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Geometry of object and ground planes for an upright viewer. An eye at height h from the ground views a point on the ground at distance d. The line of sight is at angle θ relative to earth vertical. The ground plane is rotated by θ relative to a frontoparallel plane. The distance of the fixated point along the line of sight is z0. For a point slightly lower in the visual field, but also on the ground, the distance is z1. The angular difference between the vectors z0 and z1 is ε.
