SAM: Steady Affine Motions

An affine motion is a continuous map from time value t to an affinity At. It is a SAM (Steady Affine Motion), when At = A . Although the beauty of a motion is subjective, the above equation provides one mathematical characterization and includes the screw (“universal instantaneous”) motion and the golden (“mirabilis”) spiral. Although a real matrix, A, may not exist, we show that it does for a dense set of affinities A covering a significant range of rotations and shears around the identity and that it may be computed efficiently and robustly in two and three dimensions using closed form expressions. SAMs have remarkable properties. For example, the velocity of any point remains constant, both in the global (fixed) and local (moving) frames, which facilitates the exact computation of derived entities, such as the envelope surfaces used to define the boundary of a swept volume. We say that a pattern of features Fi is steady when there exists an affinity M such that Fi = M i F0. Each M i is a frame of a SAM and may be computed as A i n , where A is the affine relation Fn = A F0 between the first and the last feature. This option makes it possible to edit directly the feature count n or the cumulative