A method for generating candidates from a digital image includes considering at least one point x that may lie on a polypoid structure, determining whether the point x satisfies a first predetermined set of conditions, for each point x that satisfies the predetermined set of conditions, identifying each neighbor point y within a predetermined distance of point x that satisfies a second predetermined set of conditions, determining a gradient vector v1 for point x and identifying a first half-line to which the gradient vector v1 belongs, determining a gradient vector v2 for point y and identifying a second half-line to which the gradient vector v2 belongs, calculating an intersection score that represents how close the first and second half-lines come to intersecting, and identifying point x as a candidate when a candidate score is greater than a predetermined value, wherein the candidate score is the sum of intersection scores for all neighbor points y.