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MillimeterStep By Step: Tropic Thunder
Sep 1, 2008 1:00 PM, By Ellen Wolff
“Comic timing” is a phrase usually associated with actor pratfalls rather than the effect of a CG helicopter crashing to the ground. But for the DreamWorks/Paramount comedy Tropic Thunder, actor-director Ben Stiller wanted the visual effects to unfold with a rhythm that synced with the antics of his cast. In one key action scene, a chopper is hit by a mortar and then explodes in a field, endangering the character played by Robert Downey, Jr. Stiller needed the CG aircraft to look real in order to sell a sense of peril, but he wanted its explosion timed for comic effect.
That assignment went to CIS Vancouver in British Columbia. “In doing visual effects for a comedy, you consider things in a different light,” says Visual Effects Supervisor Mark Breakspear. “You have to try to keep the effects looking real and find the funniest way of doing it.”
The original plate photography for this shot included a practical explosion, but it was too far in the background to imperil Downey's character. “So the first challenge was to increase the size of the explosion,” Breakspear says. CIS Vancouver began by rotoscoping the original explosion out of the plate using a combination of eyeon Software Fusion and Apple Shake.
“In some frames, the explosion was the brightest thing in the frame,” Breakspear says. “So we did a luminance key and — almost like a bluescreen — got its matte. We took a half sphere and projected the explosion onto that. You don't see the sphere; you just see what the projection comes into contact with.” This step was necessary because the shot was photographed with a fairly wide-angle lens and Breakspear wanted to ensure that there would be a believable shift in perspective. “This gave it the necessary parallax and the feeling of the explosion still being 3D.”
This version of the explosion was then tracked into the plate photography to appear closer to camera. “We used [2d3] boujou and [The] Pixel Farm software, along with good old-fashioned hand-tracking,” Breakspear says. “Because the explosion keeps getting bigger in frame, we didn't need to do much cleanup on the background. This version covered up the original explosion.”
Breakspear's team looked at footage of countless chopper crashes on YouTube before building and animating its CG model. “What tends to happen is that its tail rotor loses stability and it starts spinning out of control,” he says. “It goes too far sideways and comes crashing down, and the blades hit the ground and shatter into pieces. Ours did that, but when it hits the ground, it turns into a fireball. That's where the comedy comes in. All these individual pieces had look real as opposed to CG objects that are obscured by a practical explosion.”
“The challenge was making the chopper look like it had weight and still manage to fly in all the way towards the point where Robert Downey, Jr., was,” Breakspear says. “The faster you move a big object, the lighter and smaller it will feel.” CG artist Dan Mayer built and animated the chopper using NewTek LightWave 3D.
Continue the discussion on “Crosstalk” the Millimeter Forum.
