COMPUTING REFLECTANCE RATIOS FROM AN IMAGE

A photometric invariant called reflectance ratio is presented that can be computed from a single brightness image of a scene. The brightness variation in an image of a surface depends on several factors; the th ree-dimensional shape of the surface, its reflectance properties, and the illumination conditions. Since neighboring points on a smoothly cu rved surface have similar surface orientations, their brightness value s can be used to compute the ratio of their reflectance coefficients. Based on this observation, an algorithm is developed that estimates a reflectance ratio for each region in the image with respect to its bac kground. The algorithm is computationally efficient as it computes rat ios for all image regions in just two raster scans. In the first scan, the image is segmented into regions using a sequential labeling algor ithm. During labeling, the reflectance ratio between adjacent pixels i s used as a measure of connectivity. In the second scan, a reflectance ratio is computed for each image region as an average of the ratios c omputed for all points that lie on its boundary. The region reflectanc e ratio is also a photometric invariant; it represents a physical prop erty of the region and is invariant to the illumination conditions. Se veral experimental results are included to demonstrate the invariance of reflectance ratios to imaging and illumination parameters. A brief discussion on the application of reflectance ratios to the problems of object recognition and visual inspection is also given.