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a b Chapter 4
c d
Figure 4.46
Example of vision-based feature extraction with the different processing steps: (a) raw image data;
(b) filtered image using a Sobel filter; (c) thresholding, selection of edge pixels (d) nonmaxima sup-
pression.
– 1 1 1 –0 1
–
1
–
p
θ ≅ atan ; p = 000 ; p = –0 1 (4.86)
1
-----
1
1
2
p
2
1
111 –0 1
In the same year Sobel [29] used, like Prewitt, two 3 x 3 masks oriented in the row and
column direction. Let s 1 be the value calculated from the first mask and s 2 the value cal-
culated from the second mask. Sobel obtained the same results as Prewitt for the gradient
magnitude G and the gradient direction taken in a clockwise angle with respect to theθ
column axis. Figure 4.46 shows application of the Sobel filter to a visual scene.
