Procedural generation

Procedural generation is a widely used term in the production of media; it refers to content generated algorithmically rather than manually. Often times, this means creating content on the fly rather than prior to distribution. This is often related to computer graphics applications and video game level design.

Overview

The term procedural refers to the process that computes a particular function. Fractals, an example of procedural generation, dramatically express this concept, around which a whole body of mathematics—fractal geometry—has evolved. Commonplace procedural content includes textures and meshes. Sound is often procedurally generated as well and has applications in both speech synthesis as well as music. It has been used to create compositions in various genres of electronic music by artists such as Brian Eno who popularized the term "generative music".

While software developers have applied procedural generation techniques for years, few products have employed this approach extensively. Procedurally generated elements have appeared in earlier video games: randomly generates terrain and NPCs, creating a world roughly twice the actual size of the British Isles. Soldier of Fortune from Raven Software uses simple routines to detail enemy models. Avalanche Studios employed procedural generation to create a large and varied group of tropical islands in great detail for Just Cause.

The modern demoscene uses procedural generation to package a great deal of audiovisual content into relatively small programs. Farbrausch is a team famous for such achievements, although many similar techniques were already implemented by The Black Lotus in the 1990s.

Contemporary application

Procedurally generated content such as textures and landscapes may exhibit variation, but the generation of a particular item or landscape must be identical from frame to frame. Accordingly, the functions used must be referentially transparent, always returning the same result for the same point, so that they may be called in any order and their results freely cached as necessary. This is similar to lazy evaluation in functional programming languages.

Video games

The earliest computer games were severely limited by memory constraints. This forced content, such as maps, to be generated algorithmically on the fly: there simply wasn't enough space to store a large amount of pre-made levels and artwork. Pseudorandom number generators were often used with predefined seed values in order to create very large game worlds that appeared premade. For example, The Sentinel supposedly had 10,000 different levels stored in only 48 or 64 kilobytes. An extreme case was Elite, which was originally planned to contain a total of 2⁴⁸ (approximately 282 trillion) galaxies with 256 solar systems each. The publisher, however, was afraid that such a gigantic universe would cause disbelief in players, and eight of these galaxies were chosen for the final version. Other notable early examples include the 1985 game Rescue on Fractalus that used fractals to procedurally create in real time the craggy mountains of an alien planet and River Raid, the 1982 Activision game that used a pseudorandom number sequence generated by a linear feedback shift register in order to generate a scrolling maze of obstacles.

Today, most games include thousands of times as much data in terms of memory as algorithmic mechanics. For example, all of the buildings in the large game worlds of the Grand Theft Auto games have been individually designed and placed by artists. In a typical modern video game, game content such as textures and character and environment models are created by artists beforehand, then rendered in the game engine. As the technical capabilities of computers and video game consoles increases, the amount of work required by artists also greatly increases. First, high-end gaming PCs and current-generation game consoles like the Xbox 360 and PlayStation 3 are capable of rendering scenes containing many very detailed objects with high-resolution textures in high-definition. This means that artists must invest a great deal more time in creating a single character, vehicle, building, or texture, since gamers will tend to expect ever-increasingly detailed environments.

Furthermore, the number of unique objects displayed in a video game is increasing. In addition to highly detailed models, players expect a variety of models that appear substantially different from one another. In older games, a single character or object model might have been used over and over again throughout a game. With the increased visual fidelity of modern games, however, it is very jarring (and threatens the suspension of disbelief) to see many copies of a single object, while the real world contains far more variety. Again, artists would be required to complete exponentially more work in order to create many different varieties of a particular object. The need to hire larger art staffs is one of the reasons for the rapid increase in game development costs.

Some initial approaches to procedural synthesis attempted to solve these problems by shifting the burden of content generation from the artists to programmers who can create code which automatically generates different meshes according to input parameters. Although sometimes this still happens, what has been recognized is that applying a purely procedural model is often hard at best, requiring huge amounts of time to evolve into a functional, usable and realistic-looking method. Instead of writing a procedure that completely builds content procedurally, it has been proven to be much cheaper and more effective to rely on artist created content for some details. For example, SpeedTree is middleware used to generate a large variety of trees procedurally, yet its leaf textures can be fetched from regular files, often representing digitally acquired real foliage. Other effective methods to generate hybrid content are to procedurally merge different pre-made assets or to procedurally apply some distortions to them.

Supposing, however, a single algorithm can be envisioned to generate a realistic-looking tree, the algorithm could be called to generate random trees, thus filling a whole forest at runtime, instead of storing all the vertices required by the various models. This would save storage media space and reduce the burden on artists, while providing a richer experience. The same method would require far more processing power. Since CPUs are constantly increasing in speed, however, the latter is becoming less of a hurdle.

