The BBC’s Christmas Day episodes of Doctor Who are traditionally a flamboyant affair, usually packing in a few celebs - and of course some big visual effects to keep everyone watching after the remains of the turkey, sprouts and crackers have been cleared away.

The most recent outing saw axisVFX, the Bristol and Glasgow-based studio, complete more than 120 shots for an episode called 'The Husbands of River Song', which reunites the Doctor (Peter Capaldi) with his once and future wife, River Song (Alex Kingston) - plus tyrannical cyborg King Hydroflax (Greg Davies), and hapless servant Nardole (Matt Lucas).

The VFX work was completed across Axis's two studios. The team - led by VFX supervisors Grant Hewlett and Stuart Aitken - worked closely with director Douglas Mackinnon and producer Nikki Wilson to create a range of CG effects including galactic star-liners and flying saucers crashing on alien planets, exploding heads and giant robots.

axisVFX co-founder Grant gave us a detailed look at the creation of four shots for the episode - giving a taste of the amount of work that goes into the 60-minute show. Be careful if you’ve not yet seen the episode however, as this one definitely contains spoilers.

A rocketing robot

The team really had to keep their heads for this formidable task: Hydroflax’s giant robot body (which operates independently) decapitates Nardole to use his head as its own. The robot takes off into the night sky (below), with Nardole’s attached head screaming.

“The Matt Lucas shot is almost entirely CG,” says Grant. “The entire robot suit is 3D, the spaceship is entirely 3D, apart from the walkway, which was filmed and then projected at a different angle. All the ground is matte painting, the surroundings are matte paintings [and] the sky is a matte painting.”

For all the spinning it had to do, a 3D model of Nardole's bodiless head was required. “Matt had a prosthetic head made, which they used on set. I was able to put that on a turntable and took about 200 photos of it, which I ran through Agisoft PhotoScan and I got pretty good textures from that. We’ve got a standard head that we use, which is all rigged. We pulled that around to the right shape of Matt Lucas’ head.”

“Then we filmed him on a greenscreen doing his “Wheeeargh!” performance. You can see this on screen - he just opens his mouth in the moment when he takes off. But we had to get that actual moment on the CG head.”

“So we unwrapped his head from the greenscreen footage, then we wrapped it back onto the 3D head to get that performance - rather than having to animate a CG head and getting all the nuances of his performance.”

“Any animation or modelling in the scene is all Maya, maybe with a bit of Modo thrown in. We used Mari and bit of Mudbox for the texturing of the suit. All of the fluid simulation for the rocket smoke is all Houdini, and all of the lighting in that particular shot was also created in Houdini.”

The snow is a mixture of 3D snow and Nuke snow. “Nuke has got a reasonably good snow system,” explains Grant. “You’ve got a real camera, and a 3D ‘box’ of snow, so you’re able to apply proper defocussing for that snow, and get some of it in focus where it’s meant to be in the shot. It instantly feels a lot more believable.”

“The VFX is intercut with lots of shots of Matt on set, in the physical suit, with practical smoke FX and the back of the saucer. A lot of planning went into it early on to work how much of the physical saucer they’d have to build on set - in the end it was just the ramp and the doorway. With very efficient shooting we managed to limit the amount of shots of the saucer, but still get all the coverage we wanted on the key people.”

Flying and landing saucers

Unsurprisingly, creating full shots of the vast ship that carries River Song and Hydroflax  - which appears both in the opening sequence, and during the meeting of the Doctor and Nardole with River Song - brought with it equally massive challenges. 

“Douglas had the Forbidden Planet saucer in his mind - the typical retro flying saucer,” says Grant. “The saucer was all modelled in Maya. We had to travel over the landscape in the opening shot, so the trees and the 3D landscape are rendered in Houdini. When we first put it out to render, it was taking around 27 hours to render a frame. It had about 400,000 trees in it.

“[The meeting sequence] was a shot where everything had to work. We needed to show the geography of the saucer to the town, we needed to link the types of trees to the earlier shot where we see the Doctor and Matt Lucas walking [towards the saucer]. We needed to see that the saucer had made a big gouge in the ground, and it needed to tie in with the opening shot.

