Ikaruga

is a bullet hell shoot 'em up developed by Treasure. It is the spiritual sequel to Radiant Silvergun (1998) and was originally released in Japanese arcades in December 2001. The story follows a rebel pilot named Shinra as he battles an enemy nation using a specially designed fighter called the Ikaruga which can flip between two polarities, black and white. This polarity mechanism is the game's key feature and the foundation for its stage and enemy design. All enemies and bullets in the game are either black or white. Bullets which are the same color as the player are absorbed while the others will kill the player. The game features both single-player and cooperative modes.

Development on Ikaruga began during director Hiroshi Iuchi's off-hours while Treasure was busy developing Sin and Punishment (2000). Titled "Project RS2", the game began as a sequel to Radiant Silvergun, and borrows many elements from it as well as the polarity mechanism from Treasure's Silhouette Mirage (1997). During the game's prototype stages, the player's ammo was limited. The bullet absorption mechanism was used as a means to refill ammo. However, this was found to be weak as it created too many breaks in the action so it was later added as an additional mode in home ports. In tradition with Treasure's game design philosophy, Ikaruga was intentionally crafted to challenge the conventions of standard game design and develop a new type of shooting game. All together, five Treasure staff worked on Ikaruga, as well as three support staff from G.rev.