Computer Graphics World

July 2009 16 Devastating Simulations T he character design artwork for the Devastator robot in Transformers: Revenge of the Fallen (see "Weighty Matters," pg. 20) depicts a 100-foot-tall gargantuan robot formed from multiple pieces of oversized mining, excavation, and demoli- tion vehicles. His mouth becomes a gigantic sucking vortex, like a giant jet engine. At ILM, we used our pre-production time to prepare for the destruction that Dev- astator would inflict upon his envi- ronment, including demolition on one of the Great Pyramids of Egypt. We separated Devastator's destructive effects into individual elements—in- cluding large-scale rigid simulation, creature rigging, air/fluid simulation, and volume rendering—to be tackled by different groups. Working from video reference of jet engine intakes in snowy environ- ments, we created a geometric repre- sentation of the path the dust would take into Devastator's mouth, which we called "the vortex." We would need to control the dust vortex in a non-physical way, as though Devastator was aiming his suction for maximum destruction, or sweeping an area with the vortex. An animation rig was created for the vortex geometry, allowing the creature technical directors to drive the performance of the vortex according to the needs of the shot. e top of the rig was constrained to Devastator's head, and the bottom was ani- mated or constrained to the ground. e geometry for the vortex was run as a cloth simulation in ILM's Zeno system under the influence of a moving noise field to give it a fluid feel. is was a quick and easy way to tune the feel, turbulence, and speed of air flowing into Devastator's mouth. After we were satisfied with the gross movement and undulation of the vortex, the shot would move into dust simulation. e cloth-simulated vortex geometry was used as a guide for the particles to flow from the bottom to the top. UV coordinates were created on the mesh that the particles used directly to find a path up the vortex. e flowing particles created a vector flow field that was used in the next stage of simulation. A Dusty Proposition e dust simulations were performed by breaking up the space around the vortex into a three-dimensional grid of tiny cells in space. Each cell was capable of holding some amount of dust. At each cell, we used the speed and direction of the flow particles mentioned before to pass dust material to neighboring cells. In this way, dust material flowed up the vortex mesh from grid cell to grid cell along the particle paths. is process was referred to as the "grid simulation." A low-resolution simulation grid was used to perform the most expensive dust emission and flow-field work, while a much higher resolution grid was used to add detailed wavelet turbulence. After the data was inspected in interactive previews, it was rendered with an array-marching RenderMan shader. In most cases, the advection and rendering grid was cam- era-aligned so that expensive calculations could be concentrated Simulation By JAson smith Jason Smith is a digital production supervisor at Industrial Light & Magic. ILM faced a gigantic task when it came to creating the vortex-like destruction made by the robot Devastator. The vortex geometry was run as a cloth sim, influenced by a moving noise field (left). This geometry was then used as a guide for the particles to flow upward and downward (right).

• Cover