Reflectance ratios: An extension of Land's retinex theory

Neighboring points on a smoothly curved surface have similar surface orientations and illumination conditions. Therefore, their brightness values can be used to compute the ratio of their reflectance coefficients. We develop an algorithm that estimates a reflectance ratio for each region in an image with respect to its background. The region reflectance ratio represents a physical property that is invariant to illumination and imaging parameters and is an extension of Land's “Lightness” defined in his Retinex theory. The ratio invariant is used to automatically build representations of objects and recognize objects from a single brightness image of a scene.