Abstract Absolute Moment Block Truncation Coding (AMBTC) is a cost-effective image compression technique that replaces pixel blocks with two quantization values and a binary map to minimize storage requirements. However, it is less effective for compressing flat and complex blocks. To address this issue, a Dynamic Multi-Grouping (DMG)-based AMBTC method was proposed, which removes the binary map for flat blocks and uses ternary or quaternary maps for complex blocks. Indicators are used to track the encoding methods applied to each block. While this method improves image quality, it fails to account for the correlations among indicators and maps, leading to higher storage requirements. This paper presents a sub-grouping strategy that groups indicators with ternary or quaternary maps to exploit their correlations, and encodes them using entropy coding to reduce the encoding length. During decoding, the compressed codes of indicators can be extracted from the bitstream, and the decompressed indicators are used to reconstruct blocks of various types. Our approach can be integrated with existing DMG methods to reduce bitrate while preserving image quality. When tested on 100 images, it achieved average reductions in bitrate of 31.83% for indicators, 14.68% for ternary maps, and 8.29% for quaternary maps.

Abstract Reversible data hiding (RDH) with contrast enhancement (RDH-CE) is a special type of RDH in improving the subjective visual perception by enhancing the image contrast during the process of data embedding. In RDH-CE, data hiding is achieved via pairwise histogram expansion, and the embedding rate can be increased by performing multiple cycles of histogram expansions. However, when embedding rate gets high, human visible image degradation is observed. Previous work designed an upper bound of the embedding level for RDH-CE, which effectively avoids image over-sharping but offers limited embedding capacity. In this paper, a better tunable bound is designed to enhance the embedding capacity of RDH-CE by exploiting the characteristics of histogram distribution. Furthermore, the objective distortion introduced by histogram pre-shifting is minimized when the embedding level is no more than the upper bound, and the human visible degradation is minimized when the embedding level exceeds the limitation of the proposed upper bound. Experimental results validate that the proposed method provides appropriate upper bound of the embedding level, increases the effective embedding capacity and offers better image contrast.

Recently, a number of high-fidelity reversible data hiding algorithms have been developed based on prediction-error expansion (PEE) and pixel sorting. In PEE, prediction is made using either a full-enclosed or a half-enclosed predictor. While in PEE with pixel sorting, the local complexity (LC), which is usually assumed to be proportional to the magnitude of prediction-error (PE), is exploited to reduce the embedding distortion. However, this assumption may not always hold in all conditions. In this letter, a directional enclosed predictor is proposed to detect the locations where LC is not proportional to PE. And, a directionally enclosed prediction and expansion (DEPE) scheme is then developed for efficient reversible data hiding. With DEPE, data embedding is restricted to pixels where LC correlates to PE with a proportional relationship. Experimental results show that, compared to the full-enclosed or half-enclosed prediction schemes, DEPE significantly improves the image fidelity while providing a considerable payload.

Recently, Luo et al. proposed a new block-based reversible data hiding method that divides an image into blocks, sorts all pixels within a block and then chooses the middle sorted pixel as a basic-point. To maintain the same basic-point before and after embedding, Luo et al.'s method classifies blocks into four types and adopts four different strategies, respectively, to embed a secret message. In this paper, we propose a new method which relies on only one strategy to reach the same effectiveness. The proposed method includes a complexity controlled mechanism to filter blocks that are not suitable for hiding the secret message. High image quality is thereby retained. Based on just-noticeable difference mechanism, the proposed scheme also estimates allowable changes such that the stego-image is able to meet the human visual system. Experimental results showed that the proposed method allows more payload and higher image quality than the method of Luo et al. and other related methods.

This letter proposes an improved version of Zhang's reversible data hiding method in encrypted images. The original work partitions an encrypted image into blocks, and each block carries one bit by flipping three LSBs of a set of pre-defined pixels. The data extraction and image recovery can be achieved by examining the block smoothness. Zhang's work did not fully exploit the pixels in calculating the smoothness of each block and did not consider the pixel correlations in the border of neighboring blocks. These two issues could reduce the correctness of data extraction. This letter adopts a better scheme for measuring the smoothness of blocks, and uses the side-match scheme to further decrease the error rate of extracted-bits. The experimental results reveal that the proposed method offers better performance over Zhang's work. For example, when the block size is set to 8 8, the error rate of the Lena image of the proposed method is 0. 34%, which is significantly lower than 1.21% of Zhang's work.

This paper proposes a new data-hiding method based on pixel pair matching (PPM). The basic idea of PPM is to use the values of pixel pair as a reference coordinate, and search a coordinate in the neighborhood set of this pixel pair according to a given message digit. The pixel pair is then replaced by the searched coordinate to conceal the digit. Exploiting modification direction (EMD) and diamond encoding (DE) are two data-hiding methods proposed recently based on PPM. The maximum capacity of EMD is 1.161 bpp and DE extends the payload of EMD by embedding digits in a larger notational system. The proposed method offers lower distortion than DE by providing more compact neighborhood sets and allowing embedded digits in any notational system. Compared with the optimal pixel adjustment process (OPAP) method, the proposed method always has lower distortion for various payloads. Experimental results reveal that the proposed method not only provides better performance than those of OPAP and DE, but also is secure under the detection of some well-known steganalysis techniques.