If you know x, y, and z (which it sounds like you do) then "V" is the magnitude of the vector:
V = sqrt[x^2 + y^2 + z^2]
But you've already said that you have a "3D normal vector," which means the magnitude is 1 (and so is V then), so this question doesn't really make sense. Can you please clarify?
V = sqrt[x^2 + y^2 + z^2]
But you've already said that you have a "3D normal vector," which means the magnitude is 1 (and so is V then), so this question doesn't really make sense. Can you please clarify?