# Frequency-Based Adaptive Interpolation Filter in Intra Prediction
## 2 Previous Works
### Related Studies  
Various studies have been conducted to improve the performance of interpolation filters. There are several approaches to improving prediction performance by increasing the length of the filters or by suggesting a filter selection method. [PERSON] et al. [22] proposed a 4-tap DCT-iFs for small prediction units (PUs) for intra prediction in HEVC. [PERSON] et al. [23] used 12-point Discrete Sine Transform (DST) interpolation filters and 11-point DST interpolation filters to replace 8-point DCT-iFs and 7-point DCT-iFs for inter prediction in motion compensation in HEVC to improve inter coding efficiency. [PERSON] et al. [24] proposed 6-tap filters to replace 4-tap DCT-IF filters, where the coefficients of 6-tap filters were obtained from a polynomial regression model. [PERSON] et al. [25] proposed 6-tap DCT-iFs and 6-tap Gaussian interpolation filters for blocks larger than 32 \(\times\) 32 blocks. In [22] and [23], the interpolation filters derived from DCT and DST highlight the high-frequency responses of integer samples in the prediction block. References [23; 24; 25] extend the interpolation filter to use more integer reference samples to apply long-tap DCT-IF and Gaussian IF compared with the existing interpolation filters in HEVC and VVC. [PERSON] et al. [26] proposed a method of selecting interpolation filters, which consists of 4-tap interpolation filters and 3-tap intra smoothing filters according to the smoothness of the reference samples. In this method, [1, \(-\)2, 1] the HPF filter is used as the reference samples for the integer-slope modes to compute the difference between the filtered samples and the unfiltered reference samples, where the pixel domain difference value determines which interpolation filters are applied or if none are applied. In VVC, alternative interpolation filters for half-pixel position depending on the motion vector accuracy for inter prediction are proposed by [PERSON] et al. [27]. [PERSON] et al. [28] proposed a filter selection to characterize the reference samples based on the block-size and QP value for the 4-tap DCT-IF and SIF selection. [PERSON] et al. [29] proposed a method of selecting filters for horizontal and vertical directions in AV1. In [26; 27; 28; 29], a filter selection method is proposed for selecting an interpolation filter that is better suited to the current block to improve the accuracy of the prediction block. Moreover, there are studies that apply filtering using a trained model based on Deep Convolutional Neural Network (CNN) rather than fixed filters. [PERSON] et al. [30] proposed a CNN-based fractional interpolation filter for Luma and Chroma components for inter prediction in HEVC. [PERSON] et al. [31] proposed an invertibility-driven interpolation filter using CNN in HEVC. The authors in [30; 31] alternatively applied both DCT-IF and CNN-based iFs to generate fractional positioned \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \(nTbS\) & **2** & **3** & **4** & **5** & **6** \\ \hline _intraHorVerDistThres[nTbS]_ & _24_ & _14_ & \(2\) & \(0\) & \(0\) \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Specification of _intraHorVerDistThres[nTbS]_ for various transform block size \(nTbS\) in VVC. samples more accurately. They achieved better coding efficiency than using fixed filters in HEVC inter prediction with more parameters and complexity. These studies show that the interpolation filter plays an important role in intra and inter coding to obtain better coding efficiency in video coding. Most of the methods applied long-tap filters or alternative filters based on filter selection methods in pixel domain to improve the video coding efficiency in intra and inter coding. However, in this study, we propose a new method that uses the newly derived 8-tap DCT-IF/SIF in this paper and the existing 4-tap DCT-IF/SIF which is used in the VVC standard based on high-frequency information after 1-D transform on reference samples. Finally, the proposed method improves coding efficiency in intra coding compared with the VVC standard.