The Homing Beacon issue 78 has an article explaining some of the details that went into the climatic Clone Wars battle of Star Wars Episode 2.
It's titled ILM's Clone War Secrets:
The overwhelming final act of Episode II was reason enough for repeat viewings. The chaos of the Clone War battlefield was vividly brought to life by the digital artists at Industrial Light & Magic under the leadership of Ben Snow, Visual Effects Supervisor. With the freeze-frame clarity of DVD, fans can revel in the intricate detail and craftsmanship of these amazing shots. Here are a handful of behind-the-scenes factoids from the epic battle.
- The CG models of the Republic attack gunships had to be extremely detailed to withstand viewer scrutiny during closeups. ILM even crafted a version with a fully decked out interior, which was used as the background for new bluescreen elements of the actors aboard the gunships shot during additional photography in London. The real life gunship interior sets were left in Sydney, so these new shots required digital gunship interiors.
- To efficiently deliver a realistic explosion for the gunship that gets shot out of the sky, ILM built a mandrill of the vessel. A mandrill is an all-blue practical miniature. It was rigged with pyrotechnics and blown up. The properly shaped explosion was digitally extracted, interacting with the properly shaped wreckage, and digital artists replaced the blue gunship with the computer-generated one.
- Many of the explosions of the final ground battle were real ones rather than digital fireballs. They were shot in the backlot at ILM. Explosions were such in demand that the compositors dipped into the library of explosions built for the Naboo plains battle from Episode I to fill out the shots.
- Yoda's command center was a 1/6th scale miniature.
- Though the Republic AT-TE walkers were computer-generated, at least one 1/10th scale miniature was constructed for pyrotechnic purposes. The walker that gets blown apart by an armor-busting Hailfire missile was first shot as a miniature against greenscreen. This provided valuable reference for the animators, though the scale of the resulting miniature explosion proved unusable as a final element. Also, the miniature was shot with a static camera while the finished shot had a swooping camera move that followed the rocket: a CG walker was needed to properly move with the perspective of the shot.
- A number of subtle visual clues were incorporated into the design of the shots to help audiences keep track of who's who. The good guys -- the Republic clones -- always move from screen right to screen left, while the Separatist forces moved from screen left to screen right. The sun is behind the clones, resulting in a gloomier sky behind the Separatists. Finally, the missile contrails were color-coded to denote allegiance: the Republic rockets leave clean white trails, while the villains launch missiles that leave noxious black exhaust.
- To efficiently communicate the damage sustained by the Trade Federation core ship blasted out of the sky, two versions of the computer-generated vessel were made. One bore its standard paint job. The other was the "distressed" version, with carbon scoring damage painted across the surface. Both were animated performing the same movement, and the compositors used animated mattes to gradually reveal the damaged ship from "behind" the intact one, covering the transitions with composited fire and explosion effects.