Near the end of Doctor Strange, the characters rush to Hong Kong to save the precious Sanctum there from the Dark Dimension. But the Sanctum is already destroyed. In order to stop the whole world being swallowed up into the Dark Dimension, Strange uses the Eye of Agamotto to reverse time. The entire street that had been destroyed now reconstructs itself in front of the audiences’ eyes. At the same time, the heroes take on the group of zealots – in normal time. Here’s overall vfx supervisor Stephane Ceretti, previs and postvis supervisor Faraz Hameed from The Third Floor and ILM’s Richard Bluff and how that sequence was achieved.

Stephane Ceretti (overall vfx supervisor): Everything had to move backwards, the street is reconstructing itself, you can see people running around in the streets unfolding from the ground, the buildings reconstructing themselves while our guys are fighting within that environment. So the zealots and Mordo and Strange are fighting against themselves but also the environment because they don’t know where things are going to come from. Things are destroyed around them and suddenly they reconstruct. And some of them are getting struck, some of them are getting hit by flying cars or whatever, and it’s the big conceit of the sequence is the idea that you have to fight with the bad guys but you also have to fight against the environment. It’s not just a fight.

Faraz Hameed (previs and postvis supervisor, The Third Floor): We started with the beats of the story, then visualized the big ‘notes’ of the destruction — explosions, buildings, shockwave blasts and aftermath. Then we took those ideas and fleshed them out in reverse – adding gags that could happen during the fight. The biggest challenge was having a forwards-working fight interacting with reversing elements. VFX did some exciting tests and we used that knowledge in creating shooting and techvis plans.

Stephane Ceretti: Doctor Strange was a film where instead of destroying a city we’re actually reconstructing it. So it was kind of playing the action movie story in reverse. So that was an interesting idea – I thought, ‘Hey, we’re finishing the film without having destroyed anything, we’re actually reconstructing something.’ We were shooting people running in the streets at the same time as the guys we are fighting, but the people in the streets are going backwards. Everything’s going backwards. And sometimes it’s going backwards at 48 frames per second, so it’s not going backwards at the same frame rate as our guys are fighting. So we looked at the shots and we started to look into how to shoot that, and we did a lot of R&D with ILM and, because everybody is saying no you can’t shoot that with motion control, it’s gonna take too long. So we started to look at shots, you know, designing the shots in previs, picked some key moments to do really complex camera moves, and these shots we needed motion control so we did have motion control on these key shots.

Richard Bluff (visual effects supervisor, ILM): Every shot became a bit of a science project, but all the while it was a science project that had to be art directed. It could possibly seem like an easy job to execute the visual of a building being destroyed because we’ve seen them in so many movies now and of course ILM’s done it for many, many years. But the idea of having that building destroyed, fall into the ground, and then you don’t actually see the building get destroyed. All you’re ever gonna see is the building get put back together again. What ends up happening is you end up seeing a lot of raining debris going backwards, a waterfall going backwards of debris if you like.

Stephane Ceretti: The destruction of the place as well was gradual. We built an eight hundred feet long street of Hong Kong in London that was built from about two floors, and then ILM went to Hong Kong and they scanned the buildings and created all the environmental, the street surrounds. We also had aerial photography in Hong Kong that we used for some of our wider shots, and we built that environment to be destroyed, all completely destroyable as a CG asset with cars and bamboos and all that stuff. It’s a huge, huge undertaking in terms of asset building and special effects.

Faraz Hameed: Because the shoot was in cold wet weather late at night, we made sure the previs was absolutely clear and easy to read. And we went on set to interface with the different units. It was like shooting two films at the same time — one in reverse and one forwards. Filming was very meticulous and careful. The shooting crew did an incredible job to make sure it was shot right.

Richard Bluff: What was very clear to us was that the sound effects would play a very important role in helping the audience to understand that story throughout the sequence. Because believe it or not you could forget that that’s what’s happening in the background when you’re focusing on either close-ups or mid-shots of the actors performing, talking, fighting, or whatever it may be. So having the sound effects constantly kind of going backwards that the audience is very familiar with and then having obviously the debris tied to that or drive those was very important.

Stephane Ceretti: It actually took twenty nights to shoot that sequence because you could only do like five to six or seven setups per night because of the complexity of the multi-layers of people and elements and backwards and forward action, also trying to make sure that our actors were shot separately from the people running around them were not going into the same space as the crowd. So we had to always kind of look at the shots and play them in reverse and check that there was no mixing of people in the same place in the same moment in all the different passes. And so it was a pretty big effort to, and from a stunts point of view, special effects point of view, everything was kind of choreographed very precisely.

Richard Bluff: What we ended up discovering and what the client ended up directing us towards was making sure that each shot had a certain composition, a certain rhythm to the debris leaving frame. So it wasn’t just the start of a shot, everything lifts off the ground, out of top of frame. It was, there was a sucking of the debris out of frame. There was a spilling out of frame. There was a crashing out of frame. And each one of those moments you had to imagine what the sound would be and whether it’s, compositionally whether it was in depth or whether it was across the image whether it told the right story, framed the action, but also put our actors in the right amount of danger.

Stephane Ceretti: We also used the fact that the Alex 65 which we’ve been using to shoot the film, has a huge sensor, so we’re shooting 6K plates, and we can reframe within the plates and do a lot of post-motion in that.

Richard Bluff: There’s a moment where a building covered in bamboo scaffolding collapses. Now of course in our movie it rebuilds. During that whole rebuild moment Mordo is fighting with one of the zealots, and he’s using the scaffolding that is reforming, moving back up through the frame, and he’s using them to climb on to hunt her down. And then he uses the reforming building behind the scaffolding to then encase her behind a wall. And figuring out the timing, figuring out the cadence of that sequence and making sure it felt poetic and there was a flow was the biggest challenge. It wasn’t just hitting the sim button.