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Why is the outgoing radiance equal to 0 when the angle between n and l is <= 0?

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This is from the book Real-Time Rendering.

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McLovin
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I thought this was a very interesting question and I couldn't explain it myself. So I emailed the authors of Real-Time Rendering and one of the said that it's most likely a mistake. It really should be cos(theta_i) <= 0 gives L(v) = 0. This means that at 90 degree angle or light projected behind the object, there is no contribution to the final value.

Justin Lam
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  • I actually asked veterans in GameDev the same question and they think it's a mistake as well. Funny, I kept thinking I missed something about the equation, but it really was a mistake _in the book_! – McLovin May 24 '16 at 16:30