The effects team at Iloura built a cavernous underground world for 'Sanctum', Producer Andrew Wight's story of danger and adventure. VFX Supervisors David Booth and Peter Webb with editor Ben Joss talk about their own adventures taking the movie, shot in stereo 3D, through post production.

The film was shot on the Gold Coast in Queensland until March 2010, when the footage was brought back to Melbourne for post. Editor Mark Warner and the editorial team's first step was two weeks of assembly for the director to look at, from which the producer's edit was cut. During assembly, First Assistant Editor Ben Joss was also working with the director to cut together the VFX sequences.

Cave Research
The VFX team at Iloura was keen to sort out their pipeline and make a start on the assets they had to build. Although at that stage only 69 VFX shots were estimated, once the artists' work got underway, the count rapidly rose. By the end, David Booth and the Iloura team had completed 380 shots, of which 330 appeared in the final film.

VFX shots were needed from the beginning of the film, with a helicopter flight over the jungle forest, the aerial reveal of the cave's doline – the Esa-ala Caves' entrance, an enormous sink hole surrounded by forest - the hole itself and the characters' entry into it, and continued throughout the film, focussed mainly on extending the foreground sets, shot against green screen, with massive, detailed CG cave interiors.

Although fairly extensive practical sets had been built at the filming locations on the Gold Coast, a number of the underground caverns needed to be as large as football stadiums and reveal fantastic structural designs that would have proven monumental to construct. As the production's on-set VFX Supervisor, David was responsible for gathering all information the FX team at Iloura would need to create these extensions from the ground up.

He and Producer Andrew Wight took a trip out to South Australia where they could study a real doline set in limestone although, at 3m across, it was much smaller than the Esa-ala Caves represented in the film. Even so, on their trip, David photographed vegetation, light angles and effects in and around the doline. He also collected photo references of rock formations and textures, which the team could use to fabricate their own rock textures and surfaces to project onto their 3D cave geometry.

Working On Set
David worked on set throughout production, recording camera data and angles, lenses, lights, and advised the DP during green screen shots and the art department on what should be built physically and what could be achieved with green screen in CG.

David explained that when shooting scenes in stereo involving green screen replacement, it was important to keep the interocular distance locked during camera moves, to avoid having to repeatedly adjust the convergence point later in post. To begin work on such shots, the team's tracker would make sure the live action left and right eye images matched properly before the artists inserted the background extension in the right eye shot, which they used as their master. When everything was complete the same work was applied to the left eye. Because setting the convergence was critical to getting these shots to work, from the foreground straight through to the CG background, keeping the I/O locked during the shoot was helpful.

Terrain Challenges
David shot the helicopter footage for the forest flyover shown as the characters approach the caves at the start of the story. It hadn't been easy to capture, not least because getting the large, heavy rig into the helicopter proved a challenge. Landing spots were so far from the shooting location that stopping between takes to adjust the rig or clean the lenses was impractical – any adjustment had to be done while flying.

David said, "One of the most challenging CG sequences illustrates what the director had underestimated – to tell this story, the audience had to see the environments in detail and comprehend their scope and size and consequently, many shots from various angles had to be created throughout the film. This scene was at the vast cave entrance, as the characters abseil and parachute down to the interior below. A ledge was built for the actors to jump from, over a 40ft deep space. The VFX team had to build the cave environment, the parachute, the parachutist, birds, falling water and light shining through it – all in CG, down to the bottom."

Image Correction
David, also a skilled compositor, had gained experience with stereo images from his work on the 3D 'Cane Toads' documentary the year before, and put considerable time into correcting some of the shots that had stereo issues, ranging from adjusting convergence to coping with water splashing up from the flooded sets onto the lenses. David said, "The side by side rigs were more compact and manoeuvrable for working underwater, but these can only achieve an I/O of 60-65mm at the smallest. As a result, the close up to mid-range underwater shots frequently needed convergence adjustment." Convergence angle adjustment was not confined to underwater shots, and formed an important part of David's work on the film. It sometimes involved blowing up shots to compensate for loss of imagery after the shots have been shifted within the frame, left and right, to change the angle, but David managed to avoid this as much as possible by rebuilding the missing information from the available footage from both cameras, using Shake for the compositing. Further shot correction tasks David carried out included removing 'floaters', objects that, only when viewed in 3D, would sit uncomfortably in stereo space between the actor and viewer, causing a distraction.
Another issue was the huge amount of rushing water on set during the flooding scenes. If one lens was splashed with water, David could in most cases reconstruct the damaged image from the 'good' eye, rebuilding the frame with a blend of the two in relation to Z depth. Ben Joss remarked on this problem as well and said a drop of water appearing on one lens is quite hard to cope with in stereo and very obvious on the monitor. Very occasionally, he would need to create a 2D shot to insert into the rest of the footage.

