Ikaruga is a 2001 Shmup (Shoot'em Up) in which the player (or players) controls a two-sided (black and white) ship.
The main gameplay of the game is based around the polarity mechanic. The enemies in the game are either black or white and fire black or white bullets. Your ship is black on one side and white on the other. Throughout the game, the player will be able to switch between the two sides of his ship and absorb bullets of the same color. If you hit enemies of one color with the opposite (for instance, hit a white enemy with black bullets) you deal double damage.
Additionally, the game offers multipliers for destroying three enemies of the same color in a row. The more sets of three you can kill, the higher your score can reach.