This is simple 're-texturing'. I.e. the image of other face is projected onto your face (or the 3D reconstruction of it). To actually change, re-render and repl

This is simple 're-texturing'. I.e. the image of other face is projected onto your face (or the 3D reconstruction of it). To actually change, re-render and replace the original geometry is much much more complicated, and also involves re-creating all direct and indirect illumination, which also needs full 3D reconstruction of the surroundings and light sources. That's how it's done in movies and it involves motion capture and huge amount of manual labor. Edit: Speaking of replacing with a generated 3D face. Winklewoss replacement could be done 'simply' by double filming both actors in same position and then photoshopping the faces replacing the one with another, frame-by-frame. Face-tracking could be used to align the faces.