What is a horizontal or vertical derivative?

Question

Image processing

Hello! I read this article habrahabr.ru/post/112079/.
I can’t understand what the author means by the concept of a horizontal or vertical derivative, I quote: "... Therefore, if we take the image as the difference between the horizontal and vertical derivatives, and then apply ..."
I really hope for your help. Thank you.

Answer 1

robocraft.ru/blog/computervision/460.html