## 4 Experimental Results and Discussion
### Imperceptibility Test
It improves further by using optimized ISF. \begin{table}
\begin{tabular}{l c c c c c c} \hline \hline
**Image** & **MSE** & **PSNR (dB)** & **SSIM** & **NC** & **BER** & **Alpha** \\ \hline Grass (1.1.01) & 0.4129 & 52.03 & 0.9999 & 1 & 0 & 6.7734 \\ Bark (1.1.02) & 0.4362 & 51.74 & 0.9995 & 1 & 0 & 7.7979 \\ Straw (1.1.03) & 0.4399 & 51.71 & 0.9996 & 1 & 0 & 9.7490 \\ Herringbone weave (1.1.04) & 0.3934 & 52.18 & 0.9998 & 1 & 0 & 13.9102 \\ Woolen (1.1.05) & 0.4215 & 51.95 & 0.9992 & 1 & 0 & 6.5332 \\ Pressed calf leather (1.1.06) & 0.4194 & 51.89 & 0.9998 & 1 & 0 & 11.9004 \\ Beach sand (1.1.07) & 0.4356 & 51.79 & 0.9993 & 1 & 0 & 6.9023 \\ Water (1.1.08) & 0.3476 & 52.64 & 0.9969 & 1 & 0 & 2.9189 \\ Wood grain (1.1.09) & 0.4101 & 52.02 & 0.9983 & 1 & 0 & 2.7051 \\ Raffia (1.1.10) & 0.4186 & 51.95 & 0.9994 & 1 & 0 & 7.6084 \\ Grass (1.2.03) & 0.4472 & 52.78 & 0.9962 & 1 & 0 & 2.3681 \\ Brick wall (1.2.12) & 0.4522 & 52.54 & 0.9998 & 1 & 0 & 1.9532 \\ Tile roof (1.4.05) & 0.4119 & 51.98 & 0.9755 & 1 & 0 & 3.1567 \\ Wood fence (1.4.06) & 0.3810 & 52.32 & 0.9964 & 1 & 0 & 2.4517 \\ Metal grates (1.4.07) & 0.3897 & 52.24 & 0.9977 & 1 & 0 & 3.8203 \\.Female (4.1.01) & 0.3085 & 53.19 & 0.9774 & 1 & 0 & 1.3779 \\ Couple (4.1.02) & 0.3041 & 53.28 & 0.9360 & 1 & 0 & 0.6797 \\ Female (4.1.03) & 0.2070 & 54.98 & 0.9424 & 1 & 0 & 0.4785 \\ Female (4.1.04) & 0.2912 & 53.52 & 0.9772 & 1 & 0 & 0.7607 \\ House (4.1.05) & 0.2895 & 53.56 & 0.9681 & 1 & 0 & 1.2666 \\ Tree (4.1.06) & 0.3627 & 52.57 & 0.9731 & 1 & 0 & 1.6543 \\ Jelly bean (4.1.07) & 0.0899 & 58.59 & 0.9935 & 1 & 0 & 0.0332 \\ Airplane (4.2.05) & 0.4051 & 52.08 & 0.9697 & 1 & 0 & 0.9648 \\ Sail boat (4.2.06) & 0.4668 & 51.42 & 0.9909 & 1 & 0 & 1.6055 \\ Peppers (4.2.07) & 0.4632 & 51.49 & 0.9896 & 1 & 0 & 1.4355 \\ Moon surface (5.1.09) & 0.3321 & 52.92 & 0.9924 & 1 & 0 & 2.0449 \\ Aerial (5.1.10) & 0.4218 & 51.85 & 0.9961 & 1 & 0 & 2.6289 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: MSE, PSNR, SSIM, NC, BER (under zero attacks) and adaptive ISF for 50 test cover images taken from USC-SIPI. \begin{table}
\begin{tabular}{l c c c c c} \hline \hline
**Grayscale** & **MSE** & **PSNR (dB)** & **SSIM** & **NC** & **BER** & **Alpha** \\ \hline Lena & 0.4315 & 51.67 & 0.9853 & 1 & 0 & 1.03417 \\ Baboon & 0.4395 & 51.58 & 0.9982 & 1 & 0 & 3.93945 \\ Lighthouse & 0.3755 & 52.41 & 0.9530 & 1 & 0 & 0.59863 \\ Desert & 0.3663 & 52.50 & 0.9657 & 1 & 0 & 0.68652 \\ Grain & 0.4573 & 51.52 & 0.9867 & 1 & 0 & 2.93261 \\ MRI & 0.3009 & 53.31 & 0.9035 & 1 & 0 & 0.57421 \\ \hline Average & 0.3951 & 52.16 & 0.9654 & 1 & 0 & \\ \hline
**Color** & **MSE** & **PSNR** & **SSIM** & **NC** & **BER** & **Alpha** \\ \hline Koala & 0.1538 & 56.31 & 0.9991 & 0.9991 & 0.0022 & 2.33984 \\ Penguins & 0.1319 & 56.90 & 0.9990 & 0.9992 & 0.0015 & 0.48535 \\ Tulips & 0.9919 & 60.85 & 1 & & 0.0001 & 0.41992 \\ Tiffany & 0.0865 & 58.76 & 0.9977 & 0.9901 & 0.0308 & 0.45898 \\ Splash & 0.1080 & 57.79 & 0.9976 & 0.9899 & 0.0229 & 1.07324 \\ Skin & 0.1336 & 56.75 & 0.9999 & 0.9992 & 0.0014 & 2.07812 \\ \hline Average & 0.0112 & 57.89 & 0.9988 & 0.9949 & 0.0115 & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: MSE, PSNR, SSIM, NC and BER (under zero attacks) for different test cover images using adaptive ISF. The imperceptibility performance of the proposed scheme using optimized ISF is also analyzed using GA, ABC and FO algorithms and presented in Table 3. Comparing the result in Tables 1 and 3, it can be observed that PSNR and SSIM for grayscale and color images increased after optimization. Using GA, the average value of PSNR for grayscale images increased from 52.16 dB to 52.95 dB and, for color, it is improved from 57.89 dB to 58.42 dB. In addition, SSIM increased from 0.9654 to 0.9989 and 0.9988 to 0.9989, for grayscale and color images, respectively. Improved performance is also observed by using ABC and FO, but GA exceeds in improving the imperceptibility performance.