Image credit: © Universal Studios
Did you catch the star-studded “Twisters” film this year? Take a deep dive behind the scenes of the film with SIGGRAPH 2024 Talks contributors Ben Snow and Florian Witzel. Here, Ben and Florian discuss the technological advancements of the new film, which evolved from the 1996 “Twister”. Find out how they used special and visual effects to create the film’s iconic tornadoes.
SIGGRAPH: What tools and setups were used to create these storms?
Ben Snow (BS), Florian Witzel (FW): ILM’s team simulated each of the tornadoes and storms with newly developed, physically based fluid solvers. Our simulation framework is called ILM Pyro. ILM Pyro is designed for ease of use and detail and energy preservation in the simulation. To recreate real storm dynamics, these simulations calculate air movement, temperature, buoyancy, gravity, moisture density, condensation, and dissipation, just to name a few. The solvers do billions of calculations per frame, creating the most accurate weather system visualizations we’ve seen.
Our tools also realistically calculate the way the sunlight scatters through the clouds and atmosphere. We closely studied and observed all the research and compared it to our results and then modified the parameters of our simulations to achieve the intensity, shape, and dynamics of each tornado. So, it’s part science, part art, to create each tornado.
Additionally, we broke down each tornado into a series of basic building blocks including a funnel, a dust shroud (thin layer of dust or vapor surrounding the funnel), a debris field (dust/dirt picked up from the ground) and rigid debris objects, a wall cloud where the funnel connects to the clouds, and a shelf cloud that connects to background sky. Those components were determined by our research and visual studies of real tornado footage we selected to match for each of the sequences. They are based on what science tells us about how these storms are formed.
For the EF5 tornadoes, we have basically the same ingredients, but instead of a funnel we have a large wedge. To get the detail we needed in the simulation, we broke the wedge of the tornado up into 3D volumetric tiles, like big voxels. This allowed a lot of precision with each simulation, and our software tools allowed us to have the velocities and directional vectors carry across the boundaries of the volumetric tiles so that the final result was cohesive and whole in terms of the physics and cloud/dust behavior.
One other wrinkle was that, sometimes, we added ground gusting components to the debris field — we’d have dust spiraling around the base of the tornado. For instance, the drone POV shot of the twin tornadoes we see as they form, and for a later high-angle shot in the same sequences following one of the twins towards Tyler’s truck after they split.
SIGGRAPH: Is there a difference between a rigged tornado and an animated tornado? How did you develop them?
BS, FW: The tornadoes were animated using a rig to sketch out the overall performance, character, and continuity of the tornado in each sequence. The rig allowed our animators to give the tornado its character but also made sure no physical boundaries were broken in this blocking stage. We added indicators to ensure the size was physically accurate and velocities were not exceeded, as that would break physical bounds. Each tornado had to be a character and support the story line. The animation rig allowed us to artistically modify the appearance of each storm to get the results needed.
SIGGRAPH: What were the major differences in creating these storms for the 1996 “Twister” versus the 2024 “Twisters”? How have the graphics and technology advanced over almost 30 years?
BS, FW: On the original movie, we had some video footage and stills to study, but scientific understanding of the storms wasn’t what it is today. Even if we had better information on the science of these storms, we didn’t have the tools or the computing power to make true simulations like we can today.
On the original film, we wrote particle systems using sin functions and turbulence and layers of math that visually looked like tornadoes. The process was much more like what you’d do for an animated film. It provided an artist’s impression of a storm based on us visually matching what we were making to the crude footage we had, using these simple particle systems in combination with a lot of compositing tricks to get the final result, which was still thrilling to the audience of the time and remains effective today.
Today, we are able to utilize the computing power we had for the entire first film in one shot on “Twisters”. We’re able to use new proprietary physically based fluid solvers that do billions of calculations per frame and provide energy and detail preservation. They calculate air movement, temperature, buoyancy, gravity, plus moisture density and dissipation.
