Spider-Man still does everything a spider can, but the blockbuster
Spider-Man sequel gets maximum mileage out of a new villain, the
tentacled Dr. Octopus, known as Doc Ock for short.
The critical task at Sony Pictures Imageworks was creating a digital
double that could stand up to Doc Ock’s close-ups. "I knew from the
start this would be our biggest challenge because there were a lot of
shots where the camera was very close to the digital Doc Ock," says
visual effects supervisor Scott Stokdyk.
The studio used motion capture to create facial movements, then
considered alternatives for handling the skin. Stokdyk tried a method
made possible by the Light Stage, a system for photographically
capturing the reflectance field of a human face developed by Paul
Debevec and implemented at USC’s Institute of Creative Technology. The
images it provides can be used to render the face under arbitrary
changes in lighting and viewpoint.
Here’s how it worked: Molina was seated in a chair and surrounded with
four film cameras running at 60 frames per second. Above his head was
an armature with strobe lights firing at 60 fps that rotated around the
chair while the cameras were filming. "At the end of eight seconds, we
had 480 images from each camera with different lighting conditions in
each frame," says Stokdyk. "We derived the texture as a combination of
images and intensity that’s calculated by a RenderMan shader."
The intensity was determined from high dynamic range images of the set
shot using a fish-eye lens to create a full 360-degree environment.
Those environmental images helped the RenderMan shader determine which
of the 480 images of the actor’s face would be used in each shot.
The Doctor’s arms were more straightforward. Puppeteers collaborated
with CG artists on the snakelike tentacles that tossed people like rag
dolls across a room. Puppets were used for the arms when they directly
interacted with the actor – when he lights a cigar, or brushes his hair
back. "But when he had to do anything with more than four feet of
tentacles, we used the CG arms," says John Dykstra, visual effects
designer on the film.
The puppet arms were sent to Gentle Giant, where modelers working in
Maya created digital appendages to match, rigging them so the spacing
between the vertebrae in each tentacle could subtly change the length
of each metallic "arm" and create the appearance of shock waves upon
impact. When the character wasn’t entirely CG, Molina was shot
traveling through the air on a wire rig and the tentacles were attached