Vanishing point as the "direction point", as lines having the same directional vector, say "D", will have the same vanishing point. Mathematically, let be a point lying on the image plane, where is the focal length (of the camera associated with the image), and let be the unit vector associated with , where . If we consider a straight line in space with the unit vector and its vanishing point , the unit vector associated with is equal to , assuming both are assumed to point towards the image plane. When the image plane is parallel to two world-coordinate axes, lines parallel
Tricritical point more components. It can be shown that three is the "minimum" number of components for which these points can appear. In this case, one may have a two-dimensional region of three-phase coexistence ("F" = 2-3+3 =2) (thus, each point in this region corresponds to a triple point). This region (F=2, P=3) will terminate in two critical lines of two-phase coexistence; these two critical lines (F=1, P=2) may then terminate at a single tricritical point. This point (F=0, P=1) is therefore "twice critical", since it belongs to two critical branches.<br> Indeed, its critical behavior is different from that of a conventional