| http://www.w3.org/ns/prov#value | - (max {v ??? d, 0})GL_SPOT_EXPONENT where: v = (vx, vy, vz) is the unit vector that points from the spotlight (GL_POSITION) to the vertex. d = (dx, dy, dz) is the spotlight's direction (GL_SPOT_DIRECTION), assuming the light is a spotlight and the vertex lies inside the cone of illumination produced by the spotlight.
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