Introduction to Opening

An opening does not destroy as much information as the erosion. An opening will discard small objects, but will keep the largest ones with a very similar shape in the original and the final image.

The opening by a disc, B, is the combination of an erosion followed by a dilation by B. The opened image I is denoted or et .

Consider a disc as the structuring element. As shown in figure 1 the opening is achieved by sliding the disc inside the object and discarding all the parts where the disc cannot fit.

Figure 1: Effect of an opening

Example

Figure 2 shows another example.

Figure 2: An opening of size 1

In this case, when the rectangle is totally included in the set X, all the points of the rectangle are kept; in the case of the erosion, only the central pixel was kept. One may also notice that the largest particle has been split in two parts, and that it has lost its two small peaks. The other particle has not been modified.