# Survey on Data Hiding Based on Block Truncation Coding
## 4 Performance Analysis  
We have classified the AMBTC-based DH method so far and summarized each classification method. In this section, the performance of the proposed DH method is evaluated with six original grayscale images. The evaluation of existing methods is based on the EC and the quality of the displayed images. EC is the total number of bits embedded in AMBTC and the quality of the marked image is measured in PSNR as  
\[\text{PSNR}=10 log_{10}\frac{255^{2}}{\text{MSE}}. \tag{9}\]  
The six original images [54] used in the experiment are Lena, Boat, Baboon, Peppers, Goldhill, and Airplane as shown in Figure 7. These original images must be converted into AMBTC before experimental evaluation. The hidden bits to be used in the experiment are generated by the random number generation library function provided by Matlab. The existing methods are applied to the units of trios converted to AMBTC, that is, the bitmap and two quantization levels. Figure 8 shows the comparison of the EC of the representative DH methods (i.e., [PERSON] and [PERSON] [29], [PERSON] et al. [38], [PERSON] [32], [PERSON] et al. [41] and [PERSON] et al. [43] using six images [54]. Here, EC was measured in case that the image quality was above 30 dB. The reason is that the measured EC when less than 30 dB may not be suitable for use in real systems. Although the other researches ignore this, in Figure 8, EC was measured with this criteria. Of course, even if the experimental result maintains 30 dB, it cannot be guaranteed that the image is not absolutely broken, but for the objective of the experiment, the lower limit of the image is set to 30 dB. The methods in Figure 8 use the DBS method [29] commonly, which is to substitute bits of the same size in a \(4\times 4\) bitmap. The merit of DBS is essential for hiding a large number of bits (1 bpp), and the image quality does not deteriorate significantly even after performing DBS, because on average 50% of the original pixel values of the block are maintained after DBS is performed. The methods of [PERSON] et al. [38] and [PERSON] et al. [43], respectively, proposed a DH technique that exploits the difference between two pixels to sufficiently hide the data at the quantization level. [PERSON] et al. 's method [38] and [PERSON] et al. 's method [43] have average bpp of 1.3 and 1.28, respectively, and [PERSON] et al. 's method is slightly superior. Since [PERSON] and [PERSON]'s method [29] hides data only with the DBS method, the data hiding rate corresponds to 1 bpp. The methods of [PERSON] [32] and [PERSON] et al. [41] is 1.18 bpp and 1.16 bpp, respectively, which is superior to DBS, but it is measured to be slightly inferior in performance compared to the [PERSON] et al. 's method. AMBTC-based DH does not have enough redundant bits compared to standard grayscale images, so achieving an average of 1.2 bpp while maintaining about 30 dB is a very valuable achievement. Figure 9 compares and evaluates the performance of a typical RDH method. The methods of [PERSON] and [PERSON] [46], [PERSON] et al. [47], and [PERSON] et al. [50] are AMBTC-based RDH methods that were proposed relatively early. [PERSON] and [PERSON] [46] and [PERSON] et al. [47] are RDH methods using histograms, and [PERSON] et al. 's method [50] is a method using HC (7,4). The data hiding performance is 0.08 bpp, 0.13 bpp, and 0.24 bpp, respectively. These methods  
Figure 8: Comparisons of EC performance among existing DH methods. Figure 7: Original grayscale images for experiments. show the potential of early BTC-based RDH and can be considered valuable. On the other hand, the relatively recent RDHs ([PERSON] et al. [48], [PERSON] et al. [51], and [PERSON] et al. [52]) range from about 0.57 bpp to about 1.3 bpp. In particular, [PERSON] et al. 's method [52] utilizes multiple histograms, so it shows the best performance. Like the existing gray image-based RDH, it can be seen that the HS method is usefully used in the BTC-based RDH system.