double.Parse(String.Format("{0:0.00}", Clinear[1]*255.0)) ), 
		Convert.ToInt32( Double.Parse(String.Format("{0:0.00}", Clinear[2]*255.0)) )
		);
}
 
Happy coding.
 
Rob

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This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

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Rob2412

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GeneralReason for my vote of 5 Rob seems to really know the subject... PinmemberEric Ouellet4-Oct-11 9:34 

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