*4.1. Share Construction and Data Extraction*

To demonstrate the applicability of the proposed secret image sharing scheme, all six pairs of cover images were tested. Two examples of the experimental results are shown in Figures 5 and 6, where (a) and (b) are the cover images, (c) and (d) are the shadow images, and (e) is the recovered secret image. As shown in the figures, the difference between a cover image and its corresponding shadow image cannot be distinguished by human eyes.

(**c**) Shadow 1 (PSNR = 39.88 dB)

(**a**) Cover image 1 (**b**) Cover image 2

(**d**) Shadow 2 (PSNR = 39.89 dB)

**Figure 5.** *Cont*.

(**e**) Recovered secret image

**Figure 5.** Experimental results of cover image pair 1.

(**c**) Shadow 1 (PSNR = 39.86 dB)

(**a**) Cover image 1 (**b**) Cover image 2

(**d**) Shadow 2 (PSNR = 39.90 dB)

(**e**) Recovered secret image

**Figure 6.** Experimental results of cover image pair 2.

To evaluate the visual quality of the shadow images, we applied the peak-signal-to-noise ratio (PSNR), defined by

$$PSNR = 10\log\_{10}\frac{255^2}{MSE} \text{(dB)},\tag{31}$$

where MSE is the mean square error between the cover image *Ck* and its corresponding shadow image *Sk*, defined by

$$MSE = \frac{1}{H \times W} \sum\_{i=1}^{H} \sum\_{j=1}^{W} (C\_k(i, j) - S\_k(i, j))^2. \tag{32}$$

The visual quality and embedding capacity for the six cover image pairs are listed in Table 2.


**Table 2.** Experimental values of the proposed scheme.
