Sobel'soperator is used to assess whether there is a line at a particular location and to estmiate its direction. It can be seen as the sum of two factors, one measuring the horizontal rate of change and the other the virtical rate of change.
H = (c+2f+i) - (a+2d+g)
V = (g+2h+i) - (a+2b+c)
G = | H | + | V |
where ,a,b,c,d,e,f,g,h,i are the pixels in a 3x3 square (see diagram)
Defined on page 252
Used on pages 252, 253, 272
Sobel's operator