Morphology - Introduction to Video and Image Processing - page 97

Fig. 6.12 Opening performed using a 7 × 7 box-shaped structuring element

Fig. 6.13 Filtering a binary image where both holes and small noisy objects are present

6.3.3 Combining Opening and Closing

In some situations we need to apply both opening and closing to an image. For
example in cases where we both have holes inside the main object and small noisy
objects. An example is provided in Fig. 6.13 . Note that the structuring elements
used in the opening and the closing operations need not be the same. In Fig. 6.13
the closing was performed using a 7

× 7 box-shaped structuring element while the

opening was performed using a 15 × 15 box-shaped structuring element.

6.3.4 Boundary Detection

Doing edge detection in binary images is normally referred to as boundary detection
and can be performed as described in the previous chapter. Morphology offers an
alternative approach for binary images. The idea is to use erosion to make a smaller
version of the object. By subtracting this from the input image only the difference
stands out, namely the boundary:

− f(x,y) SE

g(x,y) = f(x,y) (6.6)

If the task is only to locate the outer boundary, then the internal holes should first
be filled using dilation or closing. In Fig. 6.14 examples of boundary detection are
shown.