Geometry by deflaring

Stray light reflected by lens surfaces creates flare which affects the image. A pronounced form of this flare is aperture ghosting, where bright spots that resemble the shape of the lens aperture are overlayed on the image. This might disrupt image analysis. It occurs when a bright narrow source (usually the Sun) is in the vicinity of the field of view, though often the source may be outside the actual viewed field. This paper analyzes the geometry of this phenomenon. It theoretically proves empirical observations, particularly the condensation of this flare around a straight line. Based on the image-formation model, we devise a very simple method for mitigating this effect, using as few as two frames taken when the camera moves. This significantly improves the images. Furthermore, aperture ghosting is shown to encode useful geometric information, specifically the location of the (often unseen) illumination source, and the optical center of the camera. Hence, our approach decodes this information as a by-product of deflaring. This is demonstrated experimentally outdoors.