Dot Gain Modelling Applied to Stochastic Screens.

The art of making frequency modulated halftones and stochastic screens is still evolving. The sucess of rendering images using such halftones is to a large extent determined by the paper and the actual algorithm used. The paper surface topography, the light diffusion properties of paper, the paper--ink interaction and many other materials properties are essential to the outcome. It has been noted empirically that the optical scattering in paper generally causes the dot gain to increase with decreasing dot size. In stochastic screens where dot size is dramatically smaller than in conventional halftoning its effect has to be compensated for. This is usually done by tranforming the C,M,Y and K channels individually so as to counterbalance the dot gain on the input side of the screening algorithm. This paper deals with the effects of the optical dot gain of stochastic patterns. The conclusions of the paper show that the compensation should be made not on the input side, before the halftone is produced but during the halftoning process itself. If the stochastic nature of the microstructure cannot be controlled during the process the optical (and physical) dot gain may cause strong variations in the resulting tone with a noisy or grainy result. It is also shown that adaptive algorithms can overcome the problem by incorporating a dot gain model in the algorithm. Iterative adaption specifically, has been suggested as one way of incorporating the dot gain into the halftoning procedure.