If the objects are convex then you can use the second output of convhulln to give you the volume directly. If not, then you have a few options:
You can just sum the images:
v1 = sum(sum(sum(original)));
v2 = sum(sum(sum(original)));
You could use a more fancy approach such as this:
And there are other ways.
For my masters thesis that did quite a bit of CT volume analysis, I just summed - it was a good enough approximation and there were many other attributes in the image that cause more error than this (reconstruction/downsampling bits/binarizing/noise, etc).
