How Arnold Renderer was used on Gravity – An Exclusive interview with Martin Preston
Head of R&D at Framestore, Martin Preston, kindly took the time out of his busy schedule to talk to us about how Framestore adopted Arnold Render as the renderer for Gravity. Arnold is now their main rendering software and has been used on 47 Ronin, Walter Mitty and Robocop… to name just a few…
Must Watch -> Making of Gravity by Framestore
We needed to make everything as physically accurate as possible and there was only one way to do that: switch renderer to Solid Angle’s Arnold.
“I really wanted to use a physical-based rendering solution because it enables you to concentrate on making beautiful images and the other things you want to be worrying about” explains VFX Supervisor Tim Webber. “You don’t need to agonize over certain aspects of reality, or worry about cheating and faking things, as they are closer to what they should be in the first place.”
“We investigated a number of options, including mixing renderers, before settling on using Arnold for the show”, says Head of R&D Martin Preston. “Once we’d made the decision we needed to add support for Arnold to all our in-house technology, and our shader team (led by Dan Evans) needed to re-engineer much of how they approached their work. Doing all that for such a big show was quite nerve-wracking, but I think we were surprised how smoothly it went. In fact while we were doing it our other shows were starting to follow Gravity’s lead in using Arnold.”
Rendering and its scheduling was a huge challenge – render times could become phenomenal, owing not just to the incredibly long shots but also the amount of geometry in each one. In fact, had it been done on a single core machine with one processor rendering would have needed to have started at the dawn of Egyptian civilisation.
“Optimisation therefore became the buzzword of the show”, says Preston. “We spent an awful lot of time learning how to make best use of Arnold, which tended to result in either us developing new technology, particularly on the interior shots, or refining our lighting setups to focus computation where it mattered.”
But with a considerable amount of careful planning Arnold was up to the task. “I don’t know what we’d do have done without it. It was a great tool in our arsenal” says CG Supervisor Chris Lawrence. The realism it provided was essential to the compositors too “I don’t think the interior shots would have looked as good if we hadn’t rendered with a raytracer like Arnold. It would have taken a lot more lighting adjustment to make it look real” says Anthony Smith. For the Earth it was “key in getting such realistic and beautiful clouds, and the water surfaces” says Earth Supervisor Kyle McCulloch.
The lighting was another major benefit to the switch. “To me, it’s all about the way that the light bounces. Arnold does it beautifully, and realistically, in a subtle way that you have to work very hard to achieve with other renderers” says Webber. The improvement was felt right from the beginning, when cinematographer Chivo was pre-lighting the film because the light was behaving in a way that he was used to.
Our computer lighting rigs were also designed to be very similar to the traditional rigs that would be used on set, with bounces and blockers, so Chivo was working with a set-up as close to what he was familiar with as possible. “It made the whole pre-lighting process realistic. I think if you were pre-lighting with another rendering solution so much would change by the end lighting that there wouldn’t be any tie-in. There were a massive number of advantages to it” says Webber.
As Arnold was so fast at raytracing polygons the modelling team found it could make much heavier models. It was more efficient to up the polygon count knowing the renderer could handle it than use lots of displacement maps that would put the detail in at render time.
With more than 2000 assets (from the ISS to the floating props inside it) displayed from almost every angle for minutes at a time, it was a godsend. “What we found was that we could just give Arnold the actual heavy model we baked from, reduce it as much as possible but keep all the detail in there, and it was very happy to render it. The ISS is 100 million polygons with no displacement maps! It’s just raw polygons for the most part with UVs” says head of modelling, Ben Lambert.
Arnold’s ability to accommodate a brute-force approach to modelling meant that Framestore needed to boost the performance of the infrastructure serving the renders. “Some of that was improvements to our file servers”, continues Preston, “and some of it was gained by us carefully optimising the way our tools handled that enormous quantity of data. Previously we’d relied on streaming data into the renderer as and when required, on Gravity we just needed to get everything we had into the renderer as fast as possible and let it handle it!
“Overall it was a big step to go away from Framestore’s normal rendering solutions, but one that certainly paid off. Probably the best decision I made on the whole movie” says Tim Webber.
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