The main purpose in the present paper is to build a Hamiltonian
theory for fields which is consistent with the principles of relativity.
For this we consider detailed geometric pictures of Lepage theories in
the spirit of Dedecker and try to stress out the interplay between the
Lepage-Dedecker (LP) description and the (more usual) De Donder-
Weyl (DDW) one. One of the main points is the fact that the Legendre
transform in the DDW approach is replaced by a Legendre correspondence
in the LP theory (this correspondence behaves differently:
ignoring the singularities whenever the Lagrangian is degenerate).