1. A method to calculate an element R(x, y) of a viewpoint-independent spatial covariance matrix of a given object in a database for infinitely dense sampling of illumination conditions, the method comprising: calculating E(u x, u y, c), an integral over all possible light directions, weighted by their respective intensities, according to the equation: E ( u x,u y,c )&equals;∫ n&egr;S 2 L ( n ) D ( n,p,q ) where n&egr;S 2 is a particular direction on the unit sphere, S 2, where L is the light intensity and n is a normal vector on the unit sphere S 2, where u x and u y are the unit vectors of the special coordinate system defined by p and q, where p and q are vectors in which the x-axis is the bisector of the angle, 2&thgr;, between them, where p&equals;(c, &plus;s, 0) and q&equals;(c, −s, 0), where c&equals;cos &thgr;, s&equals;sin &thgr;, and (x, y) is the image point; solving for the viewpoint-independent spatial covariance matrix R(x, y) according to the equation: R ( x,y )&equals;&agr;( x )&agr;( y ) E ( u x,u y,c ) where &agr;(x) is the intrinsic reflectance at the image point x and &agr;(y) is the intrinsic reflectance at the image point y; and tabulating and storing the result of said viewpoint-independent spatial covariance matrix R(x, y).