Growing a CG Forest
Karen Moltenbrey
September 7, 2017

Growing a CG Forest

Due to the advanced realism in War and the trend toward larger digital scenes, Weta Digital overhauled its foliage system to generate some of the forested backgrounds in the film. In the past, the studio employed Lumberjack, which uses dynamics as part of the construction process, to model and build trees and plants. So a Lumberjack tree would be pre-rigged with the various dynamics attached to the limbs.

The problem was, Lumberjack only handled one tree or plant at a time.

So to create the vast forests in War, Weta devised a new organic tree-growth tool. Called Totara, it is able to grow the forest and surrounding ecosystem with physical accuracy, making it more realistic. Totara builds full natural environments down to leaf-level fidelity.

The software uses competition-for-resources logic to develop naturalistic growth patterns, incorporating changes to the shape and color of the individual elements caused by aging over time. These factors are then integrated across an entire environment automatically, adapting and altering its growth to the other plant life around it. Totara takes into consideration the life cycle of a tree in regard to leaf variation and layers. As leaves decay, they will turn brown, for instance. 

Artists start with a terrain setup and sunlight paths, then distribute resources and scatter seeds across the terrain. Next, they grow the forest over 100 years or so, depending on the maturity needed. Trees will grow according to the properties of their species and will begin to compete for the resources. Branches die for lack of sunlight, undergrowth fills in. Later, artists freeze the simulation and select an area that fits their scene. 

“That forest still has all the individual trees and properties that our Lumberjack system had, and they are dynamic as well,” says Dan Lemmon, VFX supervisor. The system also heavily leverages instancing. Whereas Lumberjack instanced leaves and trees, Totara instances the branches, letting artists build a host of infinite variety of trees made of component branches. So millions of trees can fit into an environment and also fit into the memory footprint.