Blobworld

Developer

Computer Science Division, University of California, Berkeley.

URL

http://elib.cs.berkeley.edu/photos/blobworld/. A demo of Blobworld is available at http://elib.cs.berkeley.edu/photos/blobworld/start.html.

References

[CTB⁺99].

Features

The features used for querying are the color, texture, location, and shape of regions (blobs) and of the background. The color is described by a histogram of 218 bins of the color coordinates in Lab-space. Texture is represented by mean contrast and anisotropy over the region, as the 2D coordinate $(contrast, contrast \times anisotropy)$. Shape is represented by (approximate) area, eccentricity, and orientation.

Querying

The user first selects a category, which already limits the search space. In an initial image, the user selects a region (blob), and indicates the importance of the blob (`somewhat', `very'). Next, the user indicates the importance of the blob's color, texture, location, and shape (`not', `somewhat', `very'). More than one regions can be used for querying.

Matching

To match two color histograms h₁ and h₂, the quadratic form distance is used: d(h₁, h₂)=(h₁-h₂)^TA(h₁-h₂), where A=(a_ij) is a symmetric matrix of weights representing the similarity between color bins i and j. The distance between two texture descriptors is the Euclidean distance between their coordinates in representation space. The distance between centroids is the Euclidean distance. The distances are combined into a single final distance.

Indexing

Rather than actually computing the distances between the full color histogram vectors of length 218 as d(h₁, h₂)=(h₁-h₂)^TA(h₁-h₂), singular value decomposition (SVD) is used to project the histogram vectors onto a lower-dimensional subspace. The resulting points are indexed by an R^*-tree [BKSS90].

Result presentation

The retrieved images are ranked in linear order, and presented together with the segmented version showing the regions, see figure 4.

Applications

The demo on the web provides retrieval from a collection of 10000 Corel stock photos.

  

Figure 4: Blobworld query result.