// 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; }