We talked a lot about registration. Summary:
- Current surface-to-surface methods require cutting off non-common surfaces, which will be much easier with the new surface cutting module that @Sunderlandkyl is working on (that will allow cutting with plane widgets and surface curves).
- There is also the option of doing landmark registration.
- Integration of more robust surface matching algorithm from OpenCV could allow surface matching without the need to cut off non-common surfaces - this should not be too much work, but would require dedicated funding (or maybe you may ask SlicerCMF developers if it is potentially in the scope of their project).
Volume measurement: You need to register the surfaces, then close them (for example, cut off excess parts, thicken it using vtkLinearExtrusionFilter, then cut off the back side). We did this for surface-scan based volumetric fat resorption monitoring after breast lipofilling (see details in this thesis and implementation in this repository). It would be not too hard to adapt this to dental scans, but probably would require dedicated project. SlicerCMF and others, such as @cpinter might be potentially interested in collaborating or could help out if you can secure some funding.