Straight subjective contour detector

Subjective contours or illusory contours are an important aspect of human perception. Along subjective contours, image contrast is very weak or completely missing, so that no local edge detector can recover them. Their perception is induced by the presence of small pieces of edges and of tips of other long edges incident on the contour. Indeed, in real-world images, edge information of foreground objects is often partly missing due to poor contrast of the object with respect to its background. Nevertheless, the object contour is still perceived by the presence of object or background details that end up abruptly along the contour. In this paper, we handle the detection of straight subjective contours (SSC), using an a contrario approach to control the false detection rate. The algorithm exploits the tips of line segments produced by the well-known parameter-less LSD method. The subjective straight contours are obtained by grouping free tips of parallel line sets, together with aligned short edge pieces. This detection is fully automatic and is demonstrated on a set of images containing subjective contours.