  In this paper we explore how light propagates from thin elements into a
volume for viewing. In particular, devices that are typically connected with
geometric optics, like parallax barriers, differ in treatment with those that
obey physical optics, like holograms. However, the two concepts are often used
to achieve the same effect of capturing or displaying a combination of spatial
and angular information. This paper attempts to connect the two approaches
under a general framework based in ray space, from which insights into
applications and limitations of both parallax-based and holography-based
systems can be observed. We show that each display form can generate a light
distribution that can always be expressed as a rank-1 matrix. Knowledge of this
limitation is then discussed in the context of extending the capabilities of
current display forms by considering the use of partially coherent light.
