Hybrid Color Halftoning

Halftoning methods can be divided into two main categories, namely AM (amplitude modulated) and FM (frequency modulated). Some printing methods, such as flexography, are not able to produce sufficiently small dots in order to handle the highlights and the shadows of the original image by just using an AM halftoning method. The reason is that in order to reproduce the lighter tones the halftone dots should be smaller than the critical dot size in AM techniques, which the printing press is not able to produce. This will mean that the parts that are lighter than the tone corresponding to the critical dot size will be blank. In our previous works we proposed a hybrid halftoning method for grayscale images that incorporates AM and FM technologies in order to overcome this problem. The strategy was to use an FM method in the highlights (and the shadows) of the image and an AM method in the rest of the image. In the case of color images, besides the problem related to the transition area between AM and FM, one should also consider other factors. One of the factors is where to start the FM technique in different color channels. For example, if the critical dot size corresponds to 12.5%, should the FM technique be used for tones lighter than 12.5% for all color channels? Or should it be used if any one of the channels is less than 12.5%? Or should the average of all the channels decide the FM starting point? In all of the above cases, parts of the image will be FM halftoned in one or several color channels and AM halftoned in the rest of the channels. How will the final image look like in these cases? These were only a few of many possible solutions to the problem. In this paper we are going to try all possibilities of combining AM and FM in different color channels and investigate the results. We are then going to present a new approach for hybrid color halftoning, which does not only use the best solution of where FM should start in each channel but also places the different color dots in the highlights, as homogenously as possible.