DA: What was the most creatively challenging part of your work on the film?

AW: "The main creative challenge for us was in how to depict events that had never been manifested in reality before, and to keep them believable. No one has ever seen a lava bomb the size of a four story house hit the ground before, but even so, if not done right, you can pull the audience right out of the moment. This was the central challenge for us. To present these collossal events in a very real and believable context.

DA: What were the technical challenges?

AW: "The work within our sequences was amongst the biggest challenges we have ever had to undertake. The bulk of our work was effects animation and made extensive use of simulations, producing enormous datasets that then had to be fed through our pipeline into our renderers. So every aspect of our effects pipeline had to be carefully examined and optimized to handle these huge datasets. One of our shots contained simulation assets totalling 4 terabytes. Keeping this sort of data flowing through our pipelines presented a huge challenge.

"We also had to push our in-house fluid simulator, called Squirt, very hard to be able to produce the full screen pyroclastic flows seen in the movie, and rendering these also carried a price. We had to spend a lot of time optimizing the memory handling of our volume renderer, DNB, to be able to render just a single frame."

DA: Much of the film show natural phenomema destroying real world, man-made structures. How do you do this realistically?

AW: "To maintain the requisite levels of detail within [the St Peter's Square sequence] and therefore the correct levels of scale and believability, we developed heirarchical ways of breaking up buildings. We would start with a master key frame animation to nail the beats in each shot, and would then run a further simulation based around that. Once we had found the right character to the breakup, we would then further break up the geometry of each building and run further simulations. This allowed us to build up destruction down to the brick level, yet be able to manage complexity and in some ways steer it creatively, which is always a problem with simulation

"We applied a similar approach to Yellowstone and the ashcloud. We would simulate discrete pyroclastic elements to generate a library, and then use that library to effectively sculpt each ashcloud. This let us play with the structure and blocking of each shot, yet retain a believable sense of movement within it."

DA: What are you most proud of in your work on the film?

AW: "When we initially received the brief for our work, the scale and ambition for what these sequences should be was immediately apparent. It was almost like a gauntlet had been thrown down. I'm immensely proud of the crew at Double Negative for rising to, and at times exceeding, expectations and delivering two incredible sequences."

DA: What tools did you you and why?

"We used Maya, Houdini, Renderman and Shake extensively for the backbone of our work, but then used a lot of proprietary tools for the more specialized tasks. For instance, we have an in-house fluid solver which we used to generate all of our pyroclastic flows, our fire and smoke effects and water simulations. This is then rendered using our in-house volume renderer DNB.

AW: "We also have a custom dynamics engine running inside of Maya called Dynamite which we extended for this show to handle all of the heirarchical St Peters destruction, as well as the expanding fault lines in the Yellowstone sequence. The reason for using Maya as a backbone for our pipeline is that people know it, and when new people start at the company, they can hit the ground running. The reason for using so much custom software is that most of the time, in order to retain creative control over an effect, you need to remove limitations imposed by available tools, and the only way to do that is develop new software. Having custom codebases for our tools also allows each project to retain flexibility in how they want to operate and approach certain effects, adapting the tools for each new challenge."