Gamut expanded halftone prints

We propose a framework for printing high chroma and bright colors which are beyond both the display sRGB and the classical cmyk print gamuts. These colors are printed with a combination of classical cmyk inks and the two additional daylight fluorescent magenta and yellow inks. The goal is to enhance image parts by printing them with high chroma and bright colors. We first select the image parts to be enhanced. We then apply to their colors a gamut expansion that increases both their chroma and their lightness towards the colors located at the boundary of the gamut formed by the combination of classical and fluorescent inks. This expansion can be controlled by user-defined parameters. We create smooth chroma transitions between the expanded and nonexpanded image parts. We also preview the printable gamut expanded image generated according to user-defined gamut expansion parameters. The resulting prototype software enables artists to create and print their own designs. Introduction In the present contribution, we generate gamut expanded images by printing colors located beyond the display sRGB gamut using a six ink printing system combining the classical cmyk inks with the daylight fluorescent magenta and yellow inks. Daylight fluorescent colorants are widely used in products that aim at capturing the attention of human such as highlighting markers, safety jackets and traffic signs. Fluorescent brighteners are used as whitening agents in tissues and paper. We would like to use the possibilities offered by the high chroma and bright daylight fluorescent colors in order to highlight image regions of special interest. Highlighted image parts are printed with colors beyond the sRGB gamut while the remaining image parts are printed with colors present within the sRGB gamut. We establish the total fluorescent Gf gamut by the conjunction of fluorescent sub-gamuts formed by classical inks with one or two daylight fluorescent inks. We use multiple strategies for mapping the input display sRGB gamut GsRGB onto the printable destination gamut Gf. These multiple mapping strategies define how input image GsRGB gamut colors are expanded to colors beyond the GsRGB gamut while preserving the overall aesthetics of the input image. We created specific tools for exploiting the capabilities of the expanded Gf gamut. They enable selecting an input image, applying to specific regions of that image different gamut expansions, displaying a preview and printing of the resulting gamut expanded printable image. The application offers new means for designers working in fields such as photography, advertisement and magazine production. Producing gamut expanded images raises several challenges. We have to determine expansion factors increasing the chroma of input sRGB colors to colors beyond the sRGB gamut and possibly modify their lightness. The goal is to enhance given image parts with higher chroma and brighter colors. We also have to ensure the continuity of colors at the boundary between highlighted and non-highlighted image regions. In addition, we have to generate halftoned images comprising at different locations different gamut mappings between the input image and the destination image. In order to preview the printable highlighted images having colors beyond the sRGB gamut, we simulate a lower quality display for classical image parts and render the extended sRGB colors by making use of the full capabilities of the display. Such a preview enables visualizing the differences between the color expanded and non-expanded image parts. In order to compute ink surface coverages yielding the desired colors, we need to create a model predicting accurately the color of halftones comprising daylight fluorescent inks. Then, for printing a given expanded or non-expanded color we need to select an adequate set of 3 or 4 inks from the six available inks. Fading of the daylight fluorescent inks [10] can be limited by printing thick ink layers and/or by coating the printed layers with a UVabsorbing coating [9]. Ink manufacturers are developing fading resistant daylight fluorescent pigments. Related work Guyler [1] compared the gamuts of classical and combined classical and daylight fluorescent inks for offset prints by relying on Neugebauer primaries and on printed color patch measurements. In a previous publication [2], Rossier and Hersch proposed to reproduce color images by combining classical and daylight fluorescent inks by using a spectral prediction model predicting the spectral reflectances of halftones comprising daylight fluorescent inks, a gamut mapping scheme from display gamut to the fluorescent printer gamut, and a color separation of the 6 ink layers. The proposed solution was however limited to gamut reduction from display GsRGB gamut to the gamut Gf offered by the fluorescent inks. No gamut expansion beyond the GsRGB gamut was foreseen. Printing with combined classical and daylight fluorescent inks custom ink faces similar problems as printing with custom inks. There is a need to select a specific subset of inks from many possible ink subsets and to map the input gamut into the gamut achievable with multi-ink halftones. Stollnitz, Ostromoukhov and Salesin modeled the gamut of printable custom colorants by a modified Neugebauer model accounting for trapping, dot gain and multiple internal reflectances. With this model, they optimized the selection of custom inks in order to obtain a given color [3]. Tzeng and Berns used cyan, 20th Color and Imaging Conference Final Program and Proceedings Proceedings CIC Conf, Los Angeles, 2012 315 Figure 1. Reflectance factors under the D65 illuminant of (a) the daylight fluorescent mf and yf colorants and (b) the daylight fluorescent green (cyan superposed with yf) and red (mf superposed with yf) colorants (red lines), together with the classical original Epson P50 colorant reflectances (black lines), printed on a fluorescent paper containing optical brighteners (Canon MP-101). magenta, yellow, black, orange and green inks and developed an algorithm for selecting a subset of 4 inks among the 6 inks to reproduce a given reflection spectrum as accurately as possible, i.e. by minimizing metamerism [4]. Byoung-Ho Kand et. al. performed a user study which in addition to gamut compression also dealt with users performing interactive gamut expansion from print gamut to display gamut [11]. For most colors, besides memory colors, the users tried to extend the chroma of the images. Toyoshi Morioka et. al. showed that, compared with linear expansion, non-linear chroma expansion of sRGB images displayed on a wide gamut Adobe RGB monitor was preferred by users [14]. Framework for printing daylight fluorescent inks By combining the daylight fluorescent magenta (mf) and yellow (yf) inks with classical inks, we can create high chroma and bright colors. Figure 1 shows the total spectral reflectance factors [5] of four daylight fluorescent colorants together with the classical colorant reflectances, measured under a D65 illuminant by a SpectroEye Gretag-Macbeth spectrophotometer. Due to the fluorescence of the mf and yf inks, these colorants have a significantly higher chroma and are brighter than the corresponding classical colorants. They allow enhancing the chroma and lightness of parts of the printable images. Let us establish the framework for printing with combinations of daylight fluorescent and classical inks. For this purpose, we establish the sRGB gamut GsRGB and the print fluorescent gamut Gf. The fluorescent gamut comprises all colors printable with cyan, magenta, yellow, black, fluorescent magenta (mf) and fluorescent yellow (yf). We map the GsRGB gamut into the Gf gamut by gamut expansions allowing to print beyond GsRGB gamut colors. Mapping the sRGB gamut into the fluorescent ink gamut [8] requires (a) mapping the lightness range of the sRGB gamut into the lightness range of the fluorescent ink gamut by lightness adaptation, (b) creating the volume of the lightness adapted sRGB gamut and of the printable fluorescent gamut Gf and (c) mapping the lightness adapted sRGB gamut into the printable fluorescent gamut according to user-defined chroma reduction and expansion factors. Thanks to chroma expansion, original image colors may be mapped into high chroma colors partially located outside the sRGB gamut, thereby highlighting the considered image parts. Mapping the lightness range of the sRGB gamut into the ink destination gamut In order to map the GsRGB lightness range into the destination gamut lightness range, we first determine the minimal lightness L * inksMin of the inks, i.e. the lightness of the solid pure black ink. We may then either apply a linear mapping that better preserves lightness differences of the input sRGB image space but raises all sRGB lightnesses, apply a partly non-linear mapping that preserves high lightness values but maps low lightnesses into a smaller lightness range or apply an s-shape like non-linear mapping. These lightness mappings can be defined with a cubic Bézier function