With just 12 weeks to go before Paramount’s “Hugo” was scheduled to lock picture on Nov. 7, vfx supervisor Rob Legato went to director Martin Scorsese and told him the film’s elaborate opening sequence wasn’t working. The 3D images of commuters moving mechanically through Paris’ Montparnasse train station circa 1931, like interlocked gears in a giant machine, were “like a meal that’s too rich,” Legato says.

So they enlisted Industrial Light & Magic to create a virtual nighttime flyover of a digital Paris, built from scratch, which segues into a fly-through of the station’s interior assembled by the film’s primary effects vendor, Pixomondo, led by vfx supervisor Ben Grossman.

It was the largest of the scenic reconstructions during the 10-month post-production, but it was hardly the only one undertaken by Legato and his team. If they weren’t building and rebuilding virtual set elements themselves, they were enlisting vfx supervisor Craig Barron at Matte World Digital to craft entire digital backgrounds.

“Every set without exception had greenscreen in it,” Legato says.

Prior to the digitization of filmmaking, sets were the sole purview of the art department, which on “Hugo” was led by production designer Dante Ferretti. But he departed when principal photography wrapped, as production designers typically do, so the vfx artists were left to fend for themselves. “Occasionally, we would send a mock-up of what we were doing to (Ferretti), but not every time,” says Legato, who also served as the film’s second unit director. “I’ve worked with him before on ‘The Aviator,’ ‘The Departed’ and ‘Shutter Island’ — and there’s a certain level of trust that I adhere to.” Besides, Ferretti and set decorator Francesca Lo Schiavo “do such a beautiful job of designing and dressing the sets that you have all the information in front of you that you need to extend them.”

Early in pre-production, after Ferretti designed the sets and created a visual bible for the film, he met with Legato to discuss what would be built practically and what would be CGI. Then Legato used Ferretti’s sketches and blueprints to create a computer previsualization.

While shooting at Shepperton and Pinewood Studios in London, the live feed from the camera could be A-B’d with or without the previs backgrounds added, allowing Ferretti to move a portion of the set, or d.p. Robert Richardson to adjust the framing if a virtual element clashed with the practical.

But the previs was forever being tweaked to deal with the logistical challenges that cropped up with each new camera set-up. This was especially true with the “chutes and ladders” sequence in which Hugo (played by Asa Butterfield) descends from the station’s clocktower. Although it plays on screen as one continuous shot, it was filmed in bits and pieces over the entirety of the 140-day shoot. Legato had to adjust the practical sets to accommodate the bulky Arri Alexa digital 3D camera rig (“a refrigerator on the end of a stick,” he says), then in post find a way to make the passageways look realistically narrow while creating pieces of virtual set to seamlessly link the shots.

But of all the film’s 800-plus visual effects shots, the most time-consuming were the two that comprise the opening, which took over 1,000 computers months to render. In fact, the film played several industry screenings with the previs footage as a placeholder before the sequence was finally completed in the nick of time Nov. 6.

“When (Scorsese) saw it his only comment was, ‘I want more snow,’?” Legato recalls. “So I basically made the colorist be the compositor and we did it literally in 15 minutes, multiplying this (snow) element until we got something he wanted.”

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