How do you extract surface porosity of 3D images?

Dear Colleagues,
I have to quantify the porosity of a porous char, which is not uniform. I have attached a 3D image of the structure using ImageJ interactive 3D surface plot. My question is, how can I calculate the porosity of this structure?
2D imaging is not sufficient as the surface is not flat.

Other ideas are appreciated!

Thanks in advance!

Hello ha43 and welcome to the forum,
The example shown is good so as to let us have an idea of what you are trying to achieve, however a copy of the original as that you must work would help all of us to determine a best plan. The surface of the object need not be flat in most cases so your task seem do able.
How much experience do you have with Fiji/ImageJ? Also not a problem but would make it easier to explain our ideas.
Good Day,

Dear Bob,
Thanks for the reply. I’ve attached the original image; Int. char.tif (19.3 MB)
I’ve experience with the 2D interface of ImageJ, but not the 3D, which I think is necessary to determine the surface porosity of this kind of images.

If I do 2D processing of the attached image I will receive a surface porosity of appr. 20%, but it should be +90%! (As 2D processing does not encount the ‘‘empty’’ space/ depth)
Any advice or ideas are appreciated,

Hello ha43,
I sincerely apologize for the extended delay in responding, got caught up in something I cannot talk about so please forgive me.
Have you found a solution to your research as yet? If not let me know.
There are several ways to do what you wish, so it is indeed do-able.
In the mean time you should see if you have the ‘Extrude Map’ plugin, and if not then obtain it from the ImageJ/Fiji plugin site, to get started.
Again sincere apologies,