KEY POINTS:
You might not pick it straight away but some of the creatures in Bridge to Terabithia went to the same hairdresser as King Kong. The same virtual hairdresser, anyway.
Wellington's Weta Digital had just finished working on the Peter Jackson remake when they started on the US production
of Terabithia last year.
"So we had some really good technology for creating digital fur," says visual effects supervisor Matt Aitken.
Filmed in New Zealand by Narnia producers Disney and Walden Media, and based on the novel by Katherine Paterson, the film follows two kids who find friendship when they discover a fantasy world in the forest. Although the effects make up only a small part of the film, Weta were in charge of bringing the strange creatures of the children's imaginations to life.
Among those are small, fast-moving creatures known as "Squogres", "Hairy Vultures" and a kind giant with a young girl's face.
Because the book wasn't illustrated Weta had only the pencil sketches by the film's production artist, Dina Malanitchev, to go by.
"They were great illustrations - very stylised and evocative - but they were very much illustrations," says Aitken.
Using Photoshop, Weta recreated the essence of the drawings and added digital cat and squirrel fur to make them look more lifelike. Next they decided how to make them move.
One option was key-frame animation, where an animator works with a "digital puppet" on screen, changing its pose frame by frame. The other was motion capture (the same technique used for Gollum and King Kong), whereby an actor in a special suit is filmed, then applied to the digital character.
"We decided motion capture wasn't appropriate because a lot of the creatures are not human in form," says Aitken. "It would take a lot to bend the movement into a Squogre or a Hairy Vulture. But we did use it a little bit at the end when the giant places a crown on Jess' head. We wanted her to feel really natural and gentle and to have a very elegant way of moving. So we built a ramp that matched the terrain she was walking up and had a human perform that move."
For the animated Squogres - which chase the children through the forest - Weta studied the way dogs, cats and squirrels run, deciding to base their movement on dogs after putting the Squogres on a digital treadmill.
But Aitken is most proud of his "Birdcage Lady", a woman with a cage for a torso, who is revealed as one of the weird and wonderful citizens of Terabithia. "We think she should have a film of her own."
The biggest challenge was creating Terabithia itself, with its magical, moving trees.
"This was more complex than anything we'd done in terms of having believable trees and the light shining through them and the view to Terabithia and beyond.
"It's just those trees have all those damn leaves on them. They're incredibly geometrically complex and they just take a lot of processing power. It's like fur and hair - if you don't do it properly it won't look right."
* Bridge to Terabithia opens today.