A digital image printed as a binary halftone image can have an optical watermark which becomes visible only if the associated halftone image is superimposed on it. In the scheme of template-dot halftoning, such pair of images can be generated by using different halftone cells on each image at a watermark dot. To make the watermark
invisible on either image, a halftone cell should be randomly selected among those available for representing an identical halftone level. This randomness results in noisy appearance of the generated halftone images. In this paper, to reduce the degradation of halftone quality the template cells used for each halftone level are limited
to two cells. Either one of the two template cells is randomly selected and used to generate a pair of halftone images with hiding optical watermarks in them. A set of two alternative template cells used for each halftone level which yields less noisy halftone quality is investigated through computer simulation. An example of such set of clustered-dot type cells and that of dispersed-dot type cells are presented.