A commonly held assumption is that in order to solve this ambiguity, the visual system combines optic flow information with extraretinal and proprioceptive information about the observer's motion (Ono & Steinbach,
1990; Rogers & Rogers,
1992; Dijkstra, Cornilleau-Pérès, Gielen, & Droulez,
1995; Wexler, Lamouret, & Droulez,
2001; Wexler, Panerai, Lamouret, & Droulez,
2001; Panerai, Cornilleau-Pérès, & Droulez,
2002; Peh, Panerai, Droulez, Cornilleau-Pérès, & Cheong,
2002; van Boxtel, Wexler, & Droulez,
2003; Wexler,
2003; Wexler & van Boxtel,
2005; Jaekl, Jenkin, & Harris,
2005; Colas, Droulez, Wexler, & Bessière,
2007; Dyde & Harris,
2008; Dupin & Wexler,
2013). Since surfaces in the world are mostly stationary, an accurate measurement of the observer's egomotion provides a direct estimate of
ωr and thus of
σ from
def, since
σ = tan
−1(
def/
ωr). Therefore, an optimal-observer model combining sensory information (retinal and extraretinal) with a
prior for stationarity is able to accurately derive the metric structure of the two-plane configuration. Note that a prior for stationarity also implies
a prior for rigidity (Ullman,
1979; Grzywacz & Hildreth,
1987). Since static surfaces do not change their structure during the observer's motion, most 3-D transformations generated by egomotion are necessarily rigid. For example, in the case of the two-plane configuration, if the surfaces are static in the world, then the dihedral angle between the two surfaces does not change either. In the remaining part of this article, we will therefore refer to this prior as to the
stationarity/rigidity prior.