Abstract

As desktop color publishing systems become more ubiquitous, the technical community, therefore, is being driven by the high-end color market to develop new technologies of HDR imaging and Hi-Fi color printing process, to differentiate and add value to their product. The intent is, via the use of HDR contone image, to produce Hi-Fi halftone images of superior color with maximum isomerism, an increase of the attainable color gamut, and more tone and detail than those with traditional processes. Additionally, in the field of Graphic Arts, multiple-primary displays with high-definition or wide color-gamut are practically used as soft-proofing for desktop color publishing.

Therefore, as motivated by the idea to bridge the gap between high-dynamic-range (HDR) imaging and high-fidelity color reproduction in the high-end of Graphic Arts industry, the aim of this research has two tasks. Firstly, it’s to derive an HDR contone to Hi-Fi halftone-conversion characterization model for both six-colorant CMYKOG and seven-colorant CMYKORG printing systems, via fitting spectral- reflectance approach. It is optimally revised and extended from a previous research. The supersets of CMYKOG and CMYKRGB were divided into three subsets and four subsets respectively. The superset of CMYKOG includes two five-colorant subsets of CMYKO and CMYKG, and one four-colorant subset of CMYK; whereas the CMYKRGB applies three five-colorant subsets of CMYKR, CMYKG and CMYKB, and one four-colorant subset of CMYK. The spectral printer characterization carries out a polynomial regression approach with singular value decomposition method (SVD). In total, 1617 and 1716 color patches are respectively used in test targets of four-colorant and five-colorant subsets for the printing characterization processes. To avoid both unpleasant discontinuity and swap rendition effects of lightness due to some unpredictable prediction error from model derived, a fuzzy k-cmeans clustering operation on lightness for color image segmentation will also be integrated in this work. This algorithm is used to enhance transition smoothness on color-appearance of rendered images. Preliminarily a color difference formula of CIE2000 (i.e. ΔE*00) will be adopted to investigate models’ performance by using the training sets, which also establish the color gamut of imaging device considered.

Secondarily, a halftone-palate type of image display model, forhigh-fidelity soft-proofing and facsimile simulation of 6- or 7-colorants printing system, is also derived in this study. An Adobe RGB format of wide-gamut LCD display and a retina type of MacBook Pro are tested for this work. Mainly, three general classes of contone-to-halftone conversion process will be implemented and evaluated in this work. These are: 1) point-to-point process: including cluster, recursive and dispersed template; 2) chunk-to-point process: including iterative and error diffusion; 3) Hybrid point and chunk. All processes involve an iterative thresholding operation using CIEDE2000 color-difference formula, for the “find closest color”, to match the input color to the closest available hardware color palette from 32 (i.e. 25) and 64 (i.e. 26) combinations, applied in the simulation of 5- and 6- colorants printing systems respectively.

The color difference formula of CIEDE2000, via the investigation of the correspondent training set, were also adopted here to optimize the prediction and rendition performances for both halftone-conversion of printing and display processes. Efforts will be also made on advanced development, modifications and integration of previous derived works, including the three-band CCD camera module (for multispectral measurement and/or reconstruction), an Adobe RGB format of wide-gamut LCD display and a retina type of MacBook Pro, a high-definition multicolor printing process (6/7 colorants, i.e. CMYKOG, CMYKRGB), tone/gamut mapping and color appearance model, and the halftone method derived.

Both of complex and computer-graphic HDR color images will be used in an advanced evaluation of models performance by adopting a forced-choice paired-comparison experiment. Therefore, via the bridge of these two well-performed modules derived in this research, hopefully, an optimized cross-media HDR contone-to-halftone Hi-Fi color transform module on both hardcopy and softcopy will be successfully proposed to form a complete color reproduction pipeline, and used as a main workflow in the process of color management.