Coordinate system transformation and polar azimuthal orthographic projection of the center of mass of a protein residue. Every coat protein residue is represented as a point in space (black dots). Cartesian coordinates (x, y, z) are transformed into an spherical system (r, φ, ψ), setting, leaving all residues lying on the surface of a unit sphere. An orthogonal projection (parallel to the Z axis) onto a tangential plane through the north pole is then applied, ending up with a two dimensional φ-ψ polar map representation of the relative position of all coat protein residues.