I am reading about Canny algorithm in the book Academic Press - Handbook Medical Imaging Processing Analysis where it is written that the algorithm was originally developed for antiasymmetric edges $(f(x)=−f(−x)$, where the true edge point is at $(x=0)$ such as ridge, roof and step edges. I am also discussing about the options for this algorithm here.
How can you visualize ridge, roof and step edges?