or 
and is defined as: 
.
A closing fills the holes inside the particles, eliminates the small details by smoothing the boundary from the outside and connects close particles.
A closing by a disc B is a dilation by B followed by an erosion by B.
The closing of the image I will be denoted or and is defined as: .
A closing fills the holes inside the particles, eliminates the small details by smoothing the boundary from the outside and connects close particles.
| Figure 1: Effect of a closing |
As shown in figure 1, the closing is achieved by sliding the disc outside the particle and filling all the parts where the disc cannot fit into.