Section: Scientific Foundations

Modelling, construction and structuring of the feature space

Participants : Nozha Boujemaa, Mohamed Chaouch, Jean-Paul Chièze, Amel Hamzaoui, Nicolas Hervé, Alexis Joly, Ahmed Rebai, Anne Verroust-Blondet, Itheri Yahiaoui.

The goal of the IMEDIA team is to provide the user with the ability to do content-based search into image databases in a way that is both intelligent and intuitive to the users. When formulated in concrete terms, this problem gives birth to several mathematical and algorithmic challenges.

To represent the content of an image, we are looking for a representation that is both compact (less data and more semantics), relevant (with respect to the visual content and the users) and fast to compute and compare. The choice of the feature space consists in selecting the significant features , the descriptors for those features and eventually the encoding of those descriptors as image signatures .

We deal both with generic databases, in which images are heterogeneous (for instance, search of Internet images), and with specific databases, dedicated to a specific application field. The specific databases are usually provided with a ground-truth and have an homogeneous content (faces, medical images, fingerprints, etc.)

Note that for specific databases one can develop dedicated and optimal features for the application considered (face recognition, etc.). On the contrary, generic databases require generic features (colour, textures, shapes, etc.).

We must not only distinguish generic and specific signatures, but also local and global ones. They correspond respectively to queries concerning parts of pictures or entire pictures. In this case, we can again distinguish approximate and precise queries. In the latter case one has to be provided with various descriptions of parts of images, as well as with means to specify them as regions of interest. In particular, we have to define both global and local similarity measures.

When the computation of signatures is over, the image database is finally encoded as a set of points in a high-dimensional space: the feature space.

A second step in the construction of the index can be valuable when dealing with very high-dimensional feature spaces. It consists in pre-structuring the set of signatures and storing it efficiently, in order to reduce access time for future queries (tradeoff between the access time and the cost of storage). In this second step, we have to address problems that have been dealt with for some time in the database community, but arise here in a new context: image databases. The diversity of the feature spaces we deal with force us to design specific methods for structuring each of these spaces.