Flood-Fill Large Pores
In ProcessGeo, the Flood-Fill feature allows to separate large and small pores, when at least two scales of pore sizes are present in the microstructure.
In a practical case, it might be used to fill up and separate all the empty pore-space around single grains, which internally have much smaller pores. This is done with an example structure with a single grain that contains small pores. 
The value entered in Large Pore Diameter defines the size for a large pore and excludes smaller pores from being flood-filled. In the following example, the value entered for Large Pore Diameter (20 voxels) is larger than the diameter of the small pores inside the grain. As a result, the pore space around the grain is flood-filled and the smaller pores inside the grain remain untouched.
The material to fill the large pores can be selected by clicking on the material next to Fluid in Pores > Pore Diameter and choosing the material from the Material Selector dialog. Then all pores with a diameter that is larger than the previously defined Large Pore Diameter are assigned to this material. The Material ID in these pores is also changed to the next available Material ID.
Domain Boundary Conditions
Different boundary settings can have a big influence on the flood-filling results. The Domain Boundary Conditions can be set to be Symmetric, Periodic (recommended for periodic structures), Encase, or Expert. With Expert you can set boundary conditions (Symmetric, Periodic, Encase) for each direction separately. Keeping Embed Domain by Pore Diameter checked does not allow to change the domain boundary conditions. Instead, the structure is embedded with a layer of empty voxels that has the same thickness as entered under Large Pore Diameter. Thus, pores at the boundary of the original structure belong to the pore space of the embedded structure and are filled. This avoids artifacts at the original domain boundary, but slightly increases runtime and memory usage.
When the Only Fill Components with Boundary Contact option is selected, only the large pores connected to the domain boundary are filled.
The effect of different boundary conditions for flood-filling of large pores is demonstrated below. This catalyst layer like structure was generated with the Catalyst Layer GeoApp.
Symmetric boundary conditions with Embed Domain by Pore Diameter enabled
