Constraints for interpretation of line drawings under perspective projection

vector notation and then give two different representations. The first representation is the regular gradient space; it has the advantage of simplicity but contains important singularities. The second one is just a regular vector (p, 4, r) in 3-D with the constraint that p* + q* + r* = 1. (5) This can represent the normal of any plane in 3-D, with the added complexity that Eq. (5), a quadratic equation, should be included among the equations to be solved. 3.2. Shared Boundary Constraint This constraint relates the orientation of two planes intersecting using the image of the line of intersection. Similar results have been derived previously in [14]. Say two planes have normals Ni and N2 ; then the line, L = Rt + P, formed by intersection of these planes has orientation