// Get a black point of output CMYK profile, discounting any ink-limiting embedded
// in the profile. For doing that, we use perceptual intent in input direction:
// Lab (0, 0, 0) -> [Perceptual] Profile -> CMYK -> [Rel. colorimetric] Profile -> Lab
static
cmsBool BlackPointUsingPerceptualBlack(cmsCIEXYZ* BlackPoint, cmsHPROFILE hProfile)
{
cmsHTRANSFORM hRoundTrip;
cmsCIELab LabIn, LabOut;
cmsCIEXYZ BlackXYZ;
// Is the intent supported by the profile?
if (!cmsIsIntentSupported(hProfile, INTENT_PERCEPTUAL, LCMS_USED_AS_INPUT)) {
BlackPoint -> X = BlackPoint ->Y = BlackPoint -> Z = 0.0;
return TRUE;
}
hRoundTrip = CreateRoundtripXForm(hProfile, INTENT_PERCEPTUAL);
if (hRoundTrip == NULL) {
BlackPoint -> X = BlackPoint ->Y = BlackPoint -> Z = 0.0;
return FALSE;
}
LabIn.L = LabIn.a = LabIn.b = 0;
cmsDoTransform(hRoundTrip, &LabIn, &LabOut, 1);
// Clip Lab to reasonable limits
if (LabOut.L > 50) LabOut.L = 50;
LabOut.a = LabOut.b = 0;
cmsDeleteTransform(hRoundTrip);
// Convert it to XYZ
cmsLab2XYZ(NULL, &BlackXYZ, &LabOut);
if (BlackPoint != NULL)
*BlackPoint = BlackXYZ;
return TRUE;
}
// Calculates the black point of a destination profile.
// This algorithm comes from the Adobe paper disclosing its black point compensation method.
cmsBool CMSEXPORT cmsDetectDestinationBlackPoint(cmsCIEXYZ* BlackPoint, cmsHPROFILE hProfile, cmsUInt32Number Intent, cmsUInt32Number dwFlags)
{
cmsColorSpaceSignature ColorSpace;
cmsHTRANSFORM hRoundTrip = NULL;
cmsCIELab InitialLab, destLab, Lab;
cmsFloat64Number inRamp[256], outRamp[256];
cmsFloat64Number MinL, MaxL;
cmsBool NearlyStraightMidrange = TRUE;
cmsFloat64Number yRamp[256];
cmsFloat64Number x[256], y[256];
cmsFloat64Number lo, hi;
int n, l;
cmsProfileClassSignature devClass;
// Make sure the device class is adequate
devClass = cmsGetDeviceClass(hProfile);
if (devClass == cmsSigLinkClass ||
devClass == cmsSigAbstractClass ||
devClass == cmsSigNamedColorClass) {
BlackPoint -> X = BlackPoint ->Y = BlackPoint -> Z = 0.0;
return FALSE;
}
// Make sure intent is adequate
if (Intent != INTENT_PERCEPTUAL &&
Intent != INTENT_RELATIVE_COLORIMETRIC &&
Intent != INTENT_SATURATION) {
BlackPoint -> X = BlackPoint ->Y = BlackPoint -> Z = 0.0;
return FALSE;
}
// v4 + perceptual & saturation intents does have its own black point, and it is
// well specified enough to use it. Black point tag is deprecated in V4.
