The Adobe RGB 1998 specification
(http://www.adobe.com/digitalimag/pdfs/AdobeRGB1998.pdf) states that the
unadapted primaries are:
Red x=0.6400, y=0.3300
Green x=0.2100, y=0.7100
Blue x=0.1500, y=0.0600
White x=0.3127, y=0.3290
Further down, the specifications give the hexadecimal values for the
profile colorants and (V2) white point tag:
The rXYZ tag shall be set to 9c18, 348d, 2631
The gXYZ tag shall be set to 4fa5, a02c, 102f
The bXYZ tag shall be set to 04fc, 0f95, be9c
The wtpt tag shall be set to f351, 10000, 116cc
Converting from hexadecimal to decimal (openoffice calc HEXTODEC),
dividing by 65536, and rounding to 8 decimal places gives:
rXYZ 0.60974121 0.20527649 0.14918518
GXYZ 0.31111145 0.62567139 0.06321716
bXYZ 0.01947021 0.06086731 0.74456787
wtpt 0.95045471 1.00000000 1.08905029
Using iccToXML (which gives values to 8 decimal places), these are the
exact values found in Argyllcms's ClayRGB1998.icm and Adobe's
AdobeRGB1998.icc profiles.
Using LCMS2, the unadapted primaries and source white point values from
the Adobe RGB specification together produce a profile with the wrong
blue colorant Z value, being equal to 0.74455261 instead of 0.74456787.
I calculated the D65 to D50 Bradford matrix and used it to
back-calculate "prequantized" primaries from the ClayRGB profile
colorant tags.
Using LCMS2, the prequantized primaries plus the Adobe-specified white
point values (x=0.3127, y=0.3290) produce a profile with colorants that
exactly match the Adobe/ClayRGB1998 profiles.
Here's the code, if anyone wants to check my results:
cmsCIExyYTRIPLE adobe_primaries = {
{0.6400, 0.3300, 1.0},
{0.2100, 0.7100, 1.0},
{0.1500, 0.0600, 1.0}
};
cmsCIExyYTRIPLE adobe_primaries_prequantized = {
{0.639996511, 0.329996864, 1.0},
{0.210005295, 0.710004866, 1.0},
{0.149997606, 0.060003644, 1.0}
};
cmsCIExyY d65_adobe_specs = {0.3127, 0.3290, 1.0};
cmsToneCurve* gamma220[3];
gamma220[0] = gamma220[1] = gamma220[2] = cmsBuildGamma (NULL, 2.19921875);
tone_curve[0] = tone_curve[1] = tone_curve[2] = gamma220[0];
profile = cmsCreateRGBProfile ( &d65_adobe_specs, &adobe_primaries,
tone_curve );
filename = "Test-not-prequantized.icc";
cmsSaveProfileToFile(profile, filename);
profile = cmsCreateRGBProfile ( &d65_adobe_specs,
&adobe_primaries_prequantized, tone_curve );
filename = "Test-prequantized.icc";
cmsSaveProfileToFile(profile, filename);
The reason for the discrepency between using the regular primaries and
the prequantized primaries seems to be hexadecimal quantization, which
affects the illuminant values and the colorant, chromatic adaptation,
and white point tags.
The Argyllcms profile making code has hexadecimal quantization
compensation. Is there such code in LCMS2 that I've overlooked? If not,
could it be added?
Regards,
Elle Stone
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