Appropriate-scale local centers: a foundation for parts-based recognition

An image representation in terms of local centers is developed and motivated in the context of natural object recognition. Local centers are visually compact regions that have significant internal-external contrast in some measurement. Existing computational models of the concept are compared, and a particular model, the appropriate-scale ridge, is developed. In this model, local centers are defined as smoothed image extrema that are also extrema, with respect to scale, in the second spatial derivatives. The usefulness of the model is analyzed via 1D scale-space behavior and demonstrated on 2D natural images. The basic concept of a local center is also extended to color and shading data. The stability of the shading model is demonstrated on images of a moving face under varying illumination and tested via a face detection task.