# A Fast Algorithm for VVC Intra Coding Based on the Most Probable Partition Pattern List
## 3 Proposed Method
### Reference Line for Quick Decision  
VVC has increased the number of intra prediction modes to 65, and it introduced the multiple reference line (MRL). The traditional intra prediction mode only refers to the reference pixels in the adjacent left column and the upper row when completing predictions. MRL extends this to three rows and three columns. In MRL, in order to obtain the optimal intra prediction angle, MRL references line 0, line 1, and line 3. Line 0 and line 1 can provide reference information for nearby pixels and line 3 provides reference information for distant pixels. Line 2 is difficult but provides useful information, and so no reference is made. In the calculation process, the three lines are tested in turn, and then the line or column with a smaller RDC is selected as the reference line or reference column, which increases the computational complexity of the intra prediction. We randomly selected the reference sequence in the VVC standard and counted the proportion of different reference lines or columns selected as the optimal reference lines during the intra prediction process. The statistical results are shown in Figure 7, where FO, BQ, KI, BA, and JO correspond to the coding sequences FoodMarket4, BQMall, Kimonol, BasketballPass, and Johnny, respectively. Based on the statistical data shown in Figure 7, it is not difficult to determine that line 0 was selected as the reference pixel for prediction in most cases while the probability of reference line 3 being selected as the reference pixel was very small, with an average of approximately 8%, and so reference line 3 was skipped in the vast majority of cases. Figure 6: The flowchart of the proposed algorithm. Reference line 3 was added to the reference line as we hoped that it could provide different reference information from lines 0 and 1. The difference in this reference information was directly reflected in the difference between the reference pixel contents. We measured this difference by comparing the pixel content similarity (PCS) of reference line 3 and reference line 2. The calculation formulas used are shown below, where PCS\({}_{\text{H}}\) is the content similarity between reference line 3 and reference line 2, PSH\({}_{\text{V}}\) is the content similarity between reference column 3 and reference column 2, \((x,y)\) is the starting coordinate of the current prediction block, and \(w\) and \(h\) are its width and height. \[\text{PCS}_{H}= \frac{\sum\limits_{i=x}^{x+w}\left[p(i,y-4)-\frac{p(i,y-2)+p(i,y-1 )}{2}\right]}{\sum\limits_{i=x}^{x+w}\left[p(i,y-3)-\frac{p(i,y-2)+p(i,y-1)}{2}\right]} \tag{15}\] \[\text{PCS}_{V}= \frac{\sum\limits_{i=y}^{y+h}\left[p(x-4,i)-\frac{p(x-2,i)+p(x-1,i)}{2}\right]}{\sum\limits_{i=y}^{y+h}\left[p(x-3,i)-\frac{p(x-2,i)+p(x-1,i)}{2 }\right]}. \tag{16}\]  
In Formulas (15) and (16), if reference line 3 is similar to reference line 2, the difference between them and other reference lines is closer. If the pixel content similarity between reference line 3 and reference line 2 is between \(Re\) and \(1/Re\), this means that reference line 3 and reference line 2 have a high degree of similarity in pixel content. At this time, reference line 3 was difficult to use for providing useful reference information compared to reference line 2, and the calculation for reference line 3 was skipped in subsequent calculations. In order to determine the optimal threshold m, we counted the accuracy rate (\(Ac\)) of reference line 3 as the final reference line under the different thresholds. The calculation formula used for \(Ac\) is as follows:  
\[Ac=Z_{m}/Z_{vice}, \tag{17}\]  
where \(Z_{m}\) is the probability that reference line 3 will be selected as the final reference line under the \(Re\) threshold and \(Z_{vice}\) is the probability that reference line 3 will be selected as the final reference line in the original VVC.