Figure 5.

Skeleton fitting is based on 18 keypoints, distinguishing left and right arms and legs [18]. We use the nine keypoints highlighted with stars. The upper keypoint among those and the lower are used to compute height h, which is used as scaling factor for normalizing the keypoint coordinates. Then, using the normalized keypoints, different features based on relative angles and distances are computed as features. For instance, to the right, we see several examples: (1) distance in the x (column) and y (row) axes and Euclidean distance between two keypoints ($\Delta x$, $\Delta y$, $\parallel v\parallel$); (2) angle between two keypoints ($\theta$); (3) the three angles of a triangle formed by three keypoints. After normalizing by h these seven values, they become components of the feature vector ${\psi }_i$ of frame i. Computing similar values by taking into account all the keypoints we complete ${\psi }_i$.