# Robust Template-Based Watermarking for DIBR 3D Images
## 1 Introduction  
Three-dimensional (3D) content has been steadily increasing in popularity because of its excellent lifelike appearance. With the recent development of 3D display technology, many new 3D applications have appeared to maximize realism, such as head-mounted displays, 360\({}^{\circ}\) virtual reality and ultra-high definition 3D content. Consequently, interest in 3D content and in the 3D market itself have both increased greatly. Methods of representing 3D content are divided into stereo image recording (SIR) and depth-image-based rendering (DIBR). SIR stores the left and right views (as human eyes do) and provides a high-quality immersive view; however, this has many limitations, including a large data size, fixed depth, high cost and difficulty with multiple camera settings. Meanwhile, DIBR is a rendering method that creates various virtual viewpoint images using center and depth images [1; 2; 3; 4]. The DIBR method has two main advantages: (1) the DIBR system is able to easily save and transmit 3D content, because it requires less data compared to SIR; (2) the DIBR system can provide various viewpoints, since it allows us to adjust the 3D configuration. These advantages have led to DIBR technology being employed in 2D-to-3D conversion [5; 6; 7; 8; 9; 10] and auto-stereo [11; 12] and multi-view stereo [13; 14; 15; 16; 17], which provide various viewpoints depending on the user's position  
Copyright protection techniques for DIBR content have received considerable attention due to DIBR's important role and significant 3D content market growth. A typical copyright protection technique is watermarking, but many conventional two-dimensional (2D) watermarking techniques are difficult to apply to DIBR content. In the DIBR system, center image pixels are partially moved alongthe horizontal axis with distance depending on the depth image, using a process called non-linear geometric distortion. Watermarks inserted in the center image are strongly distorted and cannot be extracted. Hence, several watermarking methods have been proposed as being robust to the DIBR process. [PERSON] et al. proposed a method of embedding watermarks by predicting pixels' moving distance [18]. Protecting the center image and both the left and right images required superimposing and embedding three watermarks. This method has a low bit error rate (BER) against the DIBR process and common distortions, such as JPEG or additive noise. However, this method is vulnerable when the depth information is modified, such as in depth image preprocessing or the change of baseline distance, since the moving distance for pixels during the DIBR process is predicted with unmodified depth information. Additionally, this method is vulnerable to geometric attacks due to the characteristics of the discrete cosine transform (DCT) domain. [PERSON] et al. proposed a method of averaging the luminance of an image in the horizontal direction and then inserting a watermark in the averaged vector [19]. This method is robust against DIBR and signal processing attacks, but has the disadvantage of not considering geometric transformations. [PERSON] et al. suggested a watermarking method that employed quantization on dual tree complex wavelet transform (DT-CWT) domain coefficients [20]. The method used directional coefficients that were not significantly changed by the DIBR process. Row-wise quantization was performed to provide robustness to horizontal pixel shifts. The method showed robustness to DIBR; JPEG compression; image scaling; and DIBR configuration adjustments, such as depth image preprocessing and baseline distance changes. However, it was vulnerable to noise addition and geometric distortions. [PERSON] et al. [21], [PERSON] et al. [22] and [PERSON] et al. [23] proposed a DIBR watermarking method that used the scale-invariant feature transform (SIFT). The SIFT based-watermarking systems found similar parts between the center image and the synthesized image using a SIFT descriptor and then inserted the watermark into those parts.