Shoot 'em up (also known as shmup or STG) is a subgenre of video games within the shooter subgenre in the action genre. There is no consensus as to which design elements compose a shoot 'em up. Some restrict the definition to games featuring spacecraft and certain types of character movement; others allow a broader definition including characters on foot and a variety of perspectives. The genre's roots can be traced back to Spacewar!, one of the earliest computer games, developed in 1962. The shoot 'em up genre was later established by the hit arcade game Space Invaders, which popularised and set the general template for the genre in 1978, and the genre was then further developed by arcade hits such as Asteroids and Galaxian in 1979. Shoot 'em ups were popular throughout the 1980s and early 1990s. In the mid-1990s, shoot 'em ups became a niche genre based on design conventions established in the 1980s, and increasingly catered to specialist enthusiasts, particularly in Japan. "Bullet hell" games are a subgenre that features overwhelming numbers of enemy projectiles, often in visually impressive formations.