(© 2006 Oronzo Parlangeli & Sergio Roncato)
Fig. 1 and Fig. 2 show the same inducing pattern.
However, if the blue lines in fig 1 are seen as donors, we can easily appreciate that when they give birth to two clones as in fig 2, these clones grow apart and they show a diametrically opposed inclination: convex lines become concave and concave lines becomes convex.
A clone for each donor is sufficient to produce the effect.
Read more about the illusion and possible explanationsClones and Donors Have Opposite Inclinations (in Vision),