I would like to use Fiji (ImageJ) for some simple pre-processing operation and to create a binary mask. I am just using the GUI and I am not interested in an authomatic procedure.
I got the following mask: (this is just a slice, the actual mask is 3 D, 298 MB)

I need to extract a 3D connected object, so that the result would be:

Ideally I would like to click inside a point inside the object with a 3D equivalent of the Wand tool, select the object, invert the selection, multiply by 0. For a start, I tried to use the command Analyze – 3D objects Counter, but I guess the image is too big and complex (the operation freezes at step 2. Connecting Objects).
Is there any procedure I can follow?
In Fiji you could do the following:
Edit > Invert.Plugins > Process > Find Connected Regions.Start from point selection?and changeStop after this number of regions are foundto1.OK.There’s some documentation for that plugin (which I wrote a long time ago). It uses a very naive algorithm for finding connected regions, but it’s good enough for many purposes.