## 2 Related Work
### Panoramic Live Broadcast  
Panoramic video is also called 360-degree video, and it can facilitate more immersive feelings in people than traditional video. Audiences can observe from different directions at the same position in the video scene and feel all the information around the scene. Since the video picture contains all the information around the scene, in order to ensure that the resolution of pictures from all angles is not lower than that of traditional video, there must be more data for panoramic video, and the demand for bandwidth is therefore larger [27]. The acquisition and production of panorama requires multiple fisheye lenses for picture acquisition, and the pictures of each lens are spliced to form a frame picture. The panoramic video image acquired synchronously is then presented on a spherical surface, but its data form is not suitable for storage, transmission, compression, or other forms of processing by classical methods. Thus, projection is required [14; 28]. The projection of panoramic video is the process of mapping the three-dimensional spherical information to a two-dimensional plane. Several projection methods used in the development of the Joint Video Exploration Team (JVET) coding standards are illustrated in [14]. As the main carrier of panoramic video, the projected image contains all the contents of the captured picture, and the distortion should be reduced as much as possible during splicing to improve the quality of picture playback. At the same time, [14] studied the impact of different projection methods on coding efficiency according to existing experiments. The resolution of a panoramic video is very high. A larger network bandwidth is therefore needed while also using traditional video compression methods. In order to reduce the pressure of the network bandwidth, it is necessary to optimize the coding method for projected pictures. The panoramic live broadcast system can be divided into three types according to different encoding optimization methods [15]: full-view video stream, viewport-based video stream, and tile-based video stream. The full-view video stream transmits the whole frame, using the same encoding transmission mode as the traditional video stream. This approach requires a high processor performance and network bandwidth. Viewport-based streaming provides a high-quality transmission for the part viewed by the user based on the position of the user's current viewpoint, while the transmission quality of the rest is low. On the client side, the streaming endpoint device detects the user's head movement and receives only the specific required video frame area, dynamically selecting the video stream region of the viewport and adjusting the viewport quality. In this way, the bitrate of the video stream can be reduced. The server stores multiple adaptive sets related to a user's direction and performs matching and viewport position prediction according to the network state. Ref. [29] proposed two dynamic viewport selection approaches, which adapted the streamed regions based on content complexity variations and positional information to ensure viewport availability and smooth visual angles for VR users. Tile-based coding technology is used to divide each frame in the video stream into multiple blocks. According to the location of the viewport, the resolution between different blocks will be different. For example, ref. [28] proposed a video streaming system that used the divide-and-conquer method to separate the video space into multiple blocks and encapsulate them during encoding. Ref. [30] proposed a sight-guidance scheme based on tile-based coding aiming at minimizing the weighted sum of the average traffic load and users' watching preference derivation. MPEG-DASH's (Dynamic Adaptive Streaming over HTTP) SRD (spatial relationship description) [31] is used to describe the relationship between blocks in the 360-degree space and is tiled in the field of view (FoV). Ref.