An image processing apparatus for finding reflectivity based on a BRDF model expressing a ratio of reflection of light incident upon one point of a surface of an object to be drawn at the object surface, the image processing apparatus having an operation unit for calculating the reflectivity based on a BRDF model calculated by a quadratic-form matrix expression including a vector comprised of a light source direction vector, a viewpoint direction vector, and a normal direction vector and a matrix determining the characteristics of the BRDF model, thereby able to achieve both a variety of expression power and good operation efficiency when mounted in a programmable pixel shader and further having enough of a compactness to easily cope with a shift-variant BRDF, and a method of the same.