# Survey on Data Hiding Based on Block Truncation Coding
## 5 Conclusions  
We briefly looked at the studies and analyzed their performance using simulations after classifying the various studies on AMBTC-based DH. The cover images used in the experiment are Lena, Boat, Baboon, Peppers, Goldhill, and Airplane. The PSNRs of these cover images are 33.65, 31.57, 26.97, 34.09, 32.83, and 32.03, respectively. Therefore, hiding  
Figure 10: Comparison of the performance of PSNR vs. EC.  
an amount of data while maintaining more than 30 dB can be a very difficult challenge. However, in the case of a high-frequency image such as the Baboon image, it is difficult to use it as a cover image because the image quality does not reach 30 dB after compression with BTC. With DH and RDH, the maximum data hiding performance is 1.3 bpp. However, if data is hidden by 1.3 bpp, it may not be suitable for the purpose of confidential communication or copyright protection. Existing methods can be a good alternative if existing methods are selected according to the purpose. Most of the methods proposed so far are actually modified methods used in gray images to apply the compression method. In other words, most of the methods verified in the grayscale image were applied to BTC and showed good performance. In the future, we intend to review the papers of various methods to consider future research directions.  
Conceptualization, C.K.; methodology, C.K.; software, L.L.; validation, C.-N.Y.; formal analysis, C.-N.Y.; investigation, L.L.; resources, J.B.; data curation, J.B.; writing--original draft preparation, C.K.; writing--review and editing, C.K.; visualization, L.L.; supervision, C.-N.Y.; project administration, C.K.; funding acquisition, C.K. All authors have read and agreed to the published version of the manuscript.  
This work was partially supported by the Ministry of Science and Technology (MOST) under Grant 108-2221-E-259-009-MY2 and 109-2221-E-259-010 and by the faculty research fund of Sejong University in 2020. This work was supported by the National Natural Science Foundation of China under Grant 61866028.  
Not applicable.  
Thus you to the reviewers who reviewed this paper and the MDPI editor who edited it professionally.  
The authors declare no conflict of interest.  
The following abbreviations are used in this manuscript:  
\begin{tabular}{l l} DH & Data Hiding \\ RDH & Reversible Data Hiding \\ BTC & Block Truncation Coding \\ AMBTC & Absolute Moment BTC \\ DCT & Discrete Cosine Transform \\ LSB & Least Significant Bit \\ OPAP & Optimal Pixel Adjustment Process \\ HS & Histogram Shifting \\ ECC & Error Correction Code \\ DBS & Direct Bitmap Substitution \\ \(x_{i}\) & \(i\)th pixel of the block \\ \(\bar{x}\) & Mean pixel value \(\bar{x}\) of a \(k\times k\) block \\ \(BM\) & A block of common bitmap \\ \(\mu_{0},\mu_{1}\) & Two quantization levels of a block \\ \(T\) & \((\mu_{1}-\mu_{0})<T\) \\ \(mod\) & Modular operator \\ \(f\) & Extraction function as weighted sum modulo 5 \\ \(\oplus\) & Exclusive or operator \\ \(H\) & Parity-check matrix for HC(7,4) \\ \(H(x)\) & Histogram of an image, where \(x\) is a pixel \\ \(M\) & \(n\)-bit secret message \\ \end{tabular}