View Invariant Matching
-- Rank Theorem (1)  (Tomasi & Kanade)
•Take 3D trajectory as 3D object.
•S is a set of 3-D instants and Πs are projection matrices for different view points, then we can arrange image coordinates in observation matrix M as follows:
S is 3 × n matrix, projection matrix is 2k × 3 matrix, then the rank of M is at most 3.
M is a 2k by n matrix
If we take 3d trajectory as 3D points, we can use rank theorem to determine if two action trajectories are from the same action
The rank of the observation matrix of a 3D action trajectory is at most 3.
S is a set of 3-D points and Πs are projection matrices
for different view points, then we can arrange image
 coordinates in matrix M