Step By Step: Tropic Thunder

“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.

“LightWave has some amazing tools for solid body objects,” Breakspear says. “Our helicopter was built to be used full frame. The camera comes pretty close to it, so it's dinged up and the windows are cracked.”

To make the chopper hit the ground realistically, CIS Vancouver simulated smoke and flying dirt and grass. “When you do an explosion and simulate debris, the debris flies around based on physics,” Breakspear says. “But Ben wanted the debris to come towards camera. We worried that it would look too contrived, but Ben knew how to balance the photoreal and the comedic.”

Simulations were done using Autodesk Maya and Side Effects Houdini. And CIS developed a plug-in for Houdini to simulate the mortar explosions that bring down the chopper. “This plug-in affectionately became known as ‘Blow Me,''” Breakspear says. “Our CG mortar explosion was designed to match the practical ones on set. It took eight months to build this thing.”

Rendering of the CG chopper was done in LightWave 3D, while other CG elements were rendered with mental images mental ray and the AIR renderer from SiTex Graphics. Several 2D elements were also incorporated into the scene, including a matte painting of the crashed helicopter. Matte painter Romain Bayle used Softimage|XSI, Adobe Photoshop, and digital-camera images of fire and debris.

Compositing was tricky. “The scene occurs on a sunny day, and there's oily smoke coming out of the back of the chopper,” Breakspear says. “There's also haze from practical mortar explosions going on, so we had to reposition elements, add CG, and then get realistic camera shake and color correction.”

Compositing was done in Shake. “We've imported CG cameras into our version of Shake so that we can manipulate camera shake and depth of field,” Breakspear says. “When objects are flying past the camera, you want a combination of motion blur and depth of field to make it look photoreal. So having the ability to import cameras made a big difference.”

Color correction was done in Shake using software called ana-log, developed by CIS and Deluxe. “We can take plates from our clients and make sure that the data is protected all the way through the process,” Breakspear says. “We've developed the software to do non-destructive color correction. We don't want to burn something into the image that we can't get rid of later.”

Breakspear says the tools used for Tropic Thunder were archived right along with the images themselves. “That way, when we have another explosion to do in the future, it won't take much to reconnoiter plug-ins like Blow Me,” he says. “We could call it Blow Me Again!”

CREDIT ROLL

Director: Ben Stiller

DP: John Toll

Visual Effects Supervisors: Michael Owens, Michael Fink

For CIS Vancouver

Visual Effects Supervisor: Mark Breakspear

Visual Effects Producer: Christopher Anderson

Visual Effects Line Producer: Jinnie Pak

Visual Effects Coordinator: Steve Won

CG Supervisor: Craig Calvert

2D Supervisor: Martyn Culpitt

CG Artist: Dan Mayer

Compositor: John Cairns

Matte Painter: Romain Bayle

Color Correction Software: Ken Hayward