Appearance indexing
Although it is very hard to quantify the visual impression of an image, we conjecture that the overall visual appearance of an image may be the combined effect of the appearances of image patches of various shapes and sizes. We can imagine that all possible appearances of image patches of all possible shapes and sizes form a conceptual appearance space. Each point in the appearance space therefore corresponds to a certain combination of the object parameters (shape, size, surface texture, pose, orientation, etc) and the imaging conditions (illuminating source, viewing angle, sensor response etc.). Using real sensor data and unsupervised learning, statistically most representative appearance prototypes can be found to approximate the appearance space. Statistics of these appearance prototypes present in an image therefore characterize the image content, which in turn can be used to perform tasks such as content-based image retrieval.