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Dust and sunlight created all sorts of reflection problems in pulling off the train. Courtesy of Universal

Suddenly, CG trains are all the rage, what with Unstoppable, Source Code, Water for Elephants and now Fast Five.

In fact, for the thrilling train heist that opens the fifth Fast and Furious film, MPC Vancouver (under the supervision of Guillaume Rocheron) handled around 240 shots. These included a CG train, CG bridge, CG dust, extension to a practical train, digital doubles for Vin Diesel and Paul Walker and other digital stunts and face replacements and CG sets for the entire canyon shot nearly all by second unit in the flat Arizona desert.

Of course, there were quite a few challenges in mixing and matching practical and CG elements (since Rocheron was working on Sucker Punch, MPC's Jessica Norman from the London office was the onset supervisor in Arizona). But the primary focus was the train. While there was a real train on set with one passenger car and three baggage cars on set, MPC's job was to give the illusion of a high-speed train, which ultimately crashes into the bridge. That could be accomplished through extension by adding an extra car and the engine, or by replacing with a full-CG train, depending on the shot.

"For that train, we took thousands of pictures from different angles and different times of day to really identify how the reflections were behaving, which presented a challenge because we're in a very bright environment," Rocheron explains. "Normally, we try to avoid as many reflections as possible. You want the most flat-lit train, which wasn't doable, and because of the size we couldn't do a cross polarized shoot like we do for actors. So we had to create some artificial lighting to be able texture very flat lighting references so none of the textures would be biased with already baked in reflections or tints."

The bridge was patched together from different reference that fit with the setting and circumstance.

"And the train was made of different types of metal, so it was going on a shot for shot basis," Rocheron continues. "You have the bottom section that's made of aluminum, the center section that is a glossy metal and then the roof that is made of a different metal. We had a hard time [getting] the right values for shaders to really make sure that the reflections and the Fresnels were working correctly. Obviously, the reflections changed dramatically depending on the time of day. We're rendering everything in RenderMan and we have our core shader library, so every metal shader is made of in-house shading components with different specular models and others."

Meanwhile, the digital doubles posed an interesting challenge because the sequence is a mixture of second unit photography in Arizona with stunt doubles. And for shots where you see the actors in close-up, it was shot on a greenscreen in Atlanta with a partial set train.