Improving image quality in multi-channel printing - multilevel halftoning, color separation and graininess characterization

Sammanfattning: Color printing is traditionally achieved by separating an input image into four CMYK channels and binarizing them, employing halftoning algorithms, to designate the locations of ink droplet placement. Multi-channel printing means a reproduction that employs additional inks other than these four in order to augment the color gamut (scope of reproducible colors) and reduce undesirable ink droplet visibility, so-called graininess.One of the aims of this dissertation has been to characterize a print setup in which both the primary inks CMYK and their light versions are used. The presented approach groups the inks, forming subsets, each representing a channel that is reproduced with multiple inks. To halftone the separated channels in the present methodology, a specific multilevel halftoning algorithm is employed, halftoning each channel to multiple levels. This algorithm performs the binarization from the ink subsets to each separate colorant. Consequently, the print characterization complexity remains unaltered when employing the light inks, eliminating the usual increased computational complexity, the one-to-many mapping problem and the increase in the number of training samples. The results show that the reproduction is visually improved in terms of graininess and detail enhancement.The secondary color inks RGB are added in multi-channel printing to increase the color gamut. Utilizing them, however, implies challenges such as a print characterization and image quality. A proposed cost function, incorporated in the color separation approach, weights factors that influence the reproduced image quality, namely graininess and color accuracy, in order to select the optimal ink combination. The perceived graininess is calculated employing S-CIELAB, a spatial low-pass filtering mimicking the human visual system. By applying the filtering to a large dataset, a generalized prediction that quantifies the perceived graininess is carried out and incorporated as a criterion in the color separation.Consequently, the presented research exploits some of the improved image quality factors that are possible when employing additional inks in multi-channel printing. The results indicate an accurate print reproduction utilizing both the secondary and the light inks, in which the perceived graininess is decreased, hence improving the image quality.