Thanks to all,  I have no more doubts with that vectors :o)
But I have a small question . To compute diffuse lighting povray uses
lambertian reflection of light on surfaces, to simplify it just computes
the dot product between the incoming light  and the normal_layer, with
normalized vectors it  returns the cos of the angle bewteen that 2 vectors.
But why using fabs(Cos_Angle_of_Incidence)  ? If Cos_Angle_of_Incidence is
negative it seems that the geometric angle between normal and incoming light
is greater than pi/2. I understand that computing negative intensity isn't a
good choice :o) but why removed it by fabs(Cos_Angle_of_Incidence), this
value isn't more  linked to the 'real' angle between normal and incoming
light.

Mathieu

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