# Data Embedding in SHVC Video Using Threshold-Controlled Block Splitting
## 2 Related Work  
In this section, we briefly review methods designed for compressed video, where data embedding can take place during intra/inter picture prediction, transformation, quantization, and/or entropy coding. In general, modifying an intra prediction directional mode to its adjacent directional mode has minimal impact on the video quality, although the file size is slightly increased. Therefore, this technique has been widely exploited to embed data in the literature. For example, [PERSON] et al. [13] embed data by using the I4-blocks intra prediction mode in H.264/AVC video. They adopt matrix encoding to minimize the number of changes needed, viz., changing not more than one intra prediction mode when embedding two bits from the payload by using three I4-blocks. As newer standards, HEVC and its extension have more directional modes (i.e., more angles) for intra picture prediction, hence more data can be embedded by using the same technique. [PERSON] et al. manipulate the difference of two consecutive intra prediction directional modes or a pair of continuous planar and DC mode in HEVC to embed data [14]. To suppress the distortion caused by data embedding, [PERSON] et al. restrict the modification on the intra prediction mode by \(\pm 1\) so that the changes are imperceptible to the viewer [15]. Recently, [PERSON] et al. selected the smallest intra-coded prediction units in a coding tree unit (CTU) for data embedding purposes. In their work, a random function and an embedding threshold are defined to select CTUs for data embedding, but their proposed method suffers from low embedding capacity [16]. On the other hand, the motion vectors (MVs) of inter-coded blocks are also commonly exploited to embed data. For example, the least significant bit (LSB) of either the horizontal or vertical component of MV is modified to match one payload bit [17; 18]. Instead of using all MVs, [PERSON] only embeds data into MVs with high prediction errors in MPEG-2 video [19]. To increase the chances of successful data embedding, [PERSON] et al. exploit motion vector differences (MVD) and transformed coefficients in HEVC video [20]. Recently, [PERSON] et al. proposed to associate payload bit with a motion vector predictor (MVP), which is utilized during inter picture prediction, to embed data without changing any MVs [21]. Changing the MVP causes insignificant/nearly no distortion to the video, although the difference between the MVP and the actual coded MV (denoted by MVD) is modified to embed data. While both the intra prediction mode and MVs are attractive techniques to embed data, there are situations where data cannot be embedded into a block. It is because the syntax element exploited for data embedding is simply unavailable; for example, the intra prediction mode is non-existence in an inter predicted block, hence the intra prediction mode cannot be exploited. Taking a different approach, [PERSON] et al. manipulate the prediction block (PB) size and nonzero AC coefficients to increase the capacity [22], while [PERSON]  
Figure 1: Flow of processes in the proposed method. associates each payload bit to a split flag which is utilized in HEVC to indicate whether a block is split into smaller blocks [23]. [PERSON] et al. divide block partition modes into three groups to represent different data bits [24]. Although data are also embedded into the transformed coefficients [22; 25; 26; 27], there are several drawbacks. Specifically, embedding data into the mid-frequency and high-frequency AC coefficients usually results in bit rate overhead, while embedding data in the low-frequency coefficients results in perceptual quality degradation. In addition, manipulating DC coefficients will change the entire block of pixel values. Therefore, embedding data into transformed coefficients may cause perceptible and notably degradation, which must be implemented appropriately in order to minimize the embedding artifact as well as error propagation.