if ((cmsGetEncodedICCversion(hProfile) >= 0x4000000) &&
(Intent == INTENT_PERCEPTUAL || Intent == INTENT_SATURATION)) {
// Matrix shaper share MRC & perceptual intents
if (cmsIsMatrixShaper(hProfile))
return BlackPointAsDarkerColorant(hProfile, INTENT_RELATIVE_COLORIMETRIC, BlackPoint, 0);
// Get Perceptual black out of v4 profiles. That is fixed for perceptual & saturation intents
BlackPoint -> X = cmsPERCEPTUAL_BLACK_X;
BlackPoint -> Y = cmsPERCEPTUAL_BLACK_Y;
BlackPoint -> Z = cmsPERCEPTUAL_BLACK_Z;
return TRUE;
}
// Check if the profile is lut based and gray, rgb or cmyk (7.2 in Adobe's document)
ColorSpace = cmsGetColorSpace(hProfile);
if (!cmsIsCLUT(hProfile, Intent, LCMS_USED_AS_OUTPUT ) ||
(ColorSpace != cmsSigGrayData &&
ColorSpace != cmsSigRgbData &&
ColorSpace != cmsSigCmykData)) {
// In this case, handle as input case
return cmsDetectBlackPoint(BlackPoint, hProfile, Intent, dwFlags);
}
// It is one of the valid cases!, use Adobe algorithm
// Set a first guess, that should work on good profiles.
if (Intent == INTENT_RELATIVE_COLORIMETRIC) {
cmsCIEXYZ IniXYZ;
// calculate initial Lab as source black point
if (!cmsDetectBlackPoint(&IniXYZ, hProfile, Intent, dwFlags)) {
return FALSE;
}
// convert the XYZ to lab
cmsXYZ2Lab(NULL, &InitialLab, &IniXYZ);
} else {
// set the initial Lab to zero, that should be the black point for perceptual and saturation
InitialLab.L = 0;
InitialLab.a = 0;
InitialLab.b = 0;
}
// Step 2
// ======
// Create a roundtrip. Define a Transform BT for all x in L*a*b*
hRoundTrip = CreateRoundtripXForm(hProfile, Intent);
if (hRoundTrip == NULL) return FALSE;
// Compute ramps
for (l=0; l < 256; l++) {
Lab.L = (cmsFloat64Number) (l * 100.0) / 255.0;
Lab.a = cmsmin(50, cmsmax(-50, InitialLab.a));
Lab.b = cmsmin(50, cmsmax(-50, InitialLab.b));
cmsDoTransform(hRoundTrip, &Lab, &destLab, 1);
inRamp[l] = Lab.L;
outRamp[l] = destLab.L;
}
// Make monotonic
for (l = 254; l > 0; --l) {
outRamp[l] = cmsmin(outRamp[l], outRamp[l+1]);
}
// Check
if (! (outRamp[0] < outRamp[255])) {
cmsDeleteTransform(hRoundTrip);
BlackPoint -> X = BlackPoint ->Y = BlackPoint -> Z = 0.0;
return FALSE;
}
// Test for mid range straight (only on relative colorimetric)
NearlyStraightMidrange = TRUE;
MinL = outRamp[0]; MaxL = outRamp[255];
if (Intent == INTENT_RELATIVE_COLORIMETRIC) {
for (l=0; l < 256; l++) {
if (! ((inRamp[l] <= MinL + 0.2 * (MaxL - MinL) ) ||
(fabs(inRamp[l] - outRamp[l]) < 4.0 )))
NearlyStraightMidrange = FALSE;
}
// If the mid range is straight (as determined above) then the
// DestinationBlackPoint shall be the same as initialLab.
// Otherwise, the DestinationBlackPoint shall be determined
// using curve fitting.
if (NearlyStraightMidrange) {
cmsLab2XYZ(NULL, BlackPoint, &InitialLab);
cmsDeleteTransform(hRoundTrip);
return TRUE;
}
}
// curve fitting: The round-trip curve normally looks like a nearly constant section at the black point,
// with a corner and a nearly straight line to the white point.
for (l=0; l < 256; l++) {
yRamp[l] = (outRamp[l] - MinL) / (MaxL - MinL);
}
// find the black point using the least squares error quadratic curve fitting
if (Intent == INTENT_RELATIVE_COLORIMETRIC) {
lo = 0.1;
hi = 0.5;
}
else {
// Perceptual and saturation
lo = 0.03;
hi = 0.25;
}
// Capture shadow points for the fitting.
