Extraction and fusion of connected domains

After analyzing the connected domain of a 3D volume data, several labeled connected domains are obtained. How to obtain the volume data of the corresponding connected domain through the label? What should I do if I want to extract and merge two of the connected domains?

There are multiple ways to get label’s volume, see this old post.

One possible way to “extract” a label is using threshold image filter, or binary threshold filter. After you have extracted them into two images, you could add those images together.