Limitations of Content-based Image Retrieval

© Copyright 2008 by T. Pavlidis

Appendix D: Edge Histograms Provide No Semantic Information Either

It is not only color histograms that do not provide semantic information. Edge Histograms (another popular feature) do not provide it either. For each of the six images used in the discussion of Color Histograms and a seventh one ("Tomb") as well as their histogram equalized version I computed an edge strength histogram and then computed the normalized distance (percent of average image area) for all the 91 pairs. The results were sorted according to the computed distance. If such histograms provided semantic information, one would expect that the smallest distances would occur between an image and its equalized version. But this happened in only two cases, the images of "dogs" and "people" (marked by green). For the other five images the distance between the pair (marked in dark red) that are closest semantically is larger than that of unrelated pairs.

A Note on Details: The edge strength of a pixel is computed as the largest of the vertical and horizontal gradients at that pixel. The table has been generated by a program that looks at all possible distinct pairs of images in a folder. Only format editing was done before it was incorporated in this document.