Also the headlamps the actors were wearing tended to dominate shots in which the convergence had been changed, and David would have to reduce the detail in the lamps and adjust the shot. Furthermore, while they read reasonably well in the live action footage, they weren't so successful against green screen and had to be removed and replaced with CG beams, ensuring they were shining in the correct direction to work stereoscopically as well.

Alignment and Colour
Sanctum was Iloura's first stereoscopic production and consequently required quite a bit of R&D before starting on the actual shots. The effects team's VFX Supervisor Peter Webb explained that a sound understanding of how the brain perceives 3D and processes left and right images helped them decide how to best approach their shots. Moreover, because a viewer's experience looking at a 2D image in a cinema is slightly different to observing the 3D world, understanding the theory behind achieving a good stereoscopic image was important.

The original footage had to be prepared before it went into their VFX pipeline, checking for alignment and colour disparity problems. The success of all of their effects work would depend on correct stereo alignment and positioning of the convergence point relative to the interocular distance. The team helped David Booth with some of the image correction work, using Nuke with the OCULA plug-in. The right and left images of shots requiring set extensions and compositing in CG elements, in particular, had to be tracked before the team started work.
Their pipeline consisted of Nuke for compositing, Syntheyes for most of the stereo tracking. 3ds Max and Maya were used for the CG component and also for lighting set ups.

Aerial Views
The editor or director especially liked certain views from the helicopter footage David had shot, but decided some takes didn't run long enough or show the precise angle they wanted to see. In such cases CG extensions often helped – extending the forest in certain directions and tracking in the imagery. Aerial views of the Esa-ala Caves' entrance were a part of many of the aerial shots of course, and these had to accommodate every angle the photography adopted. This meant realistic vegetation and rock had to be completed in full detail to match the landscape.

But in fact, they ended up 'cheating' on the size of the hole in some shots in order to produce the necessary dramatic effect. In some instances they had to make it twice the actual size than in others - after all, it was going to be the focus of the entire story. Nuke with OCULA, plus a few of Iloura's own techniques, helped blend the CG into the shots while making sure the stereoscopy was consistent.

Sense of Scale
A basic challenge was communicating the correct sense of scale inside the caverns. In a stereoscopic film, achieving this partly depends on setting the interocular distance correctly on their CG camera rigs. Setting the two CG cameras too far apart will make a space appear small, so they had to ensure that the technical scale of their rigs produced the desired effect. Adjusting the relative sizes and positions of people, rocks and cavern walls within the 3D stereo space took lengthy trial and error.

Working with light, how it fell on objects and calculating the light fall-off and pinpointing distance when the camera faced out into the dark reaches of the set extensions was another way of conveying their size. Up to a point, this could be based on how light from the actors' headlamps behaved in the built sets. Atmospherics such as drifting rain and water helped to occupy the huge spaces and catch stray shafts of light.

Creating such massive set extensions as the caverns tends to make the usual use of HDR images of the set for lighting inadequate. However, given the story's underground location, the absence of a natural light source would have meant the only available light would have come from the divers' helmets, requiring the artists to creatively introduce enough additional light to make a worthwhile viewing experience while maintaining an authentic feeling.

"Our designs for the sets were just a start point," said Peter. "From there, working with the lighting and the stereo became a creative process." All of these elements were calculated into their complete light simulation, and although everything started with real-world calculations, once each sequence was in the review theatre, it had to be assessed for dramatic impact, storytelling and the director's vision. Sometimes the environments needed to be brighter, darker, or appear closer or further from the camera.

Reference material for building the CG sets came from a quite a few sources. Production supplied stills and moving footage of the environments they wanted the caverns to resemble. As an experienced caver himself, writer and producer Andrew Wight had incorporated some of his own experience in a caving disaster in the Nullarbor into this story, and was able to help influence and guide the way the CG sets' looks developed. He sat with the team and helped review their work and commented on what looked realistic and feasible in terms of lighting and structure.

To create the rock surfaces for their set extensions, they used the hundreds of high-res reference stills David Booth had taken on set and on the research trip he and Andrew had made to South Australia. Helping to define the interior structures, in the absence of a real location to scan, were Andrew Wight's photos and reference material gathered during his caving trips, which gave them a real-life start point for the looks of the environments and lighting.

But each cavern had to have its own looks and personality – St Jude's Cathedral, the Belfrey where Ben and Frank discover a Japanese tank, the huge desert-like set the team called the 'Sunless Sahara' due to the sand dunes covering its floor – and had to be referenced separately. The team also did some of their own research. Nevertheless, no single source was able to give them exactly what they wanted or needed for each of the caverns and so the caves seen in the film are created with digital matte paintings projected over CG modelled terrain.