"I think what [Natali] was after from day one was a sympathetic character that didn't draw attention to itself as a monster, but as a creation of two monsters. The look of Dren is [subtractive] -- the bald head, the eyes slightly far apart and slanted, only three fingers and a thumb on each hand, a tail that only moves when necessary to conserve energy. And the way we supported that from a visual effects standpoint was to constantly make things as subtle as possible."
C.O.R.E. did the bulk of the animation in Toronto using Houdini , with Paris-based Buf Compagnie  and Mac Guff Ligne  animating Dren as a baby/ toddler and the pre-Dren slugs, Fred and Ginger, respectively.
Munroe credits CG Supervisor Terry Bradley with devising the techniques for pulling off the different Drens. The child Dren, though, offered the most complex challenge since most of her face is CG. "We had to blend skin tones right in the middle of her face and make it work," Munroe continues. The eyes, skull and ears are CG. We had her wearing a prosthetic built by KNB for lighting reference."
Not surprisingly, the child's eyes were the biggest challenge, and it was Bradley's suggestion to once again rely on Chanéac. "When we finished editing all of the scenes of the child Dren, at the same time that we brought Delphine back for reference, we did what we called Eye ADR.
Then, as Dren evolves, she becomes more predatory, with the eyes moving more toward the head. "We didn't want to go through the expense of a complete CG rendered head with Delphine because of the number of shots where she's on screen," Munroe says. "Terry and John Mariella, one of our animation directors, came up with a technique that was absolutely brilliant in its simplicity: We brought a company in from Ottawa called XYX RGB and they scanned Delphine's head in very high-resolution detail, so we had a CG data set of her head. On set every day, we put tracking markers around Delphine's eyes to make it easier later on. Once we were finished and got the plates, Paul Waggoner, our head of tracking, then processed that footage and that would go to our compositing team, and they would remove the dots. Then our animation team took that 3D data set of Delphine's head and placed it into the shot so that it was tracked in perfectly, and then the cleaned up version of the plate was camera projected right back on that 3D head; and in realtime, Vincenzo and I could sit with Paul to [determine where to position the eyes]. Then it would be put in the render queue. There was no ray tracing or global illumination or subsurface scattering or ambient occlusion because the lighting was already in the footage. At that point, there might be some cleaning up of the edges by the compositing team. And that was our adult Dren. It was remarkably simple, effective and cost-efficient.
After a sixth-month design/pre-production phase for vfx and 12 months to produce around 500 shots, Munroe singles out producer Steven Hoban. "He convinced Gaumont and the other financiers that this was critical to the success of the film. If the effects were substandard, it would've taken you out of the movie. He gave us the time to figure them out because on a more typical production schedule I don't think we would've succeeded the way we did."
Bill Desowitz is senior editor of AWN & VFXWorld.