## 3 Action Classification Definition and the NPU RGB+D Dataset for Basketball  
### Action Classification Definitions for Basketball  
Technical actions in basketball are very complex and diverse, and proposing a standard action template to match all the actions of basketball players is impossible. Nevertheless, we can analyze the rules of basketball to get the common basic actions in basketball games. With the analysis of the changes of basketball rules of FIBA in the recent 10 years [35] as a basis, we found that shooting, free throw, dribbling, and other common actions play important roles in basketball games. Under the guidance of professional basketball coaches, we defined the actions that do not depend on the semantics of the game environment and established the judgment standard of basketball action classification according to the spatial positions of players' body parts. The basketball actions are divided into three categories: movement, ball control, and shooting, with a total of 55 complex movements. We further decomposed 55 kinds of complex actions into 32 kinds of atomic action combination, simplified the action category, shortened the longer action sequences, and reduced the difficulty of action recognition. In this paper, J represents the joint, followed by a number to indicate the joint number. For example, J1 represents the neck (The number 1 after J comes from joint 1 in Figure 1, and the following numbers after J all refer to Figure 1), J11 represents the right ankle, and the number of other joint points is shown in Figure 1. J is followed by a number in brackets, such as J(2-3), indicating the position between joint J2 and J3, that is, the position from the right shoulder to the right elbow. \"\(\angle\)\" refers to the angle of the joint, followed by three joint numbers to indicate the angle formed by the three joints. For example, \(\angle\)2-3-4 refers to the angle formed by the right shoulder, right elbow, and right wrist. The joint number is followed by \"x\", \"y\", \"z\" to represent the x-direction coordinate value, y-direction coordinate value, and z-direction coordinate value of the joint, respectively. All the classification and judgment conditions of basketball complex actions and their atomic action decomposition are shown in Figure 2.  
Figure 1: Configuration of 25 body joints in our dataset. The labels of the joints are: 0 = Nose, 1 = Neck, 2 = RShoulder, 3 = RElbow, 4 = RWrist, 5 = LShoulder, 6 = LElbow, 7 = LWrist, 8 = MidHip, 9 = RHip, 10 = RKnee, 11 = RAnkle, 12 = LHip, 13 = LKnee, 14 = LAnkle, 15 = REye, 16 = LEye, 17 = REar, 18 = LEar, 19 = LBigToe, 20 = LSmallToe, 21 = LHeel, 22 = RBigToe, 23 = RSmallToe, 24 = RHeel. R = right, L = left. The combination of 32 kinds of atomic actions can cover most of the complex basketball actions. As an attempt, we selected 12 kinds of basic atomic actions that are the most representative to collect and make the dataset. On this basis, we carried out experiments to set the benchmark of the dataset and test the effectiveness and robustness of the basketball action recognition algorithm. The basketball players' basic atomic actions cover three major aspects, including dribbling (standing dribble, moving dribble, etc. ), shooting (chest shoot, overhead shoot, etc. ), and passing and catching (chest passing and catching, overhead passing and catching). This classification not only reduces the number of categories but also avoids the problem that the same features exist in different actions, which leads to difficulties in distinguishing them. We invited 10 players from the NPU basketball team to complete these 12 types of action shooting tasks so as to ensure that the action data in our dataset conform to the basketball rules. We included the action styles of different players to enhance the representations and richness of the dataset. Detailed action classes can be viewed in Table 2.  
The above 12 classes of basketball players' actions cover all possible actions in basketball, and unmentioned actions can find some corresponding category.