A different problem is that it is not easy to develop a good algorithm for a single tree, let alone for a variety of species (compare Sumac, Birch, Maple). An additional caveat is that assembling a realistic-looking forest could not be done by simply assembling trees because in the real world there are interactions between the various trees which can dramatically change their appearance and distribution.

In 2004, a PC first-person shooter called .kkrieger was released that made heavy use of procedural synthesis: while quite short and very simple, the advanced video effects were packed into just 96 Kilobytes. In contrast, many modern games have to be released on DVDs, often exceeding 2 gigabytes in size, more than 20,000,000 times larger. Naked Sky's RoboBlitz used procedural generation to maximize content in a less than 50MB downloadable file for Xbox Live Arcade. Will Wright's Spore also makes use of procedural synthesis.

In 2008, Valve Software released Left 4 Dead, a first-person shooter based on the Source engine that utilized procedural generation as a major game mechanic. The game featured a built-in artificial intelligence structure, dubbed the "Director," which analyzed player statistics and game states on the fly to provide dynamic experiences on each and every playthrough. Based on different player variables, such as remaining health, ammo, and number of players, the A.I. Director could potentially create or remove enemies and items so that any given match maintained an exciting and breakneck pace. Left 4 Dead 2, released in November 2009, expanded on this concept, introducing even more advanced mechanics to the A.I. Director, such as the ability to generate new paths for players to follow according to their individual statuses.

An indie game that makes extensive use of procedural generation is Minecraft. In the game the initial state of the world is mostly random (although the generator is constrained in some ways to make the world appear real, for example below a certain height most of the blocks are rock), and new areas are generated whenever the player moves towards the edges of the world. This has the benefit that every time a new game is made, the world is completely different and will need a different method to be successful, adding replay value.

Another indie game that relies heavily on procedural generation is Dwarf Fortress. In the game the whole world is generated, completely with its history, notable people, and monsters.

Film

As in video games, procedural generation is often used in film to rapidly create visually interesting and accurate spaces. This comes in a wide variety of applications.

One application is known as an "imperfect factory," where artists can rapidly generate a large number of similar objects. This accounts for the fact that, in real life, no two objects are ever exactly alike. For instance, an artist could model a product for a grocery store shelf, and then create an imperfect factory that would generate a large number of similar objects to populate the shelf.

Noise is extremely important to procedural workflow in film, the most prolific of which is Perlin noise. Noise refers to an algorithm that generates a patterned sequence of pseudorandom numbers.

Cellular automata methods of procedural generation

Simple programs which generate complex output are a typical method of procedural generation. Typically, one starts with some simple initial conditions like an array of numbers. Then one applies simple rules to the array, which determine the next step in the evolution of the output—that is, the next array of numbers. The rules can be fixed, or they can be changing in time. One can let the program run, and given the right update rules and initial conditions, one can obtain a non-repeating evolution starting from an initial array.

Cellular automata are discrete computer models of evolutionary behavior which start from an array of differently-colored cells, then apply update rules to determine the colors of the next array of cells. Typically one starts with a two-color program, and a random finite array of black and white cells. Then one defines a neighborhood—for the simplest case, just the two neighboring cells of any one cell—and creates a so-called "update rule" to determine what the next cell in the evolution will be. Typical elementary update rules are of the form, "If the current cell is black and the cells to the right and left are white, then the next cell in the evolution shall be white."

It is an astonishing fact that such simple rules, even in the simplest case of two colors, can produce a complex evolution (see Rule 30 and Rule 110).

For game developers, this means that, using with finite initial conditions and extremely simple rules, one can generate complex behavior. That is, content can be nearly spontaneously generated from virtually nothing, which is the idea behind procedural generation. Evolving cellular automata is one way of generating a large amount of content from a small amount of input.

Software examples

Middleware

Acropora - a procedural 3D modeling software utilizing voxels to create organic objects and terrain.

Art of Illusion – an open source and free 3D modeler, has an internal node-based procedural texture editor.

CityEngine – a procedural 3D modeling software, specialized in city modeling.

CityScape - procedural generation of 3D cities, including overpasses and tunnels, from GIS data.

Filter Forge – an Adobe Photoshop plugin for designing procedural textures using node-based editing.

Houdini - a procedural 3D animation package. A free version of the software is available.

MaPZone/ProFX – a middleware product designed to create and generate procedural textures in games (used in RoboBlitz). ProFX has been renamed to "Substance Air."

Softimage - a 3D computer graphics application that allows node-based procedural creation and deformation of geometry.

SpeedTree – a middleware product for procedurally generating trees.

Terragen – landscape generation software. Terragen 2 permits procedural generation of an entire world.

