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Spy Kids 3-D: Game Over

Sep 1, 2003 12:00 PM, by Ellen Wolff

Chasing The Stereo Illusion

For Spy Kids 3-D: Game Over, director Robert Rodriguez had some ambitious plans. To bring something new to his hit franchise for Dimension Films and Miramax, Rodriguez chose HD stereoscopy. He shot his actors with twin 24p cameras primarily against greenscreens, and then placed them in 3D-CG videogame worlds. Among the more challenging assignments was a 100-shot bike race sequence that's 11 minutes long. That challenge fell to Hybride, a digital effects facility based in Piedmont, Quebec.

“This mega-race begins in the city and then passes through a huge dome structure and through tunnels, finally ending in a desert in the middle of nowhere,” Hybride visual effects supervisor Daniel Leduc explains. “Normally, we're laying a 3D object on live footage, but this was the other way around — live actors in an entirely CG background.”

To help map out the sequence, Rodriguez presented Hybride with a low-res previz done in Maya. “It was just an example of the framing that Robert wanted. It was not something for us to lock on,” says Leduc. “The actors were not shot with a motion control camera because Robert wanted to shoot fast and not lock himself into long preparations. The actors were shot as if it was a normal shoot, but with two cameras. We did record the 3D camera information, the convergence of the two cameras.” Leduc also put markers on the greenscreen so Hybride would later be able to track the camera movements using 3D-Equalizer.

While most of the final shots in the mega-race included multiple actors, Rodriguez preserved his options during the greenscreen sessions by shooting the actors individually as they rode a bike-like rig. “The hardest part,” recalls Leduc, “was to combine those actors and integrate them in a background that wasn't part of the set.” He notes that because the show was stereo, the actors had to be placed correctly on the z-axis. “We couldn't cheat,” he admits.

Meanwhile, the CG was being created with Softimage/XSI. According to Leduc, the stereoscopic requirements of the movie definitely affected how Hybride designed the backgrounds. “You always need to think, ‘What can I put in the scene to feel depth?’ In 2D that's easy. You can just change the color of some layer. In 3D stereo that's not the case. It's not just color, it's positioning,” says Leduc. Hybride intentionally created backgrounds that were more faded in color than the characters. “With stereoscopic images you need to find a way for audiences to focus on the correct objects — in this case, the characters,” he says. “If audiences look somewhere else in the frame they won't be able to see the stereo. By keeping the background less colorful, it's clear what they should be focusing on.”

Hybride also had to create CG cycles to replace the bike rig ridden by the actors during the greenscreen shoot. It took some engineering to make those CG models work properly with the positions of the actors' hands and feet. In addition, Leduc's team had to create CG doubles of the actors using full-body cyberscan data because sometimes they switched from a live actor to a CG double in the middle of a shot. Finally, Hybride added CG smoke and other particulate effects created with Lightwave.

Typically, racing sequences utilize motion blur to enhance the feeling of speed, but Leduc notes that this is problematic with stereoscopic images. “You need to feel the edges of an object to feel stereoscopic effect. Originally, we used normal motion blur and we were literally losing the 3D effect. So we were forced to reduce the motion blur. Of course doing that helped reduce rendering times, so I didn't complain!”

Given the huge volume of CG that had to be rendered, Hybride focused on ways to render quickly using Mental Ray. Making this easier was the videogame look that Rodriguez wanted. “Robert tried to stay far away from the photoreal. He wanted to feel those polygons!” Leduc says. So Hybride developed a videogame pipeline process that got rendering times down to as little as a minute per frame.

“We did what we called texture projection,” Leduc explains. “We did bake all the textures together and even the lighting and shadows on the models, so that's why it was really fast rendering. You can move your model in 3D and the texture will track on it. You still have to do one frame of rendering with the textures and shadows and surface reactions. But you're doing that for one frame only. Then you render 1,000 frames with the same texture, just adding the second lighting pass. So it's really fast. It's a common technique, but it's probably not common to do it on such a big scale.”

Compositing was then done mainly in Inferno running on SGI Onyx 2 hardware, and this stage brought the moment of truth. In most movies, problems can be corrected with 2D fixes during compositing, but that's not an option with 3D stereo. “Sometimes we had to go back to the modeling stage if the shape of an environment was not 3D enough. Of course, you don't see that until the end,” Leduc says.

The final hurdle involved filming out the digital images. The anaglyph process used for this movie takes the red channel from the left eye and the green and blue channel from the right eye, and Leduc notes, “The main problem was the red channel. It's really difficult to match video red to film. If your red doesn't match the red filter of the 3D glasses, you lose the stereo effect. But it was E-Film's job to figure that out!”

To do this, E-Film enlisted CIS Hollywood's Dr. Ken Jones. “As far as I know, nobody's ever done a digital output like this,” Jones says. “The first film-outs didn't separate between the two eyes. There was huge ghosting. I didn't have a solution, so I had to work out some new color table solutions. Of course, the best way get an engineer interested is to say, ‘I'll bet you can't do that.’”

Credit Roll

Robert Rodriguez - Director

For Hybride -
Daniel Leduc - Visual Effects Supervisor and Producer

Thierry Delattre, Sean Stranks - Visual Effects Supervisors

Philippe Theroux - Computer Graphics Supervisor

Marco Tremblay - Lead Modeler

Jean-Yves Audouard - Lead Animator

Joseph Kasparian - Textures & Lighting Lead

Marc Bourbonnais - Lead Technical Director

Mathieu Dupuis - Lead Compositor

For E-Film -
Dr. Ken Jones/CIS Hollywood - Filmout Supervisor

© 2008 Penton Media, Inc.