Bungie is using a completely redone version of the Halo engine for Halo: Reach, with no component left untouched.[33][9][10]
The environments, while larger than in the previous games, are far more detailed. The graphical effects have been drastically improved, allowing for a new atmospheric effects system and more dynamic lighting for dramatic shadows and moving light sources in interiors. The engine was improved with "imposters" systems; this allows very cheap textures far away, and moving closer will make them improve.[33] This was proved with the map, Forge World. Rain and fogging effects are also improved. Halo: Reach also features a new animation system, which smoothly blends movement between standing, walking and running as well as jumps, grenade throws and firing. The facial animation system has been completely revamped, including motion capture. Weapon effects are amplified to be more powerful and visceral than before, giving them a more serious tone. Human weapons will sound more powerful and emit smoke and showers of sparks. Plasma bolts now burn through material as they hit, and explosions will throw clouds of dirt and debris around.
Halo: Reach's engine is able to support up to 40 active AI characters and 20 vehicles with AI characters on screen at once, twice as many as in Halo 3. In addition, distant battles between AI characters follow automated behavior models, which convert into scripted behaviors as the player gets closer, and finally convert to full AI