  Looming, traditionally defined as the relative expansion of objects in the
observer's retina, is a fundamental visual cue for perception of threat and can
be used to accomplish collision free navigation. In this paper we derive novel
solutions for obtaining visual looming quantitatively from the 2D motion field
resulting from a six-degree-of-freedom motion of an observer relative to a
local surface in 3D. We also show the relationship between visual looming and
surface normals. We present novel methods to estimate visual looming from
spatial derivatives of optical flow without the need for knowing range.
Simulation results show that estimations of looming are very close to ground
truth looming under some assumptions of surface orientations. In addition, we
present results of visual looming using real data from the KITTI dataset.
Advantages and limitations of the methods are discussed as well.
