Zero Dark Thirty hits stores today on Blu-Ray and DVD. To mark the film’s release, we spoke with the film’s VFX supervisor, Chris Harvey.

During the conversation we discussed the difficulties of recreating secret helicopters, the satisfaction of subtle CGI and the ethics of digitally changing a film after it’s finished.

HeyUGuys: I didn’t pick out a lot of the CGI in the film, which I presume means you’ve done your job properly, but what was the heaviest CG shot in the film?

Chris Harvey: The helicopter stuff would have been the heavy stuff. It’s not heavy in the way that Avengers was, the entire world was digital and there are 15 characters running around or whatever, but 90% of the time you see the helicopters in the film they’re entirely digital. At least 90%, maybe 95%. So even when you see stuff you presume we built some and shot that, most of the time it’s all digital. So the whole crash sequence is pretty extensive. And it’s not just the helicopters that have to be added digitally, it’s all of the auxiliary effects that they kick up.

We shot real ones, but we still had to augment all of that. There’s a lot of dust, and while there might have been dust that kicked up on set we had to clear the set of any debris, again for safety, so we would have to go back and add in digital paper and cups, bottles, cans, laundry and miscellaneous garbage that gets kicked up in the rotor wash. So you’d see a helicopter flying over, and along with that there’s full-frame dust, garbage blowing around, some of the explosions are either fully replaced, or augmented, and then some shots, because they redesigned the entire crash sequence, with what happened to it, some of those shots are 100% digital, and most people don’t realise that the compound and the walls that are crumbling and the helicopter and every pixel in frame is 100% digital.

Staying on the helicopters for a second, presumably you had to model the stealth helicopters to some degree. that’s got to be a weird experience, trying to build a stealth helicopter that we’ve only seen in photos, where you’re not able to take measurements, or get light refraction information.

Yeah, that’s definitely one of the big challenges we had. You can’t go look at the real helicopter and do any measurements on it. We had what we got from production design because they built their 60’ practical ones, that we ended up using for mostly interior work, but we still had them on set, and at times they stood in and were fine. So we used those measurements for the big volumes, but for little details on how paint reacts, and how the windows react, and all that kind of stuff, we had to look at other sources.

We looked at the stealth bomber, and we looked at prototypes for stealthy helicopters, and the stealth fighter, and anything we could find that would be somewhat representative of what this might be. And really we just had to base it on that, because there’s nothing else to base it on. And to be honest, there’s some creative license taken. Clearly, a real stealth helicopter, flying in a moonlit sky, you just wouldn’t see it at all. We had to see something, so  we had to take some license so that you could actually see the helicopter, and while it wouldn’t be a spectacle, and wouldn’t stand out, there’s still some aesthetics you want to see in there.

On the subject of stealthy effects that you can’t see, what are you most proud of? What would no-one know was digital unless you pointed it out?

There are a couple of shots of the helicopter, not by themselves the most amazing shots, with the nature of the framing and what happens, there’s no reason I think anyone would think that they’re digital, and we’re pretty proud of those. The shot of them just starting to lift off, and it’s really a close-up of the wheel, and the wheel housing, you just see the weight of the wheel.

There’s another shot, a medium close-up of the pilot, looking through the window, signalling someone, and it’s just the nose of the helicopter, again you think it’s probably just the practical one, and it’s all digital as well.

Was it a practical pilot, or did you create a digital person?

It was a practical pilot. It was a Black Hawk pilot, we were filming a real Black Hawk. We just lifted him out of that, and replaced everything around him. So the background is real, and the pilot is real, but everything around him is digital. It’s just one of those shots that’s just a nice looking shot, but you’d never think it was a digital effect, because why would you?

You touched on the idea of changing things after the event, now there’s obviously been a lot of discussion about that one way or another, recently, but how often are you called upon to not only fix a shoot, but to help a director when they change their mind and they haven’t got something in the can?

All the time. It’s pretty common, and it’s not really new; that’s been the nature of digital effects for a long time, the more we do it, the more they realise they can do it. I think a good director will limit that as much as possible because they’re going to get what they want on set, on the shoot, but filmmaking is a creative process, and there’s always going to be times when you’re like, ‘you know what, this isn’t working in the way I expected now that the edit’s come together, and something about it is wrong’. Quite often there are good changes, there need to be changed.

The crash sequence that we changed, Kathryn had a really good reason to change it. She wanted a film that was fitting information she had available as close as possible to reality. She got new information about the crash, and what we filmed was wrong, it’s not what happened, and we had to change it. That’s cool. That’s a commitment to telling what she wanted, and the honesty of her story. I respect that, so it was a case of ‘let’s make that change, we’ll figure out how to do it’.

Elaborating on that a little: we’re talking about that for the Blu-Ray release. What if, hypothetically, in the future someone has made a film about a historical event, and in the time between theatrical release and Blu-Ray release, new information comes to light. Do you think it’s a good idea, and do you think that it’s viable, for people to amend the product digitally between theatrical release and Blu-Ray release?

It does happen. I’ve been on films when shots have been added or changed after the film’s come out. In some cases they do it between a North American release and a worldwide release. Sometimes new shots will appear, or changes will appear in shots, because they have an extra week and want to change something, and they can do the one cut of the film and not the other. I don’t know, it’s a tough one. In some ways, should you change a major story point? If you tweak a shot or add a shot because it’s the director’s cut versus what was allowed in theatres, sure, but fundamentally should you change the film? I don’t know, it’s an interesting question.