The steps in determination of porosity:
On the left upper there is the starting image.
First a brightness
value is set. All image parts below this value are marked red.
Then all objects with diameter smaller than 20µm are removed.
The result of both operations is displayed mid left.
All remaining objects are enlarged by adding a hem of size 11µm. As a result of this morphological operation some objects are joined. This is to be on the save side not to omit large holes which may have more than one exits in the 2 dimensional view.
The object situation after this operations is shown left bottom. The blue objects are counted and sorted into a table shown right bottom.
