for romb by Infra H.D.C.

lambda := sqrt( sqr(x2-x1) + sqr(y2-y1) + sqr(z2-z1) ) / c
x3 := (x2-x1) / lambda + x1
y3 := (y2-y1) / lambda + y1
z3 := (z2-z1) / lambda + z1