On the capacity of orientation modulation halftone channels

Clustered-dot halftones are extensively utilized in hardcopy printing. Modulation of the dot orientation in these halftones offers an avenue for data embedding which has been exploited in a number of different methods. We consider the capacity of these channels, modeling them as binary orientation input channels with vector valued output detection statistics. We derive upper bounds on the capacity for three channel conditional distributions corresponding to sub-Gaussian, Gaussian and super-Gaussian distributions. Using experimentally estimated channel parameters our bounds reveal that channel capacity has noticeable variations as a function of gray level. Highlights, shadows and mid-tones offer negligible capacity, on the contrary the regions between highlights and mid-tones or shadows and mid-tones offer high capacity for data embedding.