“The foreground trees were shot on location. The centre part of the saucer, where River walks down, is real; the rest of it is matte painting [see our tutorial].

“It was initially generated in 3D [from the opening shot] then painted over [for the second shot], to create the depth, and add the two moons, and the ice on the top of the saucer and so on.”

The latter had a knock-on effect, however, as axisVFX then had to go back and match the ice on the top of the saucer in the opening shot.

“It was a bit of a pain, because it’s about 1,500 frames long,” says Grant. “The snow on the top of the saucer is generated in Houdini. The saucer is an asset in 3D, which we used in all the different shots. We knew we would have to change things, so we kept everything linked and consistent. We use publishing  quite a lot; for instance, once that saucer is changed in the matte painting shot, it becomes version 15.

"The one in the opening sequence might be version 14, so we needed to go back to that sequence, update the asset and re-render it and recomposite it.”

A shattered floor

From there, the ride doesn’t get any smoother for the team. As the ship they’re on is hit by a meteor, the floor beneath Doctor and River Song cracks so dramatically, they fall through. 

“That’s actually quite a tricky shot,” explains Grant. “On the day [director] Douglas shot it on a Jimmy Jib, which was not what I was expecting. 

“The Jimmy Jib is less like a crane than a see-saw, so when the camera goes up it also goes away at an angle, rather than straight up. It was also shot on an 8mm lens, a massively wide-angle lens with loads of distortion on it. It’s got a chequered floor, in which you can see every single bit of distortion."

This made things more complex for axisVFX as needed to ensure that the VFX the team created matched the bulging geometry of the footage, so that the end result looked realistic. 

To further complicate matters, the show also had a greenscreen that couldn’t be lit properly because it was on the floor and, as Grant puts it, had "actors all over the shot.”

“The actual camera move is just the Doctor and River Song standing on a greenscreen on the floor, while the camera moves up on the Jimmy Jib. They kneel down in the middle of that move. Then we shot another version of that, which was similar, but not exactly the same, because we didn’t have motion control.”

“We had to get those two plates to work together, then we had to shrink them down, so the actors look like they’re falling away through the cracked floor, then put the dust in. It was a bit of a jigsaw to put all that together.”

The cracking floor was achieved using tricks developed by the team’s FX lead Joe Thornley Heard: Bullet Solver - which helps collisions look real - and image-based cracking, which uses patterns to create a believable crack. "The smoke and dust is generated from the pieces as they fall, using Pyro FX in Houdini,” adds Grant. 

“It was one of the harder shots; it doesn’t have that many pieces of CG in it, but it was just tricky and technically difficult.”

Darillium 

As well as creating the planet Darillium, which is known for its ‘Singing Towers’ - a natural feature of the landscape that has great significance to the story - the team had to convincingly crash a ship into it. 

“When they’ve crash-landed, we see the planet - which we built in 3D - a couple of times,” explains Grantt. “Then there’s the big shot at the end, when we spin around the Doctor and River and reveal the towers, which is their final moment together.”

“That’s a 2.5D matte painting,” he continues. “The towers were sculpted in ZBrush and painted and projected. Once everything is in place, it was reduced down to cards in Nuke and it all sits together, with the camera move used to speed everything up.

“It was all rendered onto different elements - then a lot of work was done by the composting team, putting in depth cues and [adding] subtle grading, flares and other photographic elements to make it look realistic. That’s quite tough because the couple are silhouetted - you’re seeing through their hair. So there was a lot of [technically difficult] rotoscoping to get it to work.

“Then it’s just getting the feel of the light right on the characters, making it feel like it’s a big space, getting the right atmosphere and depth. Colours played a big part in this, and the quality of the keying around the characters.”

“Focus was my bugbear. If focus isn’t right it can really throw you off. You just need to be bold with your decisions. You need to imagine filming it in that place on that planet, and know your stuff when it comes to what you would see in a photograph like that: what part of the landscape would be blurred out, at which point would it defocus, and [allow for] general photographic sensibilities.”

“There was a lot of subtlety in that shot,” he concludes. “Apart from the massive lens flare, of course.”