# Intra Prediction-Based Hologram Phase Component Coding Using Modified Phase Unwrapping
###### Abstract  
In this paper, we propose a method for compressing the phase component of a full-complex hologram. The JPEG (Joint Photographic Experts Group) Pleno is undergoing standardization for compressing full-complex holograms. If the full-complex hologram is compressed in the form of amplitude and phase components, the three-dimensional information of the hologram may be better preserved. Therefore, in order to solve the disadvantages of the method of independently compressing real and imaginary parts, we propose a method for directly compressing phase information. We select the HEVC (High Efficiency Video Coding), which has the best performance in compressing holograms from previous studies, as the anchor codec, and propose an algorithm for converting the phase information into the form suitable for the HEVC. Since the phase component is very random, we propose a modified phase unwrapping technique to improve this. In addition, in order to make good use of the property of HEVC Intra coding, the phase unwrapping considering Intra prediction is applied, and the most suitable HEVC Intra coding condition is searched. Compared with the result of compressing the phase using the HEVC, the hologram was improved by 2 dB or more and the reconstruction result was improved by more than 4 dB at a compression ratio of 80:1. If the compression ratio is increased, the proposed method has better results. hologram compression; phase unwrapping; HEVC Intra prediction; full-complex hologram +
Footnote †: journal: Applied Sciences  
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imaginary numbers (IM), or amplitude (AM) and phase (PH). Since these pairs display the same information, but show different characteristics, there is a possibility that the effect on compression will be different. In the previous study, the compression effect of the hologram did not appear in the case of the AM/PH pair, and the compression ratio of about 3:1-4:1 was shown in the case of the RE/IM pair [6]. A compression method using hologram resampling was also tried [8]. The classical form of resampling studies have been mainly conducted before [6], and because of the low compression ratio, few studies have been published recently. In terms of research results, the bilinear interpolation was the best result, followed by the bicubic interpolation, and finally the nearest neighborhood interpolation method [6]. Research that analyzed the effect of quantization on holograms can be seen in several research cases so far. In the past, uniform quantization was mainly used, and scalar quantization, which was considered in units of hologram pixels, was mainly considered [6]. Recently, research on vector quantization that performs quantization in block units is also being conducted [9]. In addition, scalar quantization and a uniform quantizer as well as a non-uniform quantizer were studied. Looking at the research results using normalized standard deviation (NSTD), the uniform quantizer shows good performance in the case of assigning low quantization bits (high compression), and the K-medians clustering method in the case of assigning high quantization bits (low compression) showed good performance. However, the difference is very small. Overall, the K-means clustering method showed low performance for all quantization bits. The results of this study show that vector quantizers with high computational complexity and complex algorithms do not necessarily show good performance for quantization of holograms. Recently, various studies have been carried out to compress holograms using standard codecs such as JPEG, JPEG2000, AVC, and HEVC, and benchmarking the compression efficiency between them [10; 11; 12]. There are several common considerations in the way standard codec is used. The first is to convert a complex hologram in the form of floating-point into 8-bit integer data for input to the standard codec. The second is to adjust the parameters of the codec. Each codec has various parameters that can be adjusted.