As erosions and dilations, the key mechanism under the opening operator is the local comparision of a shape, the structural element, with the object that will be transformed. If, when positioned at a given point, the structural element is included in the object than the whole structural element will appear in the result of the transformation, otherwise none of its points will appear.
The following figures present a binary image and the result of its opening by a small Euclidian disk.
The following figure presents a colored composition of the original image, its opening by a small disk and the structural element positioned in some critical points.
The next three figures show a binary image, the erosion by the 3x3 square and the corresponding opening, that is, the dilation of the result of the erosion.
It is worth to note in the figures above the folowing properties of openings:
This last property can be used to build granulometries, that is, the separation of objects and their parts by their size relative to a family of homothetic structural elements.
The next two figures show a gray scale image and its opening by a small flat disk.