perspective, and players must use ranged weapons to take action at a distance. The player's avatar is typically a vehicle or spacecraft under constant attack. Thus, the player's goal is to shoot as quickly as possible at anything that moves or threatens them to reach the end of the level with a boss battle. In some games, the player's character can withstand some damage or a single hit will result in their destruction. The main skills required in shoot 'em ups are fast reactions and memorising enemy attack patterns. Some games feature overwhelming numbers of enemy projectiles and the player has to memorise their patterns to survive. These games belong to one of the fastest-paced video game genres. Large numbers of enemy characters programmed to behave in an easily predictable manner are typically featured. These enemies may behave in a certain way dependent on their type, or attack in formations that the player can learn to predict. The basic gameplay tends to be straightforward with many varieties of weapons. Shoot 'em ups rarely have realistic physics. Characters can instantly change direction with no inertia, and projectiles move in a straight line at constant speeds. The player's character can collect "power-ups" which may afford the character's greater protection, an "extra life", health, shield, or upgraded weaponry. Different weapons are often suited to different enemies, but these games seldom keep track of ammunition. As such, players tend to fire indiscriminately, and their weapons only damage legitimate targets. Types Shoot 'em ups are categorized by their design elements, particularly viewpoint and movement: Fixed shooters restrict the player and enemies to a single screen, and the player primarily moves along a single axis, such as back and forth along the bottom of the screen. Examples include Space Invaders (1978), Galaxian (1979), Centipede (1980), and Galaga (1981). In Pooyan (1982), the fixed axis of movement is vertical, along the right side of the screen. Multidirectional shooters feature 360-degree movement where the protagonist may rotate and move in any direction such as Asteroids (1979) and Mad Planets (1983). Multidirectional shooters with one joystick for movement and one joystick for firing in any direction independent of movement are called twin-stick shooters. One of the first games to popularize twin-stick controls was Robotron: 2084 (1982). Space shooters are a thematic variant of involving spacecraft in outer space. Following the success of Space Invaders, space shooters were the dominant