# 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)
### Picture Partitioning  
The way in which a video frame is split into a set of non-overlapping blocks is called picture partitioning. These non-overlapping blocks are arranged into a quadtree structure, where the root is called a coding tree unit (CTU) [16], and each CTU is further partitioned into smaller blocks. Figure 1 shows the division of a \(1280\times 720\) pixel frame into 240 CTUs, split into 20 columns by 12 rows, where the last row is composed of incomplete CTUs. The complete CTUs are composed of \(64\times 64\) pixels. Two of the major differences between HEVC and JEM are the way in which a CTU is further partitioned and the size of the CTU itself. In HEVC, the maximum CTU size is \(64\times 64\) pixels, and there is the option to further recursively partition it into four square coding units (CU) whose sizes range from \(64\times 64\) pixels (i.e., no partitioning) to \(8\times 8\) pixels. The leaf blocks in a CU quadtree form the roots of two independent trees that contain prediction units (PUs) and transform units (TUs). A PU can have the same size as the CU, or can be further split into smaller PUs of up to \(8\times 8\) pixels. The PUs store the prediction information in the form of motion vectors (MVs). In intra-prediction mode, HEVC uses a quadtree structure with only square PUs, while in inter prediction mode, asymmetric splitting of PUs is possible, giving up to eight possible partitions for each PU block: 2N \(\times\) 2N, 2N \(\times\) N, N \(\times\) 2N, N \(\times\) N, 2N \(\times\) nU, 2N \(\times\) nD, nL \(\times\) 2N and nR \(\times\) 2N. 1,0,0 Figure 2 shows an example of a CTU partition in HEVC and the relationship between CU partitioning, PU partitioning and TU partitioning. The picture partitioning schema is modified in JEM in order to simplify the prediction and transform stages, and further partitions of CUs to form PU and TU trees are avoided. The JEM partitioning schema, called quadtree plus binary tree (QTBT), offers a better match with the local characteristics of each frame [17]. The highest level is a CTU, as in HEVC,  
Figure 1: Division of a \(1280\times 720\) pixel frame into CTUs: 240 (\(20\times 12\)). Figure 2: HEVC QT partition schema and the relationship between CU, PU and TU partitioning. but the main change is that block splitting below each branch is a binary partition giving the leaves. The size of the CTU is larger than in HEVC, with a maximum of \(256\times 256\) pixels, and only the first partition needs to be square partitioned. Lower partitions can be partitioned further in a quadtree schema, but at the desired level the binary tree ends the partitioning schema. There are two types of splitting in the binary tree: symmetric horizontal and symmetric vertical. The binary tree leaf node is the CU, which is used for prediction and transformation with no further partitioning. Hence, in most cases, the CU, PU and TU have the same size. An example of QTBT is shown in Figure 3; here, the quadtree has two levels (continuous line), after which the binary tree starts (dotted lines). In JEM, a CU can have either a square or a rectangular shape, and consists of coding blocks (CBs) of different color components; for example, a CU may contain one luma CB and two chroma CBs in the YUV420 chroma format. In HEVC, inter prediction for small blocks is restricted in order to reduce memory accesses for motion compensation, i.e., bi-directional prediction for \(4\times 8\) and \(8\times 4\) blocks is not allowed, and \(4\times 4\) inter-prediction is also disabled. In QTBT, these restrictions are removed, which increases the computational cost of the JEM codec. The CUs are not partitioned further for transforming or prediction unless the CU is too large for the maximum transform size. The maximum transform size is \(128\times 128\) pixels, which improves the coding efficiency for higher resolution video, e.g., 1080 p and 4 K sequences.