  John's differential equation and its canonical form, the ultrahyperbolic
equation, plays important role in lightfield imaging. The equation describes a
local constraint on the lightfield, that was first observed as a
"dimensionality gap" in the frequency representation. Related to the
ultrahyperbolic equation, Asgeirsson's theorems describe global properties.
These indicate new, global, constraints on the lightfield. In order to help
validate those theorems on real captured images, we introduce a coordinate
system for the lightfield, which suits better the Asgeirsson theorems, and
analyze behaviour in terms of the new coordinates.
