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Mechanics Revisited
Lecture 2
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Purpose of Today’s Lecture
� Give a review of formal design elements � Not everyone here has had the Intro Games course � And for the rest of you, it has been over a year
� Develop a deeper understanding of mechanics � Understand the important of interactions � Understand the analysis challenges
� Set us up for the later lectures on mechanics � Why do we need game grammars?
Mechanics Revisited 2
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Actions
� Verbs that describe what the player can do � Walk � Run � Jump � Shoot
� Does not need to be attached to an avatar � Build � Swap � Rotate
Mechanics Revisited 3
Action Platformer
(left or right) (walk, but faster!) (up; jump/run for left or right) (left or right)
(RTS or simulation) (Bejeweled clones) (Stacking games)
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� How do verbs, goals relate? � Imagine there no challenges � What verbs must you have?
� Example: Platformers � Goal: reach exit location � Only need movement verbs � Killing enemies is optional � Other actions are secondary
� Design Goal: Primary only � Secondary verbs lead to bloat � Add features with interactions
Primary Actions
Mechanics Revisited 4
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� Often in puzzle platformers � Platformer verbs + something
� “Innovation on the cheap”
� Verb that alters “geography” � Access hard-to-reach areas � Directly overcome challenges � Not directly needed for goal
� But do this sparingly! � Indies have one new verb! � Other features are interactions
Secondary Actions are Acceptable
Mechanics Revisited 5
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� Not a direct action of player � Outcome of the game state � Can happen without controller
� Example: collisions � Accidental or player forced � May be bad (take damage) � May be good (gain power-up)
� Other Examples: � Spatial proximity � Line-of-sight � Resource acquisition
Interactions
Mechanics Revisited 6
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Game Mechanics
� Game mechanic � Relationship between verbs and interactions � Often call this relationship the “rules” � Gameplay is manifestation of these rules
� Example: Joust � Verbs: Flap; go left or right � Interaction: Collision with opponent � Rule: If hit opponent, lower player dies
Mechanics Revisited 7
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Gameplay Example: Joust
Mechanics Revisited 8
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� Can we limit to one verb? � Mechanics are all interactions
� Common in mobile, tablet
� Due to lack of input modes
� Example: Sneak Beat Bandit � Has only one verb: move
� Rhythm game; move to beat
� All movement on rails
� If obstacle in way, turn
� Line-of-sight mechanics
Design Goal: Verb Minimalism
Mechanics Revisited 9
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Beat Sneak Bandit
Mechanics Revisited 10
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Avoid Verb Proxies
� Proxy: verb that activates another verb � “Use an item” (what does the item do?)
� “Shoot” (what does the weapon do?)
� Make your verbs outcome oriented � Fire standard projectile (like shoot, but says what it shoots)
� Fire freezing beam (what is does and how it is applied)
� Important questions to ask � Does it help me reach a goal?
� Does it overcome a challenge?
Mechanics Revisited 11
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Avoid Verb Proxies
� Proxy: verb that activates another verb � “Use an item” (what does the item do?)
� “Shoot” (what does the weapon do?)
� Make your verbs outcome oriented � Fire standard projectile (like shoot, but says what it shoots)
� Fire freezing beam (what is does and how it is applied)
� Important questions to ask � Does it help me reach a goal?
� Does it overcome a challenge?
Mechanics Revisited 12
Behavior is defined by interaction of projectile with the environment
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Combining Actions
� Verbs can combine in interesting ways � Run and jump in a platformer � Strafing fire in a shooter
� Typically result of the interactions � Each verb interacts with environment in different way � Combination of two give extra feature for “free” � This is an example of emergent behavior
� Not all combinations are emergent � Example: Double jump is not a feature of interactions � This type of verb combination is a distinct action
Mechanics Revisited 13
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Running Jump
� Can move while in midair � Just horizontal movement � Not realistic; it is a game � Many platformer challenges
assume this type of control
� Different than a long jump � Less height than reg. jump � No control once in the air � Would be a distinct action
Combining Actions
Strafing Fire
� Based on “real life” property � Bullets travel in straight line � Movement changes origin � Walking side-side makes a
spray (used in covering fire)
� But some features are gamy � Bullets slower than life � Character faster than life � Creates interesting effects
Mechanics Revisited 14
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Running Jump
� Can move while in midair � Just horizontal movement � Not realistic; it is a game � Many platformer challenges
assume this type of control
� Different than a long jump � Less height than reg. jump � No control once in the air � Would be a distinct action
Combining Actions
Strafing Fire
� Based on “real life” property � Bullets travel in straight line � Movement changes origin � Walking side-side makes a
spray (used in covering fire)
� But some features are gamy � Bullets slower than life � Character faster than life � Creates interesting effects
Interaction
Interaction(?)
Mechanics Revisited 15
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Combining Actions
Mechanics Revisited 16
Is this an example? Why or why not?
