ITK/Release 4/Modularization/Add an external module (external module)
An external module is distributed outside the ITK main repository, but it could be built into ITK as a module once downloaded into the local copy of ITK source tree.
The organization of an external module should be the same as an internal module: Add a module(internal module)
Users could download an external module and put it into local copy of ITK source tree under: "ITK/Modules/External/".
To build an external module, simply rerun CMake step to configure the new external module together with other enabled ITK modules.
An example is Lesion Sizing Toolkit: