Trigonometric circle
sin(0) = 0
sin(PI/2 rad) = 1
cos(0) = 1
sin(a) = opp/hyp
cos(a) = adj/hyp (Trigo + Thales)
if you know the cosine, you don't know the sign of the angle !
u.v = 0 <=> (u,v) orthogonal, perpendicular (Maths)
u.v <= epsilon <=> (u,v) orthogonal, perpendicular (Computing)
(or u = 0, or v = 0 ...)
Length of the perpendicular projection of v on u ??
Length = u.v / || u || = (u/||u||).v
