Step Edge
•the response of the two derivatives is the same at the gray-Ievelstep (in most cases when the transition into a step is not from zero, the second derivative will be weaker).
•The second derivative has a transition from positive back to negative.
•In an image, this shows as a thin double line or "double-edge“.
•Note that if the gray level of the thin line had been the same as the step,
•the response of the second derivative would have been stronger for the line than for the step.