You either shrink picture horizontally by one pixel or vertically by one pixel; seams must be 8-connected and always advance by 1 pixel in the opposite directio

You either shrink picture horizontally by one pixel or vertically by one pixel; seams must be 8-connected and always advance by 1 pixel in the opposite direction to the one being removed. It's a heuristics and by making it that way you can guarantee exactness of 1px removal. These days it's a popular algorithm to be implemented as a midterm project in computational photography courses at top schools.