An image stored in image database systems is assumed to be associated with some content-based meta-data about that image, that is, information about objects in the image and absolute/relative spatial relationships among them. An image query for such an image database system can generally be handled in two ways: exact picture matching and approximate picture matching. We address the approximate picture matching problem of central interest in this paper, and present a stepwise approximation of

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... ractable spatial constraints in an image query. Especially, this stepwise approximation may be pre-processed on an image query before an advanced picture matching algorithm is invoked. We then work out details of the stepwise approximation algorithm by analyzing, one by one, all possible 16 cases for results of the object matching step. Our analysis turns out that only 13 cases are valid, while the other 3 cases are identified impossible for finding an exact picture-matching between a query picture and a database picture. The worst-case running time complexity is given for each of them. In order to reduce the number of database pictures being matched by a user query, we also provide two suggestions to help enhance the effectiveness of image retrieval at the additional time cost.