Contrast-enhancing and deterministic tone mapping method in natural image hiding scheme using halftone images

Abstract

This paper presents a novel tone mapping method for a natural image hiding scheme using halftone images, where a natural image can be visually decoded by overlaying two natural halftone images. In this scheme, there is a tradeoff between noise and contrast, and a generally applicable setting of tone mapping before halftoning is required. However, in a conventional method, the coefficients for the tone mapping cannot be found automatically. It is difficult even for experts to manipulate the tradeoff with the coefficients. To solve this problem, we propose a deterministic tone mapping method that can intuitively control the tradeoff. To realize this, we introduce a noise metric which can be understood intuitively instead of affine parameters used in the mapping system. To maximize the dynamic range at a given noise level and to make tone mapping deterministic, we clarify the condition and introduce the general functions to obtain coefficients from the geometric constraint of the tone mapping region. The proposed method enables any user to generate images with the highest possible contrast at a given noise level deterministically from any natural images such as own photos just by setting the noise level. Experimental results show the validity of the proposed noise metric and also show that the generally applicable tradeoff point through various images that are used as a guideline to set the noise level. By using the tradeoff point, the average dynamic range is expanded by a factor of 1.4 compared to the noiseless case.