In this paper, we present a planar shape recognition approach based on hidden Markov model and autoregressive parameters. This approach segments closed shapes into segments and explores the characteristic relations between consecutive segments to make classification at finer level. The algorithm can tolerate a lot of shape contour perturbation and moderate amount of occlusion. Also, the overall classification scheme is independent of shape orientation. Excellent recognition result has been reported. A distinct advantage of the approach is that the classifier does not have to be trained all over again when a new class of shapes is added.
I. I N T R O D U C T I O NTwo dimensional shape classification is an important problem in computer vision and image processing. There currently exist many shape classification techniques. Some of the earlier techniques are summarized in 111. They include Fourier descriptors, the class of space domain or curve fitting methods featuring the chain code approach, the polygonal approximation scheme, the category of decomposition technique including the medial axis transform (MAT), decomposition at concave vertices, decomposition by clustering, and the scale domain representation. All of these methods have their advantages and limitations.Another very effective technique is the Autoregressive (AR) model approach [2-41. 100% correct classification results are reported in [3] and [4]. The major disadvantage is that the schemes are very sensitive to shape occlusion, even slight occlusion. In this case, only 35% to 70% recognition rates were reported. The schemes are also sensitive to shape contour distortion or perturbation.The reason for this is that these schemes model the whole shape with only one set of predictive parameters. If the shape contains a large number of sample points, and the contour varies radically, the shape may seem rather unpredictable. In this case, it is reasonable to think that an AR This work has been supported in part by National Science Foundation Grant MIP-8908082 model with finite number of parameters is not adequate for the whole shape. This problem inspires the consideration of another important technique in pattern recognitionthe hidden Markov model.Hidden Markov Model (HMM) explores the relationship between consecutive segments of a pattern to be classified. Each segment is relatively smaller, and therefore is easier to be characterized. HMM has been used successfully in many applications. For example, Rabiner e l a1 have applied HMM to speech recognition 15). Kundu e t a1 have developed a handwritten word recognition scheme using HMM [SI. In these applications, better results are achieved compared to other methods that treat the patterns as a single set of features.In this paper, we will present a technique combining HMM and AR model to recognize closed 2-D shapes. Our approach is to segment the 1-D representation sequence of a closed shape into several pieces, characterize each piece with AR parameters and get a vector sequence for each shape, and then ap...