Video game graphics  A variety of computer graphic techniques have been used to display video game content throughout the history of video games. The predominance of individual techniques have evolved over time, primarily due to hardware advances and restrictions such as the processing power of central or graphics processing units. Some of the earliest video games were text games or text-based games that used text characters instead of bitmapped or vector graphics. Examples include MUDs ("Multi-User Dungeons"), where players could read or view depictions of rooms, objects, other players, and actions performed in the virtual world; and roguelikes, a subgenre of role-playing video games featuring many monsters, items, and environmental effects, as well as an emphasis on randomization, replayability and permanent death. Some of the earliest text games were developed for computer systems which had no video display at all. Text games are typically easier to write and require less processing power than graphical games, and thus were more common from 1970 to 1990. However, terminal emulators are still in use today, and people continue to play MUDs and explore interactive fiction. Many beginning programmers still create these types of games to familiarize themselves with a programming language, and contests are held even today on who can finish programming a roguelike within a short time period, such as seven days. Vector graphics refers to the use of geometrical primitives such as points, lines, and curves (i.e. shapes based on mathematical equations) instead of resolution-dependent bitmap graphics to represent images in computer graphics. In video games this type of projection is somewhat rare, but has become more common in recent years in browser-based gaming with the advent of Flash and HTML5 Canvas, since these support vector graphics natively. An earlier example for the personal computer is "Starglider" (1986). Vector game can also refer to a video game that uses a vector graphics display capable of projecting images using an electron beam to draw images instead of with pixels, much like a laser show. Many early arcade games used such displays, as they were capable of displaying more detailed images than raster displays on the hardware available at that time. Many vector-based arcade games used full-color overlays to complement the otherwise monochrome vector images. Other uses of these overlays were very detailed drawings of the static gaming environment, while the moving objects were drawn by the vector beam. Games of this type were produced mainly by Atari, Cinematronics, and Sega. Examples of vector games include "Asteroids", "Armor Attack", "Aztarac", "Eliminator", "Lunar Lander", "Space Fury", "Space Wars", "Star Trek", "Tac/Scan", "Tempest" and "Zektor".