A physically valid BSDF gives the same value when incoming and outgoing directions are swapped. Bidirectional algorithms depend on it: without it, a path built from the light differs from the same path built from the camera.
supersedescorrects · extends
classifiesspecializes · part-of
substitutes forapproximates · alternative-to
depends onrequires · validates
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validatesis a test for BSDFswapping the directions must not change the value