motion is an important clue for industrial inspection, video surveillance, and service machines to localize and recognize products and objects. because blur co-occurs with motion, it is desirable for developing efficient and robust motion blur detection algorithm. however, existing algorithms are inefficient for detecting spatially varying motion blur. to deal with the problem, this paper presents a theorem, according to which, motion blur can be efficiently detected and segmented. according to the theorem, the proposed algorithm requires a simple filtering operation and variance computation. classification as either blurred or unblurred pixel can be done by substituting the variance into the proposed simple formula and checking the sign of the resulting value. moreover, a geometric interpretation and two extensions of the algorithm are given. importantly, based on the geometric interpretation of the indicator function, we develop a one-class classifier, which is more effective than the indicator function and has comparable computational cost of the indicator function. experimental results on detecting motion-blurred cars, motorcycles, bicycles, bags, and persons demonstrate that the proposed algorithm is very efficient without loss of effectiveness.