# Load Balancing Strategies for Slice-Based Parallel Versions of JEM Video Encoder
## 2 Description of the New Characteristics of the Joint Exploration Test Model (JEM)
### Spatial Prediction  
In order to be able to capture the finer edge directions presented in natural videos, the directional intra-modes in JEM have been extended from 33, as defined in HEVC, to 65. The addition of planar and DC modes gives a total of 67 different prediction modes for JEM. These denser directional intra-prediction modes (see Figure 4) are applied to all PU sizes and both luma and chroma intra-predictions. The partitioning schema described in the previous section is directed by a rate-distortion optimization (RDO) algorithm that recursively searches for the best possible partitioning schema in terms of an R/D estimation. This algorithm tries all directional intra-modes for each of the possible partitions, (i.e., no partitioning, vertical partitions, horizontal partitions and quadtree partitions), at each recursion level, to find the one with the lowest cost. For a CTU in which the underlying scene is smooth, this recursion ends rapidly, and the number of trials for the 67 directional modes is therefore much lower than if the CTU belongs to a highly textured area within the scene. Thus, the computational effort is not evenly distributed over the CTUs in a video frame, and depends on the content of the scene. In JEM, the list of most probable modes (MPMs) is extended from the three used in HEVC to six, and the selection procedure for these modes is also changed. In HEVC, the method proposed in [18] was adopted in the standard for building the MPMs list. The 35 intra-modes are divided into two groups: three MPMs and 32 remaining modes. The three MPMs are derived based on the modes of the PUs to the left of and above the current PU. The new procedure followed in JEM [19] uses five neighbors of the current PU: left, above, above left, below left and above right, as shown in Figure 5[19]. \begin{table}
\begin{tabular}{l c c} \hline
**Characteristics** & **HEVC** & **JEM** \\ \hline CTU size & \(64\times 64\) & \(256\times 256\) \\ CTU partition & Quad-Tree with separate & Quad-Tree+Binary-Tree \\  & tress for CU, PU and TU & QTBT (shared by CU, PU, TU) \\ \hline Inter-Prediction & No bi-directional & Bi-directional allowed \\  & & for \(4\times 8\) and \(8\times 4\) sizes \\ \hline Max transform unit size & \(32\times 32\) & \(128\times 128\) \\ \hline \end{tabular}
\end{table} 
Table 1: Differences between HEVC and JEM with respect to picture partitioning. The improvements in JEM described in the following paragraphs, which increase the complexity of the encoder, can also lead to a load imbalance when parallel processing the slices of a frame. In addition to the changes in the JEM encoder mentioned above, there are also differences in the entropy coding of the MPM list between HEVC and JEM, as explained in [17; 19], which lead to a reduction of the contexts used in the entropic encoder to signal the MPM index from nine to three, corresponding to the vertical, horizontal or non-angular class MPM modes. The interpolation filters are also changed in JEM with respect to HEVC [15]. In HEVC, a two-tap linear interpolation filter is used to generate the intra-prediction block in the directional prediction modes (i.e., excluding the planar and DC predictors). In the JEM, four-tap intra-interpolation filters are used for directional intra-prediction filtering. Cubic interpolation filters are used for blocks smaller than or equal to 64 samples, and Gaussian interpolation filters are used for larger blocks. The filter parameters are set based on the block size, and the same filter is used for all modes. Another improvement to JEM is made in the boundary prediction filters [15]. In HEVC, after the prediction block has been generated for the vertical or horizontal intra-modes, the leftmost column or the top row of the predicted block are adjusted further using the values of the boundary samples. In JEM, the number of boundary samples is increased from one to four (rows or columns) in order to obtain the predicted value using a two-tap filter (for  
Figure 4: Extended (red) prediction modes in JEM [17].