Visual effects are usually one of the last things added to a film, but for the recent stereo 3D rendering of 2000AD icon Judge Dredd – Dredd 3D – the production model was altered to integrate the VFX process from the very beginning – allowing production and even the script to be changed based on what lead house Prime Focus World came up with.

We caught up with VFX supervisor Jon Thum from Prime Focus World, whose previous credits include Quantum of Solace and The Matrix.

DA: Tell us a bit more about the script-to-screen production model on Dredd 3D.

JT: "We had a relatively low budget for such a VFX-heavy film, so it was approached in a different way from the start, and was very much a collaboration between the filmmakers (DNA Films) and Prime Focus. As VFX supervisor, I was involved very early on in the design stage and we were able to develop our assets alongside the conceptuals for the film.

"When the production designer came on board we aligned our art department with his.? It was important to us to ground our visual effects in the real world, so when South Africa became the most practical and cost effective shooting location we started basing our Mega City One concepts on parts of Johannesburg and Cape Town, and those concepts held true right through to the final shots on film.?The film was shot in stereo for the most part, so we had an added complication for the VFX, with accurate camera tracking being crucial to the process.? We had a digital workflow with most footage shot on Red, SI 2K and Phantom cameras.?

DA: Could you give us some concrete examples of how creative choices made in pre-production helped to make the film overall and/or the VFX better.

JT: "When designing the megablocks we came up with the idea of exterior spaces hanging off the sides of the blocks. These appendages allowed us to break up the straight lines of the buildings and create more interesting aesthetics.

"Initially devised as basketball courts, they then became a skatepark for our hero megablock Peach Trees. Once this idea was in place, the script changed to allow some story points to revolve around the skatepark. It also gave us a great scale reference when used as a foreground for views of the city.

"Another idea that came about concerned the ground floor atrium of our megablock. We couldn’t find any location big enough for the scale we were looking for and didn’t want to build such a large set.

"I suggested using an exterior space that we could shoot at night, and found a suitable location In Cape Town by looking on Google Earth. It worked very well despite the actual shoot day turning out to be one of the windiest days of the year."

DA: What were the most important things you drew on from the comics?

JT: "The producer and writer Alex Garland was the main creative force behind the film and was a big 2000AD fan. He was in regular contact with [Judge Dredd creator] John Wagner about the direction we were taking and John had input on all the conceptuals we were making for the film.

"I did read some of the comics myself, but the look for Mega City One came out of our desire to ground the film in the real world. There are however lots of references from the comics in the film that we were able to place in some of the sets and visual effects shots, through shops, posters, building names etc."

DA: How would you characterize the look of Mega City One?

JT: "It had to be gritty and real; it had to feel like a dangerous place to be. Monolithic architecture from Eastern Europe and the Italian film Gomorrah were strong references for us in designing the look and feel of the megablocks. Then we used some of the seedier streets of Johannesburg and Cape Town as foregrounds for us to composite in our Mega City One backdrops.

"When we turned our helicopter shots of Johannesburg into Mega City One, we cut whole blocks out of the city and replaced them with jammed freeways to add to the crowded claustrophobic feel of the city, and extended the horizon to infinity. We also added in surveillance drones to enhance the oppressive atmosphere, which also allowed us editorially to tie in some of our action sequences with the Halls of Justice communications centre. Consequently, our establishing shots of Mega City One became part of the story.?"

DA: What's your favourite sequence?

JT: "The main draw for me when I read the script were the slo-mo sequences. Although in the end they weren’t technically the hardest shots to achieve, they were at the centre of the story and had to work well for the film to work well. We were interested in capturing the unexpected and from that perspective wanted to shoot as much live action as possible. We looked at lots of slo-mo stock footage to get a feel for what would look great, but also tell the story. It had to be hypnotic for the drug to make sense and we wanted the violence to somehow look beautiful.

"We knew we'd have to add VFX for the blood and gore when people are being shot, so for reference we fired bullets at blood bags and prosthetic skin and noticed the stringy nature of the blood and the shock wave impact of the bullet on skin. We shot as much of it as possible in layers so we could control the action and in VFX we added particulates in the air to add depth to the stereo. The slo-mo 'look' was developed over a period of time and consisted of separating out the colours and objects in the scene and taking them in different directions with hue and saturation. Add to that some sparkles in the highlights and some heightened stereo and we had our drug look.

DA: Dredd has been praised for its punchy use of stereo 3D. What VFX techniques did you use to help with this effect?

JT: "All of the film’s creatives were stereo novices, so the 3D was a learning process for everyone. What we did at first was look at the rulebook, then with that in mind threw it away and did some test shoots to establish a style and try something different. We also did previs in stereo for some of the more difficult sequences, and this helped us all learn how the film might look and cut with the constraints of stereo. Production design, and to an extent VFX design, was done with stereo in mind, in particular to add as much layering to our scenes as possible i.e. midground props and elements to fill our 3D sets.

"There was also manipulation in post. Stereo depth in all of our VFX shots was pushed to the max with foreground elements added where appropriate but even some of the non-VFX footage was enhanced to create more depth. With the Prime Focus World stereo post conversion team at hand, we were able to adapt the process that we use on mono footage to work on stereo footage."

DA: What's the secret to a realistic, thrilling stereo 3D explosion?

JT: "Because we're talking about stereo, the explosion has to fill the 3D space, so it’s about camera placement (close to the explosion) and the right camera interaxial distance to get the most depth out of the scene without hurting the eyes. Most of stereo shot design is about this because the more you fill the space the more you feel the depth.

"To make the explosion real, try to shoot it real – if you can’t then get some good reference. Like I said before, it’s all about the unexpected. If you make the perfect CG explosion, it might not ring true."

DA: What was the greatest creative challenge and how did you overcome it?

JT: "In addition to Mega City One we also had to create the huge 200-floor atrium around which most of the action revolves. A corner set was built that was three floors high and about half the intended width of the atrium and this set had to be reused over and again to represent the various stages of Dredd’s ascent though the building.  That meant we had a lot of VFX shots to produce – close to 300 out of a total shot count of 650, ranging from small set extensions to complete CG builds.

"We couldn’t possibly build the CG model with all the detail needed for every eventuality, so for this we frequently used the techniques of paint and reprojection. With this method we had a basic textured build of the atrium that could be redressed to represent the different floors. We would render the CG but then paint on top extra details such as graffiti and scorching to the concrete, vents and shop signs to the walls. The painted frame would then be reprojected back onto the geometry and rerendered. This makes for very efficient turnaround and allows for quick creative iterations on the final shot."