Identification of classes

We assume that the image space consists of three classes, building, non-building, and intermediate, which are denoted as $\Omega_1$, $\Omega_2$, and $\Omega_3$, respectively. The intermediate class is added to account for the fact that some natural outdoor images are too ambiguous to reliably classify as either building or non-building, even for human operators.

Our representation of the images which pertain to these classes is better served by the following example. In figure 2 the sixth image in the first row and the second image in the second row belong to the building class. The first and the second images in the first row belong to the non-building class. The last image in the first row and the fourth image in the second row are examples of the images in the intermediate class.