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Based on physical laws and considered to be physically plausible, BRDFs have two properties: reciprocity and conservation of energy. The reciprocity property says that if the sense of the traveling light is reversed, say, swapping the incoming and outgoing directions, the value of the BRDF remains unchanged. Conservation of energy property states that the total quantity of scattering light during the light-matter interaction cannot exceed the original quantity of light arriving at the surface.