  We explore connections furnished by the Funk metric, a relative of the
Hilbert metric, between projective geometry, billiards, convex geometry and
affine inequalities. We first show that many metric invariants of the Funk
metric are invariant under projective transformations as well as projective
duality. These include the Holmes-Thompson volume and surface area of convex
subsets, and the length spectrum of their boundary, extending results of
Holmes-Thompson and \'Alvarez Paiva on Sch\"affer's dual girth conjecture. We
explore in particular Funk billiards, which generalize hyperbolic billiards in
the same way that Minkowski billiards generalize Euclidean ones, and extend a
result of Gutkin-Tabachnikov on the duality of Minkowski billiards.
  We next consider the volume of outward balls in Funk geometry. We conjecture
a general affine inequality corresponding to the volume maximizers, which
includes the Blaschke-Santal\'o and centro-affine isoperimetric inequalities as
limit cases, and prove it for unconditional bodies, yielding a new proof of the
volume entropy conjecture for the Hilbert metric for unconditional bodies. As a
by-product, we obtain generalizations to higher moments of inequalities of Ball
and Huang-Li, which in turn strengthen the Blaschke-Santal\'o inequality for
unconditional bodies. Lastly, we introduce a regularization of the total volume
of a smooth strictly convex 2-dimensional set equipped with the Funk metric,
resembling the O'Hara M\"obius energy of a knot, and show that it is a
projective invariant of the convex body.
