I've never really bought into the Kanizsa Triangle as an illusion. If you where to take an actual triangle and block out the same areas with tree branches and such. No one would call it an illusion if you noticed the triangle.
the jastrow illusion would work if one looks at the objects in a 3 dimensional way: the 'above' object is in the back of the 'below' object. so the front object and the back object are then parts of a 3 dimensional model, i would say. if my eyes do not deceive me. :)