Ürban PAD – a software for creating and generating procedural urban landscape in games.

World Machine - a powerful node-based procedurally generated terrain software with a plugin system to write new, complex nodes. Exports to Terragen, among other formats, for rendering, as well as having internal texture generation tools. See

Space simulations with procedural worlds and universes

Elite (1984) - Everything about the universe, planet positions, names, politics and general descriptions, is generated procedurally; Ian Bell has released the algorithms in C as text elite StarFlight (1986) Exile (1988) - Game levels were created in a pseudorandom fashion, as areas important to gameplay were generated. (1993) - Much as the game Elite had a procedural universe, so did its sequel.

(1995)

Mankind (1998) - a MMORTS where everything about the galaxy, systems names, planets, maps and resources is generated procedurally from a simple tga image.

Noctis (2002)

Infinity: The Quest for Earth (In development, not yet released)

Vega Strike - An open source game very similar to Elite.

Games with procedural levels

;Arcade games

The Sentinel (1986) - Used procedural generation to create 10,000 unique levels.

Darwinia (2005) - Has procedural landscapes that allowed for greatly reduced game development time.

;Racing games Fuel (2009) - Generates an open world through procedural techniques

;Role-playing games ''Captive (1990) generates (theoretically up to 65,535) game levels procedurally

Virtual Hydlide (1995)

(1996)

Diablo (1998) and Diablo II (2000) both use procedural generation for level design.

Torchlight, a diablo clone differing mostly by art style and feel.

procedurally generates a large game world, including civilization structures, a large world history, interactive geography including erosion and magma flows, ecosystems which react with each other and the game world. The process of initially generating the world can take up to half an hour even on a modern PC, and is then stored in text files reaching over 100MB to be reloaded whenever the game is played.

Dark Cloud and Dark Cloud 2 both generate game levels procedurally.

(2007)

The Disgaea series of games use procedural level generation for the "Item World".

Nearly all roguelikes use this technique.

;Strategy games

(2000) - Uses procedural generation for all levels and scenarios.

Seven Kingdoms (1997) - Uses procedural generation for levels.

Xconq - an open source strategy game and game engine.

Minecraft - The game world is procedurally generated as the player explores it, with the full size possible stretching out to be nearly eight times the surface area of the Earth before running into technical limits.

;Games with yet unknown genre

Subversion (TBA) - Uses procedural generation to create cities on a given terrain.

;Third-person shooters Just Cause (2006) - Game area is over , created procedurally

Almost entirely procedural games

.kkrieger (2004)

Synth video game (2009) - 100% procedural graphics and levels

Games with miscellaneous procedural effects

ToeJam & Earl (1991) - The random levels were procedurally generated.

(2002) - Water effects are generated on the fly with procedural animation by the technique demonstrated in NVIDIA's "Water Interaction" demo.

RoboBlitz (2006) for XBox360 live arcade and PC (Textures generated on the fly via ProFX)

Spore (2008)

Left 4 Dead (2008) - Certain events, item locations, and number of enemies are procedurally generated according to player statistics.

Borderlands (2009) - The weapons, items and some levels are procedurally generated based on individual players' current level.

Left 4 Dead 2 (2009) - Certain areas of maps are randomly generated and weather effects are dynamically altered based on current situation.

Star Trek Online (2010) - Star Trek Online procedurally generates new races, new objects, star systems and planets for exploration. The player can save the coordinates of a system they find, so that they can return or let other players find the system.

(2010) - A free, procedurally generated arena-shooter

Minecraft (2009) - All the worlds are pseudo-infinite and procedurally generated from a random seed.

See also

Artificial Creativity

Procedural modeling

Procedural texture

Procedural animation

Scenery generator

Fractal landscape

Cellular automata

Digital morphogenesis

L-System

Perlin noise

Random dungeon

Generative Modelling Language

Iterated function system

Generative art

Synth video game

Shape grammar

References

External links

The Future Of Content - Will Wright keynote on Spore & procedural generation at the Game Developers Conference 2005. (registration required to view video).

Darwinia - development diary procedural generation of terrains and trees.

Filter Forge tutorial at The Photoshop Roadmap

Procedural Graphics - an introduction by in4k

Texturing & Modeling:A Procedural Approach

Ken Perlin's Discussion of Perlin Noise

The HVox Engine: Procedural Volumetric Terrains on the Fly (2004)

Procedural Content Generation Wiki: a community dedicated to documenting, analyzing, and discussing all forms of procedural content generation.

Procedural Trees and Procedural Fire in a Virtual World: A white paper on creating procedural trees and procedural fire using the Intel Smoke framework

A Real-Time Procedural Universe a tutorial on generating procedural planets in real-time

Category:3D computer graphics