The quadrifocal variety

Multi-view Geometry is reviewed from an Algebraic Geometry perspective and multi-focal tensors are constructed as equivariant projections of the Grassmannian. A connection to the principal minor assignment problem is made by considering several flatlander cameras. The ideal of the quadrifocal variety is computed up to degree 8 (and partially in degree 9) using the representations of GL(3)×4 in the polynomial ring on the space of 3×3×3×33×3×3×3 tensors. Further representation-theoretic analysis gives a lower bound for the number of minimal generators.