Nishikado conceived the game by combining elements of Breakout (1976) with those of earlier target shooting games, and simple alien creatures inspired by H. G. Wells' The War of the Worlds. The hardware was unable to render the movement of aircraft, so the game was set in space, with a black background. It had a more interactive style of play than earlier target shooting games, with multiple enemies who responded to the player-controlled cannon's movement and fired back at the player. The game ended when the player was killed by the enemies. While earlier shooting games allowed the player to shoot at targets, Space Invaders was the first where multiple enemies fired back at the player. It also introduced the idea of giving the player multiple lives and popularized the concept of achieving a high score With these elements, Space Invaders set the general template for the shoot 'em up genre. It became one of the most widely cloned shooting games, spawning more than 100 imitators with only the most minor differences (if any) from the original. Most shooting games released since then have followed its "multiple life, progressively difficult level" paradigm, according to Eugene Jarvis. Golden age and refinement (late 1970s to early 1980s) Following the success of Space Invaders, shoot 'em ups became the dominant genre for much of the golden age of arcade video games, from the late 1970s up until the early 1980s, particularly the "space shooter" subgenre. In 1979, Namco's Galaxian—"the granddaddy of all top-down shooters", according to IGN—was released. Its use of colour graphics and individualised antagonists were considered "strong evolutionary concepts" among space ship games. Atari's Asteroids (1979) was a hit multi-directional shooter, allowing the player to shoot in any direction by rotating the game's spacecraft. The Space Invaders format evolved into the vertical scrolling shooter sub-genre. SNK's debut shoot 'em up Ozma Wars (1979) featured vertical scrolling backgrounds and enemies, and it was the first action game to feature a supply of energy, similar to hit points. Namco's Xevious, released in 1982, was one of the first and most influential vertical scrolling shooters. Xevious is also the first to convincingly portray dithered/shaded organic landscapes as opposed to blocks-in-space or wireframe obstacles. Side-scrolling shoot 'em ups emerged in the early 1980s. Defender, introduced by Williams Electronics in late 1980 and entering production in early 1981, allowed side-scrolling in both directions in a wrap-around