n = 0;
for (l=0; l < 256; l++) {
cmsFloat64Number ff = yRamp[l];
if (ff >= lo && ff < hi) {
x[n] = inRamp[l];
y[n] = yRamp[l];
n++;
}
}
// No suitable points
if (n < 3 ) {
cmsDeleteTransform(hRoundTrip);
BlackPoint -> X = BlackPoint ->Y = BlackPoint -> Z = 0.0;
return FALSE;
}
// fit and get the vertex of quadratic curve
Lab.L = RootOfLeastSquaresFitQuadraticCurve(n, x, y);
if (Lab.L < 0.0) { // clip to zero L* if the vertex is negative
Lab.L = 0;
}
Lab.a = InitialLab.a;
Lab.b = InitialLab.b;
cmsLab2XYZ(NULL, BlackPoint, &Lab);
cmsDeleteTransform(hRoundTrip);
return TRUE;
}
// Calculates the black point of a destination profile.
// This algorithm comes from the Adobe paper disclosing its black point compensation method.
cmsBool CMSEXPORT cmsDetectDestinationBlackPoint(cmsCIEXYZ* BlackPoint, cmsHPROFILE hProfile, cmsUInt32Number Intent, cmsUInt32Number dwFlags)
{
cmsColorSpaceSignature ColorSpace;
cmsHTRANSFORM hRoundTrip = NULL;
cmsCIELab InitialLab, destLab, Lab;
cmsFloat64Number MinL, MaxL;
cmsBool NearlyStraightMidRange = FALSE;
cmsFloat64Number L;
cmsFloat64Number x[101], y[101];
cmsFloat64Number lo, hi, NonMonoMin;
int n, l, i, NonMonoIndx;
// Make sure intent is adequate
if (Intent != INTENT_PERCEPTUAL &&
Intent != INTENT_RELATIVE_COLORIMETRIC &&
Intent != INTENT_SATURATION) {
BlackPoint -> X = BlackPoint ->Y = BlackPoint -> Z = 0.0;
return FALSE;
}
// v4 + perceptual & saturation intents does have its own black point, and it is
// well specified enough to use it. Black point tag is deprecated in V4.
if ((cmsGetEncodedICCversion(hProfile) >= 0x4000000) &&
(Intent == INTENT_PERCEPTUAL || Intent == INTENT_SATURATION)) {
// Matrix shaper share MRC & perceptual intents
if (cmsIsMatrixShaper(hProfile))
return BlackPointAsDarkerColorant(hProfile, INTENT_RELATIVE_COLORIMETRIC, BlackPoint, 0);
// Get Perceptual black out of v4 profiles. That is fixed for perceptual & saturation intents
BlackPoint -> X = cmsPERCEPTUAL_BLACK_X;
BlackPoint -> Y = cmsPERCEPTUAL_BLACK_Y;
BlackPoint -> Z = cmsPERCEPTUAL_BLACK_Z;
return TRUE;
}
// Check if the profile is lut based and gray, rgb or cmyk (7.2 in Adobe's document)
ColorSpace = cmsGetColorSpace(hProfile);
if (!cmsIsCLUT(hProfile, Intent, LCMS_USED_AS_OUTPUT ) ||
(ColorSpace != cmsSigGrayData &&
ColorSpace != cmsSigRgbData &&
ColorSpace != cmsSigCmykData)) {
// In this case, handle as input case
return cmsDetectBlackPoint(BlackPoint, hProfile, Intent, dwFlags);
}
// It is one of the valid cases!, presto chargo hocus pocus, go for the Adobe magic
// Step 1
// ======
// Set a first guess, that should work on good profiles.
