an effective and efficient image retrieval system is needed for finding useful images in large databases or on Internet.Because of the complexity of images
the traditional information retrieval techniques
which are based on value and/or text comparison
are not sufficient.Then the research is shifted to the techniques of content-based image retrieval (CBIR).In this paper
some key issues on CBIR are discussed:image features extraction
similarity measures
the expression of query conditions
retrieval techniques in compressed-domain
the performance evaluation
and so on.Some future research directions are also given in the conclusion.