# Intra Prediction-Based Hologram Phase Component Coding Using Modified Phase Unwrapping
## 3 Phase Compression
### Structure of Codec  
Figure 6 shows the structure of the proposed compression codec. Figure 6a is the structure of the encoder, and Figure 6b is the structure of the decoder. In each figure, the gray part corresponds to the part to compress the phase.  
First, we observe the encoding process of Figure 6a. In consideration of the Intra prediction of the HEVC, the segmentation process is performed to divide the hologram into sub-holograms. The separated sub-holograms are unwrapped by considering the direction of the Intra prediction. Unwraped sub-holograms are normalized to an 8-bit width for input to the HEVC. The HEVC can also accept inputs in 10 bits, but we only consider 8-bit inputs to simplify the discussion. Finally, compression is performed using the HEVC.  
Next, we observe the decoding process in Figure 6b. The decoding is the reverse of the encoding process. The result decoded using the HEVC is transformed into an unwrapped phase after de-normalization. The unwrapped phase returns to the original phase values after the rewrapping process. Finally, the decoding result can be obtained by combining the separated sub-holograms.  
In H.264/AVC, Intra coding is based on spatial extrapolation of samples from previously decoded image blocks, followed by discrete cosine transform (DCT)-based transform coding [19]. HEVC utilizes the same principle, but further extends it to be able to efficiently represent wider range of textural and structural information in images. The following aspects were considered during the course of HEVC project leading to the selected Intra coding design [19].  
HEVC Intra coding is based on predicting the current block (\(R_{x,y}\) in Figure 7a) by spatially extrapolating the previously decoded left and upper blocks (\(P_{x,y}\) in Figure 7a). Intra coding of HEVC can support a larger block size than AVC and supports a consistent prediction method for various blocks. In addition, it is possible to efficiently predict not only a structure with a directionality (Angular mode, \(N=2\)\(\sim\)34 in Figure 7b) but also a uniform region (Planar and DC mode, \(N=0,1\) in Figure 7b). HEVC divides an image into coding units (CUs) for efficient coding, and performs Intra coding in units of prediction units (PUs) having various sizes and shapes from \(4\times 4\) to \(32\times 32\). For residual components generated after Intra prediction, DCT is performed in units of transform units (TU). A boundary smoothing filter can be used to reduce the discontinuity between blocks occurring during Intra prediction [19]. The HEVC supports two special coding modes such as the I_PCM and the transform skipping mode. I_PCM mode is used when efficient coding cannot be performed by other modes. In the I_PCM mode, prediction, transform, quantization, and entropy coding are all bypassed, and predefined bits are allocated. In the transform skipping mode, only transform is skipped.  
Figure 6: Proposed structure (**a**) encoder, (**b**) decoder. of the hologram codec.  
We use the HEVC as an anchor codec for compressing the phase information of the still hologram and the Intra prediction of HEVC. Therefore, the phase component of the hologram is transformed into a form suitable for the Intra prediction of the HEVC. As described above, since residual coding is performed on the difference value between \(P_{x,y}\) and the original phase after generating \(P_{x,y}\) using \(R_{x,y}\), the spatial feature of the unwrapped phase must be in a form that is good for predicting \(P_{x,y}\) from \(R_{x,y}\).