For our lighting, instead of the rough and approximated shadowing we had in our renders on “Twister”, our rendering tools realistically calculate the way sunlight scatters through clouds and the way lighting and visibility of the scenes is affected by the air and moisture conditions of the storms.
SIGGRAPH: On the other hand, were there any visual storytelling strategies used in the original film that were implemented for “Twisters”?
BS, FW: What hasn’t changed is the need to use our artistic judgement. We’re still telling a story and depicting the action of the script and what director Lee Isaac Chung wants to show the audience. Our tools had to give us the control and flexibility to guide the performance of the tornadoes and art direct their look.
In the first film, the animators would animate rough splines and tornado meshes in Softimage software to show the path of the tornadoes. On “Twisters”, the animators had a similar tool, but it allowed a lot more control. On the new film, our tool sets gave the ability to create a lot more variety in our tornadoes and show the evolution of the storms much more realistically than in the past.
SIGGRAPH: What advice would you give to someone who plans to submit to Talks at SIGGRAPH 2025?
BS, FW: Every talk is very different and can be approached from many different angles, but I would say that, if you are passionate about your project and focus on sharing your unique experience and the creative problem-solving you used, you are on a good path. Also, sharing behind-the-scenes insights and practical takeaways is always great for the audience to see.
SIGGRAPH 2025 Talks submissions are now open! Take Ben and Florian’s advice, and plan for a Talk that provides behind-the-scenes insights and practical takeaways. Submit now.
Ben Snow has been working in visual effects for 35 years. He joined the burgeoning computer graphics industry as a runner in 1989 having studied computing and film at the University of Canberra. He set up the computer animation department for Conja in Sydney, where he worked on commercials and broadcast indents and openers. He left Australia to join Industrial Light & Magic in 1994, where his first project was to help create the three-dimensional computer graphics image of the Enterprise B for Star Trek: Generations. This version had to intercut seamlessly with the motion-control model shot on ILM’s soundstage, one of the first such endeavors for a feature film. He went on to do R&D, lighting and effects development on films like Twister, Mars Attacks! and The Mummy (1999). A visual effects supervisor since 1998, Ben Snow’s credits include Pearl Harbor, Galaxy Quest, Star Wars: Episode II – Attack of the Clones, King Kong (at Weta digital), Iron Man 1 and 2, Noah, and Dungeons and Dragons: Honor Among Thieves, amongst many others. Ben’s film work has garnered him four Oscar nominations and between film and real time projects his work has earned himself ten Visual Effects Society nominations. His most recent film is Twisters. Ben has also written and directed Virtual Reality projects with ILMxLAB, including the Vader Immortal trilogy for Meta Quest, Rift and Sony PS-VR, and a 360 immersive clip for Star Wars: The Force Awakens for Lucasfilm and Meta. He supervised Star Wars: Secrets of the Empire, a location-based virtual reality experience made in collaboration with Lucasfilm and The VOID. Ben continues to work on both immersive entertainment and film projects.
Florian Witzel is an award winning Visual Effects Supervisor at Industrial Light & Magic. Most recently, Florian oversaw over 1,000 effects-heavy shots in multiple offices globally for Twisters. He worked in close collaboration with the director and client from set through post-production to bring the screenplay to life. He is well known for his collaborative backing of clients’ visions and storytelling with compelling, innovative visuals, and, from there, leading teams to push the envelope of visual effects both creatively and technically to achieve new moments in filmmaking. In his 15+ years at ILM, he has innovated and supervised the effects work of Marvel’s Avengers, Doctor Strange, and Ant-Man franchises, with five of these films earning Academy Award nominations for their visual effects. Other representative work includes the Emmy-nominated Season 3 of The Mandalorian, Space Jam: A New Legacy, Transformers, Battleship, Pacific Rim, Lucy, and Rango. Florian has been honored with eight nominations for Visual Effects Society and Annie Awards, with two wins. He is an active member of the Academy of Motion Picture Arts and Sciences, and Visual Effects Society, as well as a speaker/contributor to international conferences and publications on visual effects in the motion picture industry.
