Encoding and Generating
Videogame Mechanics

Tutorial session
IEEE Computational Intelligence and Games 2012

Mark J. Nelson
ITU Copenhagen

Encoding? Generating? Mechanics?

• Encoding: a representation of a game
• Generating: producing new games (or game variants)
• Game mechanics: roughly, a game's logic

Game generation versus PCG

• Procedural content generation focuses on assets: levels, items, terrain, etc.
• Game generation focuses on... non-assets.
  • Core of the game in some sense
  • Mechanics?

This tutorial's approach

• A "correct" approach to explain isn't settled yet
• Survey of systems and papers
• End with some proposals for future work and connections

I'll be pulling up a few of the papers during the talk. To follow along:

• Annotated bibliography at: http://www.kmjn.org/notes/generating_mechanics_bibliography.html

First cluster of systems: board games

A significant chunk of work, including the earliest.

• Board games have nice formal structure
• Discrete, turn-based
• Established culture of inventing formal variants

Metagame

• Defines a generative space of games more general than chess
• "Symmetric, chess-like games"
• Motivation: worry that chess-playing AI had delved too deeply into special-case code, away from fundamental advances in intelligence

Metagame encoding

• Ontology: board, spaces, pieces with movement, capture, and promotion rules
• Goals: stalemate, eradicate, or arrival
• Symmetric by construction

• "...a tradeoff between generality, where we prefer classes which can describe the widest variety of games, and structure, where we prefer a class where the individual games appear to have similar underlying structure"

Metagame generator

Probabilistic grammar, which is possible due to the encoding.

• Roughly balanced by symmetric construction
• The defined space of games is dense in playable games (though still needs some checks)

• Modularity-breaking references mix logic among the generated branches
• (Some) high-level controllability

Metagame control knobs

• Rule complexity
• Decision complexity
• Indirect: search complexity
• Indirect: locality

Balanced board games

• Metagame aims for rough balance by construction.
• We could explicitly check for balance by simulation
• Proposal: A game is balanced if a general gameplaying engine playing itself gets roughly equal win rates on each side

Significantly smaller representation space than Metagame, in a subset of Zillions of Games ZRF format.

Space searched with a genetic algorithm.

Evolutionary game design

Ludi proposes a much more ambitious framework:

• Criteria other than balance, e.g. aesthetics
• Search a much larger space, via recombination of game elements
• Distinguish variants from new games (game distance)

Second cluster of systems: graphical logics

Vaguely an old-school arcade style:

• 2d objects that might move
• Of several kinds
• Player typically controls one (often single-player)
• Collisions might happen and have effects
• Some kind of goal

Designing graphical-logic games

What makes an interesting graphical-logic game?

• Compared to board games, less existing formal structure and theory for evaluation
• Concepts of "balance" and "strategies" in 1-player games are different

Evolving graphical-logic games

Proposal: "a game is fun if it is learnable but not trivial"

• Related to theories of fun and curiosity (1-player balance?)
• Implementation: A game is good if a controller can be learned to beat it, but not a very trivial controller
• Evolve rules and evolve controllers

Sculpting game spaces

Proposal: declaratively specify the boundaries/contents of a game space

• Flexibility: less fixed up-front ontology of elements
• Constraint-based: include or exclude games based on written criteria
• Aim is generative-space sculpting rather than optimization
• Implementation: answer-set programming

Multi-faceted generation

Proposal: Integrate PCG concerns such as level and NPC generation into the rule-generation system

• Possible synergy as elements play off each other
• Larger search space (unless further constrained)
• Internal and external criteria
• Interest in modularity, possibly in mixed-initiative design

Could we generate something like this?

Factoring game design

Four design-knowledge areas:

• Abstract mechanics: order-fulfillment
• Concrete representations: sliding beer, customer position
  • May have their own mechanics, e.g. collision-detection
• Input mappings: control bartender, hold/release button
• Thematic elements: in a bar, serving customers

Game-generation systems

What do game-generation systems generate?

• So far, mechanics at some level
  • Some more abstract, some more concrete, usually not strongly separated

Generating concrete representations

Given an abstract game, can we assign audiovisual elements and layout to its abstract state/events, and make it playable?

• So far, "game compiler" systems
• Specified ontology, rule-based compilation
• Ties into code-generation, automatic programming

Generating thematic mappings

Proposal: an auto-skinning system

• What is the relationship between mechanics and the skin?
• Mechanics define nouns and verbs
• Constraints on mapping skins to those nouns/verbs

Generating rhetorical games

Can the mapping be more than just "making sense"?

• User supplies a concept map they want a game about
• Codify rhetorical strategies in simple games
• Devise appropriate mappings

Connections to other areas

• Game-mechanic encodings as game ontology
• Formalized game mechanics for design support and verification

Game ontology

Game-mechanic encodings are formal specifications about what games "are" and "contain", at least in a particular domain.

Related investigations:

• Less-formal catalogs of components: Game Ontology Project, even TVTropes
• Historical investigations of game-element genealogy
• Discussions around specific genres

Design-support systems

Also may have formal encoding of game mechanics.

• Aim to give feedback about a design.
• May be editable, but not primarily focused on variation, or automatic design.

Proposed challenge:
extensible, reusable domains

Modular/extensible ontologies?

• Can we codify things like "graphical logic", "combat system", and associated mechanics?
• Cyc of game design? A formal TVTropes? Something else?
• Some existing concepts: ludemes, operational logics
• My own quick-and-dirty vocabularies/mechanics
• How would generation algorithms adapt to cope?

Submit to FDG 2013!

If you're interested in doing or seeing more of this kind of work, consider submitting to and attending Foundations of Digital Games 2013.

• Chania, Crete. 14-17 May 2013.
• http://www.fdg2013.org/
• Workshop proposal deadline: 28 October
• Paper deadline: 10 December