In Disney's Mulan, filmmaker Niki Caro brings the epic story of China's legendary warrior to life. One of the main suppliers of visual effects was Sony Pictures Imageworks.
The Imageworks team led by VFX supervisor Seth Maury, animation supervisor Julius Kwan, compositing supervisor Sandra Balej and VFX producer Jarrod Nesbit. The team was commissioned to deliver sequences from the epic battle sequence in Mulan's third act. The epic battle was filmed in a valley on New Zealand's South Island. The plates had a lot of natural surroundings. The artists had to add a mountain that housed the great avalanche, and also the foothills that led to the mountain. They added mountain extensions to the battlefield itself with 2D and 3D features, as well as rocks and steam springs to give the appearance of thermal pools in the ground. Imageworks was hired to produce 20 to 30 heroic shots of the avalanche sequence, which was one of the largest visual effects sequences in the film. The SPI artists divided the avalanche into two different types: wide-angle shots and more haunted shots.
To achieve the avalanche effects, Imageworks built a new system in Houdini called Katyusha to simulate snow and ice.
In addition to taking a volumetric approach, the team is committed to simulating many passes to allow flexibility in compositing. The team rendered using the internal version of Arnold from SPI.
One hundred percent photorealism was the ultimate goal. The SPI artists pointed to tons of real avalanche material. The artists used real life as a starting point, but for creative reasons, artists sometimes had to slow down or speed up the pace of the avalanche.
For Mulan's escape, Imageworks had to add two different types of horses to the film. One of these was Mulan's Blackwind, which was built with improved tools and built in detail to withstand close to the camera. The CG Blackwind was a very detailed model that used a new muscle simulation system under the horse's skin. The development team also created a new hair remover called Fyber, which is more interactive and less data intensive, so artists can iterate faster.
Sean Faden, VFX supervisor on the production side, encouraged Imageworks to create what he called the "Pepsi Challenge". He ran slow-motion footage of Mulan on Blackwind and drove through a ravine alongside the CG Blackwind. Artists shot the footage of Mulan on the horse, adjusted the lighting, and adapted the CG horse to that horse.
Artists based lower resolution background horses on the Blackwind model and varied colors, manes, tails, and other characteristics. They created as many variations as possible to make sure the crowd scenes never repeat themselves.
The CG Blackwind is featured in battlefield shots too chaotic and dangerous to be shot with a real horse, and in shots in the following sequence where Mulan competes against her friend and ally Chen Honghui (Yoson An) when he is swept in an avalanche. For this sequence, Mulan was shot on a green mechanical box and replaced with a CG horse. The face, mane and body are directly in front of the lens in some shots.