if (Intent == INTENT_RELATIVE_COLORIMETRIC) {
cmsCIEXYZ IniXYZ;
// calculate initial Lab as source black point
if (!cmsDetectBlackPoint(&IniXYZ, hProfile, Intent, dwFlags)) {
return FALSE;
}
// convert the XYZ to lab
cmsXYZ2Lab(NULL, &InitialLab, &IniXYZ);
} else {
// set the initial Lab to zero, that should be the black point for perceptual and saturation
InitialLab.L = 0;
InitialLab.a = 0;
InitialLab.b = 0;
}
// Step 2
// ======
// Create a roundtrip. Define a Transform BT for all x in L*a*b*
hRoundTrip = CreateRoundtripXForm(hProfile, Intent);
if (hRoundTrip == NULL) return FALSE;
// Calculate Min L*
Lab = InitialLab;
Lab.L = 0;
cmsDoTransform(hRoundTrip, &Lab, &destLab, 1);
MinL = destLab.L;
// Calculate Max L*
Lab = InitialLab;
Lab.L = 100;
cmsDoTransform(hRoundTrip, &Lab, &destLab, 1);
MaxL = destLab.L;
// Step 3
// ======
// check if quadratic estimation needs to be done.
if (Intent == INTENT_RELATIVE_COLORIMETRIC) {
// Conceptually, this code tests how close the source l and converted L are to one another in the mid-range
// of the values. If the converted ramp of L values is close enough to a straight line y=x, then InitialLab
// is good enough to be the DestinationBlackPoint,
NearlyStraightMidRange = TRUE;
for (l=0; l <= 100; l++) {
Lab.L = l;
Lab.a = InitialLab.a;
Lab.b = InitialLab.b;
cmsDoTransform(hRoundTrip, &Lab, &destLab, 1);
L = destLab.L;
// Check the mid range in 20% after MinL
if (L > (MinL + 0.2 * (MaxL - MinL))) {
// Is close enough?
if (fabs(L - l) > 4.0) {
// Too far away, profile is buggy!
NearlyStraightMidRange = FALSE;
break;
}
}
}
}
else {
// Check is always performed for perceptual and saturation intents
NearlyStraightMidRange = FALSE;
}
// If no furter checking is needed, we are done
if (NearlyStraightMidRange) {
cmsLab2XYZ(NULL, BlackPoint, &InitialLab);
cmsDeleteTransform(hRoundTrip);
return TRUE;
}
// The round-trip curve normally looks like a nearly constant section at the black point,
// with a corner and a nearly straight line to the white point.
// STEP 4
// =======
// find the black point using the least squares error quadratic curve fitting
if (Intent == INTENT_RELATIVE_COLORIMETRIC) {
lo = 0.1;
hi = 0.5;
}
else {
// Perceptual and saturation
lo = 0.03;
hi = 0.25;
}
// Capture points for the fitting.
n = 0;
for (l=0; l <= 100; l++) {
cmsFloat64Number ff;
Lab.L = (cmsFloat64Number) l;
Lab.a = InitialLab.a;
Lab.b = InitialLab.b;
cmsDoTransform(hRoundTrip, &Lab, &destLab, 1);
ff = (destLab.L - MinL)/(MaxL - MinL);
if (ff >= lo && ff < hi) {
x[n] = Lab.L;
y[n] = ff;
n++;
}
}
// This part is not on the Adobe paper, but I found is necessary for getting any result.
if (IsMonotonic(n, y)) {
// Monotonic means lower point is stil valid
cmsLab2XYZ(NULL, BlackPoint, &InitialLab);
cmsDeleteTransform(hRoundTrip);
return TRUE;
}
// No suitable points, regret and use safer algorithm
if (n == 0) {
cmsDeleteTransform(hRoundTrip);
return cmsDetectBlackPoint(BlackPoint, hProfile, Intent, dwFlags);
}
NonMonoMin = 100;
NonMonoIndx = 0;
for (i=0; i < n; i++) {
if (y[i] < NonMonoMin) {
NonMonoIndx = i;
NonMonoMin = y[i];
}
}
Lab.L = x[NonMonoIndx];
// fit and get the vertex of quadratic curve
Lab.L = VertexOfLeastSquaresFitQuadraticCurve(n, x, y);
if (Lab.L < 0.0 || Lab.L > 50.0) { // clip to zero L* if the vertex is negative
Lab.L = 0;
}
Lab.a = InitialLab.a;
Lab.b = InitialLab.b;
cmsLab2XYZ(NULL, BlackPoint, &Lab);
cmsDeleteTransform(hRoundTrip);
return TRUE;
}
