Thread: Re: [Lcms-user] XYZ to RGB Conversion

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I apparently am not setting the illuminant and observer. I thought I was 
but, in the larger code I have routines that convert Lab to RGB and I 
thought I was changing the observer and illuminant for this as well 
(different profile).  The cmsCreateLabProfile allows me to set the white 
point,, the cmsCreateXYZProfile doen't have an argument for the white 
point.  Excuse my lack of experience here, but how do I set the illuminant 
and observer.

-Bob

"Case, Isaac" <Isa...@xe...> 
09/30/2009 10:30 AM

To
Robert V Canning/AE/DuPont@DuPont
cc

Subject
RE: [Lcms-user] XYZ to RGB Conversion

Where/how are you setting the illuminant and observer?

-Isaac
From: Robert V Canning [mailto:Rob...@us...] 
Sent: Wednesday, September 30, 2009 10:23 AM
To: Case, Isaac
Subject: RE: [Lcms-user] XYZ to RGB Conversion

Thanks Isaac, that was the problem, I forgot the divide by 100.  The 
conversion seems to be in D50, I set the white point to D65 at 10 degrees 
but the results always seem to be in D50 space 2 degree.  I am comparing 
it to Lindblooms calculator and I get the D50 answer not D65.  Any clue? 

Thanks again, 

Bob 

"Case, Isaac" <Isa...@xe...> 
09/30/2009 09:50 AM 

To
Robert V Canning/AE/DuPont@DuPont, <lcm...@li...> 
cc

Subject
RE: [Lcms-user] XYZ to RGB Conversion

What range of values are you passing in as XYZ. I just tried and got 

X:0.5 Y:0.5 Z:0.5 -> R:190 G:185 B:207 

However, if you are expecting a range of 
X from 0 to  95.047 
Y from 0 to 100.000 
Z from 0 to 108.883 

You’ll need to divide by 100 for everything. 

-Isaac Case 

From: Robert V Canning [mailto:Rob...@us...] 
Sent: Wednesday, September 30, 2009 8:11 AM
To: lcm...@li...
Subject: [Lcms-user] XYZ to RGB Conversion 

Hi, 

       I want to be able to convert an XYZ value to RGB.  I took the 
following steps:

- created a XYZ profile used as input. 
- created a sRGB profile used as output 
- intent is set to perceptual 
- created a transform 
- loaded the XYZ values 
- did the transform 

The problem is no matter what the XYZ value I always get 255 for the RGB 
values.  What am I doing wrong? 

Thanks for any help. 

void XYZtoRGB(double X, double Y, double Z, unsigned char &red, unsigned 
char &green, unsigned char &blue) 
       cmsCIEXYZ XYZ; 
       BYTE output[3]; 

       XYZ.X = X; 
       XYZ.Y = Y; 
       XYZ.Z = Z; 

       cmsHTRANSFORM hXYZtoRGBTransform; 
       cmsHPROFILE hXYZProfile; 
       cmsHPROFILE hsRGBProfile; 
       hXYZProfile = cmsCreateXYZProfile(); 
       hsRGBProfile = cmsCreate_sRGBProfile(); 
       // Rendering intent is perceptual 
       hXYZtoRGBTransform = cmsCreateTransform(hXYZProfile, TYPE_XYZ_DBL, 
                               hsRGBProfile, TYPE_RGB_8, 
                               0, 
                                cmsFLAGS_NOTPRECALC); 

       cmsDoTransform(hXYZtoRGBTransform, &XYZ, output, 1); 

       red = output[0]; 
       green = output[1]; 
       blue = output[2]; 
       cmsDeleteTransform(hXYZtoRGBTransform); 
       cmsCloseProfile(hXYZProfile); 
       cmsCloseProfile(hsRGBProfile);         
} 

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Thread: Re: [Lcms-user] XYZ to RGB Conversion

An ICC-based CMM for color management

lcms-user