Measuring porosity inside a cross-section of a tube

This macro (source code at the end of the document) was written to measure the porosity inside an axial crosssection
in a tube. The macro makes four assumptions:
1. The cross-section is perpendicular to the tube axis
2. The tube thickness is uniform
3. No artefacts except noise are present in the images (e.g. beam hardening)
4. The solid appears in light gray over a dark background
From the set of cross-sections to analyze, the macro first thresholds the images, find the region inside the tube
and computes the porosity in that region.