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So the Diffuse exitance of the Lambert equation is: DiffuseColour(x)(Irradiance*cos(theta)) I think I understand this. Its just measuring the light power at a plane parallel to the light direction to get the Irradiance over a unit area. Which is the maximum amount of light energy a surface could recieve, due to the fact its perpendicular with the light direction. This is then scaled by the actual angle of the surface to the light. But then the next step is to divide this by pie. Why is this so?

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Pi is a mathematical constant, pie is a dessert. Critical distinction :)

To answer your question, we do this division in order to account for the fact that while reflected radiance is given over the entire hemisphere (and uniformly so for Lambertian shading) we're interested only in what's coming our way and not all that extra stuff reflected off into other directions. You generally won't see this division too often in real-time shaders simply because it's possible to account for this in the light intensity, but it is implied.

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Hah ! Im always doing that :P

I thought it may have something to do with the integral over the hempisphere. Would anyone mind going over how it is derived though?

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