HAPI vs H3D
H3D API makes it easy to develop applications utilizing haptics and graphics features. The X3D and Python level of H3D API can be used for rapid prototyping in the early development stage. If needed it is then possible to program in C++ for more advanced/optimized behaviour of the application. For this reason H3D API is optimal to use when developing new applications from scratch.
HAPI on the other hand is ideal to use when adding haptics to an already existing application that may or may not use graphics. The application must be written in C++. Using only a few simple commands the existing application will suddenly have support for several commercial haptics devices.
It is simple to add support for custom made haptics behaviour for both H3D API and HAPI.