Hi All,
I am having an issue with little CMS built in Lab profiles that I have a hard
time understanding. When I try to represent the Gamut of my display from its
profile by using the API in different Lab spaces created with two different
white points (in this case D50 and D65), I would expect to observe some
differences.For instance, The White point (1.0, 1.0, 1.0) could not be
converted to Lab (100, 0.0, 0.0) in both spaces, as the reference illuminants
used for the conversions are different.
I wrote the code sample below to enlighten my confusion:
/* hProfile is a display profile read from a validated Icc profile
beforehand*/
cmsHPROFILE labD50, labD65;
labD50 = cmsCreateLab4Profile(cmsD50_xyY());
cmsCIExyY white; cmsCIEXYZ white_xyz; white_xyz.X = 0.95047;
white_xyz.Y = 1.00000; white_xyz.Z = 1.08883; //D65
cmsXYZ2xyY(&white,&white_xyz); labD65 = cmsCreateLab4Profile(&white);
cmsHTRANSFORM TD50 = cmsCreateTransform( hProfile,
TYPE_RGB_DBL, labD50, TYPE_Lab_DBL,
INTENT_ABSOLUTE_COLORIMETRIC, // same with relative 0 );
cmsHTRANSFORM TD65 = cmsCreateTransform( hProfile, TYPE_RGB_DBL,
labD65, TYPE_Lab_DBL, INTENT_ABSOLUTE_COLORIMETRIC, // same
with relative 0 );
const int SIZE = 64; int Errcount = 0; double sum = 0.0; for
(int r = 0; r < SIZE; ++r) for (int g = 0; g < SIZE; ++g)
for (int b = 0; b < SIZE; ++b) {
cmsCIELab LD50, LD65; double RGB[] = {
double(r)/double(SIZE-1), double(g)/double(SIZE-1),
double(b)/double(SIZE-1) };
cmsDoTransform(TD50,RGB,&LD50,1);
cmsDoTransform(TD65,RGB,&LD65,1); sum += abs(LD50.L -
LD65.L) + abs(LD50.a - LD65.a) + abs(LD50.b - LD65.b); if
(LD50.L != LD65.L || LD50.a != LD65.a || LD50.b != LD65.b) {
Errcount++; } }
As a result, absolutely no difference has been observed.
Could someone please give some more precision about those built in profiles?
Did I misunderstand something?
Best regards,
Vincent. ------------------------------------------------------------------------------
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