premise of adjacent cubes or polyhedra whose sides connecting them form portals. The scenes that are drawn are the cube the player ship is in and the areas of other cubes the camera can see through their portals, and the process is repeated as the player enters a different polyhedron. Cubes can be deformed so long as they remain convex. Respectively, these prevent overdraw and accelerate portal rendering, making the engine practical to run on even inexpensive personal computers of that time. The precedeing Descent introduced a dynamic light-sourcing scheme by which the environment could be lit with flares, while newly added in Descent II is the ability to shoot out sources of light which will darken rooms as well as a headlight powerup which can continuously illuminate the space in front of the player. The game's directors, Mike Kulas and Matt Toschlog, explained they upgraded the game's artificial intelligence to contain instructions mimicking the tactics they saw players use in multiplayer mode, such as dodging behind walls and sneaking up from behind. The C-written code composing the AI was expanded to about 4,000 lines, a task the two described as being difficult. Releases The Descent II demo was released in December 1995, becoming one of the most downloaded PC games in January 1996. Unlike the preceding Descent whose shareware release could be upgraded to the full version while leaving the existing shareware files intact including player saves, the Descent II demo was a self-contained program that was not upgradable to the full version. Like the original Descent the demo version of Descent II presented the story as still screens with text and also uses the in-game engine for the mine escape sequence; while the full version replaces all of these with full-motion video pre-rendered cutscenes incorporating voice acting. The demo features eight of ten weapons from the first Descent and six of ten weapons from the full Descent II. The Descent II demo featured the first three regular levels of the game (the teleporter to the secret level was disabled), after completing the third level the player moves from Zeta Aquilae to a new star system with the story to be continued. The full release (see below) adds a fourth regular level (with a boss in lieu of the reactor) as well as the secret level to the Zeta Aquilae system, making it in line with the next five