Hello, thank you for your response.
We are following actors with different 3D objects (basic spheres etc from the primitive list) to project on them much like followspots. Sometimes full body shapes, and sometime just head shapes.
The goal is for the lighting designer to be able to change the colour, shape etc of the light being projected on the actor at different times in the show.
We are also using some Moving Light Tracking, but the versatility of the projector might be more successful since the content can be much more interesting and variable than just the coloured light from a moving light.
It sounds like the alpha mask would be a good solution for us to try.
We are onstage tomorrow, I will work with this.