The reason we use dedicated hardware is because any video based solution is dependent on having good source images and we make use of some hardware to get good quality video.
Having a headmounted camera ensures the face is always visible and gives the actor a lot of freedom compared to sitting in front of a static camera or within a confined space that is 3D scanned at a high framerate.
The headmount also allows you to add a little light to the setup to ensure consistent illumination when moving around.
Working in the visible light spectrum means the actor has some annoying lights shining into his/her face, so tracking in the infra red spectrum is nice as the light is invisible to the human eye.
The tracker can work with any camera, it’s just that we chose one with a low weight and a particularly high frame rate, as humans can move pretty quickly during blinking and speaking 60 fps really is a must have if you want to do minimal cleanup.
So as you can see the reason for the hardware is to get a very high base quality with the minimum amount of restrictions for the actor.
I’ve experimenting with a blendshape rig as well (and btw it’s bloody difficult to perform all FACS poses) however we found a method like that always throws away data.
Either you stuff the system with all the possible pose/shape relations you can think off (FACS is only a start) and they end up fighting with eachother, or you limit the amount of poses/shapes at the expense of going into interpolation on some of the frames.
It’s a fine balance, the biggest advantage of this method is that the shapes can be finely art directed, however the interpolations are more difficult.
With a direct rigging approach tuning the retargeting may be a bit more tricky but once that is done you never throw away any performance data, in the worst case scenario the outcome may be a bit distorted but the motion detail is always there giving you a lot of options for motion cleanup or keyframe augmenting.
And off course you can do a hybrid approach, extend a direct approach with some pose/shape tweaks, which is what the Alien creature uses.
But even though this is interesting stuff, I’ll probably stop ranting for now