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Quat

volume rendering and local illumination

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Quat    568
For local illumination of volume rendering, we assume the light reflects at the isosurfaces. And the common way to get the normal at a point on the isosurface is from the gradient. However, I have a question: How do we know if the gradient vector will be an inward or outward facing normal? I noticed that I was not getting correct lighting, but when I reversed the direction of the gradient, the lighting came out correct.

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Pragma    395
The gradient of a function always points in the direction in which the function increases. If your function is positive inside objects and negative outside, then the gradient faces inward. Otherwise it faces outward.

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