2. Colour Halftoning for Variable Dot Size Printers

Due to the proliferation of low-cost color printers during the last few years, converting full-color images to halftoned bi-level separations remains an important issue. A halftoning technique should faithfully reproduce the original color image, while satisfying several quality criteria such as absence of visible artifacts and Moires, providing reliable color reproduction as well as good small detail and texture rendition. Digital halftoning techniques are not limited to classical methods, such as clustered-dot dithering, dispersed dot dithering and error-diffusion methods. In order to obtain the best results, recent algorithms combine the respective advantages of dithering and error diffusion methods. One of the most spectacular progress in color reproduction technology has been made in the domain of ink-jet printers, where microdrops are used to achieve a larger dynamic range without increasing the spatial resolution. Colour rendition techniques specially adapted for these new printing devices are presented. A further advance in printing technology consists of incoporating into the printer more than the traditional 4 process colors. Advanced color separation techniques are required in order to print multicolor images with more than the Cyan Magenta Yellow and Black process colors. One of these advanced techniques combining color separation and halftone generation is error-diffusion in color space. Finally, we give a brief overview of the colour calibration standard proposed by the International Colour Consortium.