Sobel's operator

Page numbers are for draft copy at present; they will be replaced with correct numbers when final book is formatted. Chapter numbers are correct and will not change now.

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 256

Used on Chap. 12: pages 256, 257, 276, 286