Creating a delicious destruction pipeline

When it comes to creating disaster movies for the big screen, Sony Pictures Imageworks has a wealth of experience to draw from. In Cloudy With a Chance of Meatballs, this experience is put to the test as rockets, thunderstorms and earthquakes are replaced with flying hamburgers, monster-sized pancakes and spaghetti twisters. To bring this visual effects smorgasbord to life, Imageworks built a comprehensive destruction pipeline using virtually all of Houdini’s VFX tools. Paths of Destruction To manage the mayhem, the destruction pipeline needed to include effects such as the shattering and destroying of buildings, ground impact, breaking glass, food avalanches and more. These systems were built in Houdini using a combination of Houdini digital assets and custom plug-in operators. “Houdini is an amazing tool for prototyping an effect quickly,” says Dan Kramer, DFX supervisor, Sony Pictures Imageworks. “Generally you can achieve whatever you want with the tools provided and wire together a proof of concept. For one-off effects, this is generally as far as you need to go, but with more reusable VFX Houdini lets you refine the solution into a Digital Asset and, in some cases, turn these into plug-ins to speed the process up.” The destruction pipeline started off with tools built in Houdini for shattering closed volumes into many pieces. Shattered volumes could either be diced up using a 3D voronoi diagram or defined by painting crack patterns on objects then projecting those though the model to split it up. Once split, each object was assigned a simplified convex hull representation to act as that piece’s collision body which was then run though an ODE simulation. “We’ve integrated ODE into Houdini and added quite a few features, all of which are controlled and setup though Houdini,” says Kramer. “For example, convex body collisions, breakable joints and semi-rigid joints to build ‘soft’ body dynamics were all defined using a series of Houdini digital assets to set attributes to pass along to our RBD system. Along with the RBD part of destruction is all the secondary debris, from heavy particulates to airborne dust and leaves which used Houdini particle operators working alongside our custom systems. We utilised every part of Houdini to achieve these shots.” Low Level Access In some cases, the team would write custom nodes into Houdini as is the case with the ODE rigid body solver. Houdini makes it easy to build up an interface for the customized system and allows that system to leverage Houdini’s built-in tools by passing data from custom nodes to any part of the native Houdini package Imageworks even built their own crowd system into Houdini for background characters. This system is made up of channel operators in Houdini managing lots of animation clips and a custom particle operator to steer characters though a scene given specific goals and animation cycles to choose from. “For some packages, in order to get access to the low level data you are forced to write plug-ins to get to your goal. This can be time consuming and takes too long for a proof of concept,” says Kramer. “Using plug-ins also limits the number of artists who can design an effect because the number of artists who know how to code is limited. As with all Houdini workflows, we keep our systems as procedural as possible which allows us to make changes anywhere in the pipeline without losing work. Rarely do we find ourselves getting into a situation where we have to start a shot over in Houdini,” he added. Imageworks made extensive use of Digital Assets to package up their VFX systems. Early on in the show the leads would build the systems into assets, which were then used by the bulk of the VFX team. In this way, a simplified UI is presented to the artists while only exposing the parts they need to tweak. It allows for easy maintenance and versioning when pushing work out to lots of artists. Additionally, Imageworks often turns assets created for one production into generic assets that can be readily used in future productions. Transferring Knowledge The team was quite large with over 30 VFX artists at one point that ranged from novice to expert users. The more experienced users build up the pipelines and package them into Digital Assets while the less experienced artists focus on using the interface provided to them by the leads. In this way, the artists are productive right away while being exposed to Houdini a little bit at a time. At Imageworks, the team uses a proprietary scene and geometry format that all their packages can read and write. They’ve written loaders and savers for all major packages including Houdini as well as proprietary software such as their lighting tool ‘Katana’ and their sprite renderer ‘Splat’. Houdini is used to generate the geometry and export that data to in-house rendering systems where all the assets are rendered though Arnold or Renderman depending on the project. In this way everything lives in the same world and integration is fairly easy. The box office success of Cloudy With a Chance of Meatballs is a testament all the ingenuity and production know-how at Sony Imageworks. With Houdini’s node-based workflow, Imageworks has created a comprehensive set of destruction tools that has helped streamline this process and puts them in a good position for their next VFX blockbuster.

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