In SIGGRAPH Production Session, VFX Supervisor Stefen Fangmeier and Prime Focus Co-founder Merzin Tavaria Talk CG in B&W

This Friday, North American movie audiences can buy a return ticket to Sin City, courtesy co-directors Robert Rodriguez and Frank Miller, who have mounted a sequel to the groundbreaking 2005 release that pushed the visual envelope in its effort to replicate the style of Miller's expressionistic, monochromatic comic books on the movie screen. Sin City: A Dame to Kill For promises more of the same aggressive visual style, with an added third dimension (the film was shot in ARRIRAW with an Alexa M stereo rig, though the VFX were all completed in 2D and post-converted to stereo).

At SIGGRAPH in Vancouver, BC, last week, VFX supervisor Stefen Fangmeier and Prime Focus co-founder and Chief Creative Director Merzin Tavaria discussed the VFX challenges during a Tuesday morning Production Session. The duo was limited in terms of the footage they could share from the film, which is released this Friday, drawing on clips already featured in the film's trailer. Watch it below.

The show features a total of 2700 VFX shots, and 491 of them — mostly the more difficult shots — were completed in Vancouver, where Fangmeier was based. He recalled working all day in Vancouver, then starting shot reviews with Prime Focus VFX supervisor Tim McGovern, who was working in Mumbai, at 10 p.m. "It's amazing how effective you can be with three hours on CineSync and Skype," said Fangmeier, who came on board the project in October 2013 — too late to be present during the shoot, but early enough to have a profound influence on the film's look. In fact, Fangmeier told the SIGGRAPH audience, because the live-action shoot was almost entirely against green-screen backgrounds "in the most minimalistic way you can imagine," the VFX department got to try its hand at production design. "We got to pick the curtains, the tile, the artwork on the walls," he explained. "For all these locations, we got to pick what was on the wall and what the floor would look like."

Tavaria discussed the effort in Mumbai, where the Prime Focus VFX roster expanded from around 250 to 600 artists, and the number of compositors grew from 40 to 150. Moreover, the schedule was compressed — Prime Focus spent about eight months on the project, but Tavaria said the bulk of it was done in five months — so that different stages of work seemed to be happening simultaneously. In fact, layout work was still going on a month before VFX shots were scheduled to be delivered. "We were doing pre-production in production," Tavaria said, adding that senior stereoscopic supervisor Justin Jones didn't start work until six weeks before the 2D delivery.

Read on for more on what went into specific shots from the film, as revealed at the duo's SIGGRAPH presentation.

The blocking for the film's VFX was done at Prime Focus in Mumbai under the supervision of layout head Kyle Jefferson. This shot was revised from the original conception, moving the exploding car off to screen right rather than having it dominate the center of the screen behind Marv (Mickey Rourke), whose image was flipped to help make the new composition work. One difficulty in executing various shots for the film was deciding how to best complement Frank Miller's trademark minimal look with photorealistic VFX elements. For instance, Fangmeier said, the telephone pole in this shot had originally been rendered in a very spare style that became more realistic as the imagery evolved. "When you're going for photorealism, where you're going is much clearer," Fangmeier said of the balancing act. "What saved us is that most of the fine-tuning was done in compositing."

"Except Kadie's bar," added Tavaria. "Which absolutely killed us." 

The set for Kadie's Club featured actors sitting on real chairs at real tables (or at least half-tables), but the vast majority of the environment was CG. The club was built in Autodesk Maya, composited in The Foundry Nuke, and rendered out in V-Ray, Fangmeier said.

The evolution of another image of the inside of Katie's Club was actually dictated by the MPAA, which balked at the original shot composition. With Bruce Willis's head on the left-hand side of the picture, it looked like the woman on stage was pointing a gun at the back of his head — an MPAA-enforced taboo in movie trailers. The shot was reconfigured to put Willis on the right-hand side of the picture, and the shot passed without objection. 

In more violent moments, VFX was used to make stuntwork more realistic. In this scene depicting the shadow cast by a fistfight, the violence was made more believable by extending a character's arm so that the stage punch actually seemed to connect, hard, with the other character's head. 

For a scene in which Roark (Powers Boothe) flips some cards across a gaming table that slice a miniature figure of Johnny (Joseph Gordon-Levitt) into pieces, reference green-screen photography of Gordon-Levitt's reaction was captured. Because the photographic elements wouldn't mesh cleanly — for one thing, they were captured with entirely different lensing — Johnny was turned into a digital double in the finished shot, with his pose based on the live-action reference performance. 

For the background of a scene featuring Nancy (Jessica Alba) in a graveyard (shot on green-screen, natch), Fangmeier searched Getty Images for time-lapse skies that he could purchase to easily give the environment a dramatic background. The result "was a little Tim Burton-ish," he admitted, "but Robert Rodriguez liked it."

Jessica Alba received practical make-up FX for scenes where scars appear on her face, but they were completely replaced in VFX so they could be treated with more "finesse," Fangmeier said. The work often involved relighting her face to create more stylized shots with lots of individually isolated elements — but because the scars appeared in "at least 300 shots" in the film, Fangmeier said, they didn't all receive the same high degree of stylization. Still, a small army of artists was standing by, ready to jump on the roto work if the directors wanted to see the image pushed further.

As Johnny writhes on the ground, his twisted and broken fingers are a CG effect. Fangmeier said shots like this were unpleasant to look at, but working with footage of Ava (Eva Green) made up for it. "I've never worked on a movie with that many nude shots or semi-nudes," he told the SIGGRAPH audience. "So that was interesting."

Throughout the film, roto arists isolated myriad elements of the live-action plates so that color could be applied to eyes, lips, blood, etc. "I had never done anything so stylized," Fangmeier said.

To help convey the scope of the work, Tavaria described "two types of roto" on the film. First, he said, was the traditional extraction of objects shot against green-screen backgrounds. Then there was what he called "style roto," which included detailing out the whites of the eyes, red lips, or just one side of a person's face for relighting purposes. 

In one shot, Fangmeier said, Eva Green's bra was painted out. "It was an amazing amount of clean-up," he recalled. "Even Jessica Alba — we did little touches here and there for her dance sequence."

All of that detail work in VFX, Fangmeier said, meant that the film went into the DI suite almost fully graded, following a final render pass in Nuke. "I saw in black and white for seven months, pretty much seven days a week — it was a bit of a slog, being creatively stuck in a black-and-white world," he said. But now, he's ready to revisit the project at the multiplex. "I want to see it in a real theater to see what the fans will think."