visual looming navigation cue: A unified approach

This research introduces a unified approach to visual looming. Visual looming is related to an increasing projected size of an object on a viewer's retina while the relative distance between the viewer and the object decreases. Psychophysicists and neurobiologists have studied this phenomenon by observing vision and action in unison and have reported subject's tendency to react defensively or using this information in an anticipatory control of the body. Since visual looming induces senses of threat of collision, the same cue, if quantified, can be used along with visual fixation in obstacle avoidance in mobile robots. In quantitative form visual looming is defined as the time derivative of the relative distance (range) between the observer and the object divided by the relative distance itself. The visual looming is a measurable variable. Following the paradigm of Active Vision the approach in this research uses visual fixation to selectively attend a small part of the image, that is relevant to the task. Visual looming provides a time-based mapping from a "set of 2-D image cues" to "time-based 3-D space". This research describes how visual looming, which is a concept related to an object in the 3-D world, can be calculated studying the relative temporal change in the following four different attributes of a sequence of 2-D images: (i) image area; (ii) image brightness; (iii) texture density in the image; (iv) image blur. From a simple closed form expression it shows that a powerful unified approach can be adopted in these methods. An extension of this unified approach establishes a strong relationship with the Weber-Fechner law in Psychophysics. The four different methods explored for the calculation of looming are simple. The experimental results illustrate how the measured values of looming stay close to the actual values. This research also introduces one important visual invariant $\Re$ that exists in relative movements between a camera light-source pair and a visible object. Finally, looming is used in the sense of a threat of collision, to navigate in an unknown environment. The results show that the approach can be used in real-time obstacle avoidance with very little a-priori knowledge.