Sobel operator, which has been written in the textbooks of digital image processing and machine vision for many years, has never been questioned and cared about its invention background and history. Recently, I gave students a lesson on "photoelectric image processing", and I want to introduce the origin of this operator. I checked many files, but I couldn't find the original file. Google academic search, a lot of information, but inconsistent. Some are marked as periodical papers, others as publications, and the publication time is also inconsistent (Gonzalez's textbook Digital Image Processing is marked at 1970).

This seemingly simple edge detection operator has been used by researchers and developers in this field for decades. How did it come about? I came across a post in which Irwin Sobel Beier, the founder of the operator, gave a detailed account of its origin and definition years after the operator came into being.

It turns out that this famous Sobel edge operator was not published publicly, but was put forward at a doctoral seminar (1968) ("A3X3 anisotropic gradient operator for image processing"). Later, it was published as a footnote in the monograph "Pattern Classification and Scene Analysis" published by 1973.