An upper bound on the entropy of constrained 2D fields

An upper bound on the entropy of constrained 2D fields is presented. The constraints have to be symmetric in (at least) one of the two directions. The bound generalizes (in a weaker form) the bound of Calkin and Wilf (see SIAM Journal of Discrete Mathematics, vol.11, p.54-60, 1998) which is valid only for processes with symmetric transfer matrices. Results are given for constraints specified by run-length limits and minimum distance between pixels of the same color.