COLOR MANAGEMENT- P11

COLOR MANAGEMENT- P11: ICC White Papers are one of the formal deliverables of the International Color Consortium, theother being the ICC specification itself – ISO 15076: Image technology color management –Architecture, profile format, and data structure. The White Papers undergo an exhaustiveinternal development process, followed by a formal technical review by the membership and aballot for approval by the ICC Steering Committee.
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COLOR MANAGEMENT- P11284 Profile Construction and Evaluation Finite precision of computation and rounding operations give rise to relatively smalldifferences on roundtripping. These will depend largely on the CMM used to apply the profile,but the profile generator can minimize such differences by using 16-bit data in LUTs. Insituations where the magnitude of roundtrip errors is too high for a particular application,higher precision floating point LUT encodings using DToBx and BToDx tags could beconsidered. In a BToAxTag, output values are given at uniformly spaced intervals for the entire PCSencoding. Many of the values in the PCS will be outside the gamut of the data encoding, and theBToAxTag maps these out-of-gamut colors to coordinates within the range of the dataencoding. In the AToBxTag, all input values have an output value in the PCS, and no mappingof out-of-gamut colors is required. In the colorimetric intents only the values within the gamutof the data encoding can be roundtripped accurately, while in the perceptual and saturationintents only colors within the Perceptual Reference Medium used in the profile BToAx andAToBx tags can be roundtripped accurately. The effective device values are the set of device values that results from mapping the entirePCS to the output encoding. It is only possible to roundtrip to within this set of values. TheAToBTag maps these device values to the PCS, which enables the effective gamut to bedetermined. The profile can only accurately roundtrip within this gamut. For example, a given CMYK value has a unique PCS value in the AToBxTag, but when thisPCS value is converted back to the data encoding with the BToAxTag, the resulting CMYKvalue will depend on the black generation and ink limiting choices made during profilegeneration and not on the starting CMYK value. Since the colorimetric intents in a profile must be measurement based, it is implicit that theBToA1Tag is required to be an accurate inverse of the AToB1Tag. A preview or proof of colorsin the data encoding is normally obtained by applying the colorimetric intent AToB1Tag,regardless of which intent was used in the BToAx direction. In most situations, the AToB0Tagand AToB2Tag should be the inverse of the BToA0Tag and BToA2Tag respectively, to allowdata to be converted consistently between PCS and data encodings. Exceptionally, a profile generator will encode a transform in a BToAxTag that isintentionally different than the inverse of the AToBxTag. Such a profile may still conformto the ICC specification, but to avoid unexpected results it is recommended that such non-invertible transforms are only generated where required for a particular application and thatthe user is made aware of the non-invertibility of the profile, possibly through the profiledescription.32.2 Using a Roundtrip Test to Evaluate Profile QualityBy performing a roundtrip test on a set of sample colors within the effective gamut, it is possibleto obtain an indication of the accuracy of inversion in a profile. This test can be carried out for allrendering intents. If an AToB1Tag is based on measurement data for values in the device encoding andrepresents an accurate conversion from data encoding to PCS (as required by the specification),then a roundtrip test can also be used to deduce the accuracy of the BToA1Tag. Independently of the transform accuracy, the accuracy of inversion may also indicate therelative smoothness of the transform.Inverting ICC Profiles 285 Data 1 PCS 1 Data 2 PCS 2 Data 3 PCS 3 A2B1 B2A1 A2B1 B2A1 A2B1 Figure 32.1 Roundtrip test for colorimetric intent PCS 1 Data 2 PCS 2 Data 3 PCS 3 B2A1 A2B1 B2A1 A2B1 Figure 32.2 Roundtrip test for perceptual and saturation intents in v4 profiles A valid roundtrip test would be to start with a test set of colors in the data encoding andconvert them between data encoding and PCS as shown in Figure 32.1. The evaluation consists of comparing the PCS 2 values with PCS 3, the additional steps incomputing PCS 1, and CMYK 2 values being required to ensure that only colors inside theeffective gamut of the data encoding remain in the test set in PCS 2. Ideally the out-of-gamutcolors should be removed from the test set to avoid weighting the test with a large proportion oftest colors on or close to the gamut boundary. For v4 profiles containing tags indicating the use of the Perceptual Reference Medum Gamutfor either the perceptual or saturation intents, the effective gamut is the PRMG. A validroundtrip test for the intents where the PRMG is indicated would be to start with colorssampling the PRMG and roundtrip them as shown in Figure 32.2. If not using the PRMG, theactual effective gamut should be used for the roundtrip test, as shown in Figure 32.1. More details of evaluating v4 profiles is given in Chapter 33. A MATLAB function toperform a roundtrip test can be downloaded from the resources area on the ICC web site.33Evaluating Color Transformsin ICC ProfilesICC input and output profiles contain transforms between device data encodings and the ICCPCS. These transforms should be either accurate or pleasing, depending on the chosenrendering intent. The following recommendations are provided to assist in the evaluation of the colorimetricand perceptual rendering intent transforms in ICC v4 profiles. Any tolerances provided areguidelines and may not be suitable for all appli ...