Plant rigging in Houdini
FX supervisor David Stephens explains Blizzard’s approach to creating plant animation
“We developed a unified approach to the plant animation using Houdini. The first stage of the process would take a completed plant model and derive the branching structure through an iterative search algorithm. This worked surprisingly well – even for trees or bushes with tens of thousands, or even hundreds of thousands of branches. Next, the process [team] would figure out the attachment point of leaves, flowers, buds, etc. relative to the branch structure,” says David.
This meant the team could then build a hybrid ‘rig’ for the plant, explains David: “Portions of the hierarchy would be simulated using the Houdini wire solver and all other portions animated procedurally. We developed a rich suite of tools to define the simulation and procedural animation behaviors.”
It meant the team could then create a large library of plant animation cycles
for each plant at varying wind intensities. “These plant cycles could then be dropped in by the Layout department to create general ambient motion for the forest and plant cover,” says David, explaining: “For more complex shots requiring character interaction or specific choreography, the plant rigs could be loaded into a live scene with collision objects and simulated in Houdini’s Dops module. The plants on Bastion himself were simulated in this way for most shots.”