Designing Template Cells Suitable for Secret-Sharing Halftone Images by Quantitative Analysis

A method for sharing secret figures in halftone images involves the random use of template cells. In this method grainy appearances of halftone images can be reduced by limiting available template cells into two for each halftone level. A method for obtaining the cell pairs of the best halftone quality is proposed. To sort out such cell pairs from all the possible ones, quantitative measures for evaluating halftone quality are developed. To evaluate flat appearances of halftone, a level variance of a cell pair is defined. Experimental results of subjective evaluation show correlations between the level variances and the subjective qualities. Other measures are also defined to evaluate continuous appearances between consecutive levels. The method is demonstrated with cells of 4 by 4 dots. An example of the best cell pairs is then presented for practical use.