Fig. 2
From: Line and circle detection using dense one-to-one Hough transforms on greyscale images
Direct detection of lines and circles from partial derivatives. In the two images, the derivatives have been estimated at a few chosen locations (shown by the little red circles) from the grey levels using a scale estimation of σ=3.0. a Line deduced from the gradient direction (here, the segments with arrow represent the gradient vectors). b Circle deduced from the isophote curvature and the gradient direction: here, the segments with arrow have the direction of the gradient vector, their magnitude is the inverse of the absolute value of the isophote curvature, and their sense is the same as the gradient for the positive curvature (blue circles) and its opposite for the negative curvature (circles in cyan)