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Interactions and Limitations
� You cannot always perform an action � Shooting may require ammo � Cannot (always) jump in mid air
� Limitation: requirement to perform action � Boolean test (like an if-then) � Only one limitation per verb � If more than one, split into more verbs � Example: double jump is different from jump
Mechanics Revisited 17
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Game State
� Game State: values that represent the game world at a specific moment in time
� Interaction: function between game states
� Many of these values are spatial � Represent location of various game objects � Also physical values like velocity, acceleration
� Other changeable state values are resources � Governed by your game economy
Mechanics Revisited 18
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Extrinsic
� State affects other objects
� Often detection mechanics � Line-of-sight � Proximity
� But can include collisions � Damage an opponent � Move an object
Mechanics Revisited 19
Spatial Interactions
Intrinsic
� State affects the player
� Affect player resources � Player takes damage � Player transfers gold
� Can effect player position � Bounce off obstacle � Attraction to magnet
Most difficult to handle
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� Current pos. → new pos. � Which leads to new position � Unless player can escape…
� Game state is a vector field � Each pos. a velocity vector � Indicates pos. at next frame � Implicitly defined by code
� So you can use science of vector fields to understand � With several caveats
Mechanics Revisited 20
Intrinsic Effects Have Feedback Loops
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� Convergent Singularities � Vectors lead to line or point
� Sucks player into a location
� Removes player agency
� Divergent Singularities � Conflicting directions
� Player “trapped” there
� Worse than convergent
Mechanics Revisited 21
Singularities: A Design Problem
Particularly a problem in “pure physics” games.
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� Seeing fields helps design � What is stable (convergent)? � What is instable (divergent)?
� Have to draw “possibilities” � What if player is here? � Rerunning physics sim at
multiple player locations � Very compute intensive
� Also, possibility of whom? � Just player or other objects?
� Visualization Ideas? Mechanics Revisited 22
Visualizing Vector Fields
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Numeric State
� Does not change: attributes � Example: item strength
� Part of data-driven design
� Can change: resources � Example: ammo, health
� Often limit actions
� Also mark “partial” success
Mechanics Revisited 23
Non-Spatial Interaction
Symbolic State
� Lock-and-key mechanics � Do you have item X?
� Possession needed to pass
� Also for “slot” mechanics � Item gives attribute boost
� Only one of item type
� Creates dilemma challenge
Most difficult to handle
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Balancing Resources
� Sources: How a resource can increase � Examples (player): ammunition clips, health packs � Example (external): spawn points
� Drains: How a resource can decrease � Examples (player): firing weapon, player damage � Examples (external): monster death
� Adjust sources and sinks to “balance” economy � Together, determine “price” of resource � Price of resource should reflect its “power”
Mechanics Revisited 24
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Underpricing
� Cheap, powerful actions � Players favor these verbs � Limits play variety
� Examples: � Buff spells in most RPGs � Dragon Age cold spells
Mechanics Revisited 25
Design Problem: Pricing Resources
Overpricing
� Player never use resource � Heroes IV: non-heroic units � Raise Dead in classic RPGs
� Waste of designers’ time
� Player “penalized” for fun
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Underpricing
� Certain resources never used � D&D 3.5: Hideous Laughter better
than Daze � Might and Magic: Buff spells
better than damage
� Can produce monotonous play
Mechanics Revisited 26
Design Problem: Pricing Resources
Overpricing
� Expensive, weak actions � Usage is “penalized” � Waste of designers’ time
� Examples: � Shredder ammo in ME2 � Raise Dead in early D&D
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Underpricing
� Cheap, powerful actions � Players favor these verbs � Limits play variety
� Examples: � Buff spells in most RPGs � Dragon Age cold spells
Mechanics Revisited 27
Design Problem: Pricing Resources
Overpricing
� Expensive, weak actions � Usage is “penalized” � Waste of designers’ time
� Examples: � Shredder ammo in ME2 � Raise Dead in early D&D
� Resource usage determines difficulty � Resident Evil: Availability of ammunition � D&D 3.x: 20% resource per encounter
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� Simple combat mechanic � Each round, swap damage � Enemy dies when health is 0
� Player goes until health is 0 � There is healing in game � …but too sparse to go forever
� Two primary characters � Paladin: can lessen damage � Vampire: drains blood to heal � Which is better?
Mechanics Revisited 28
Resource Analysis: Dungelot
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Bad Design: “Engines”
� Actions combine to make resources free � Spend one resource to get another � Use new resource to get old one back
� Example: Dragon Age � Resources: Health, Mana � Small health loss; regain much mana � Small mana loss; heal much damage � Solution? Cool-down time
Mechanics Revisited 29
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� Resource mechanics often economic transactions � State is available resources � Action trades old for new � New state: result of “purchase”
� How can we see change? � State is high dimensional � Paths are non-exclusive
� Need special models � Resource flow graphs? � Petri Nets?
Mechanics Revisited 30
Resources Have a Visualization Problem
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� Level design about progress � Sense of closeness to goal � Choice of “paths” to goal
(dilemma challenge) � Path choice can relate to
play style and/or difficult
� Easier to design if discrete � Flow-chart out progression � Edges are mechanic(s)
� But these state values are continuous (sort of)
Mechanics Revisited 31
Progression
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� Design is discretization � Impose flow chart on state � Each box is an equivalence
class of game states
� Spatial Discretization � Contiguous zones � Example: past a doorway
� Resource Discretization � Range of resource values � Example: build threshold
Mechanics Revisited 32
Discrete Progression
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� Is the discretization correct? � Each game mechanic takes
one state to another state � Is state reachable in chart? � Is edge expected mechanic?
� Very hard problem!
� Idea: design building blocks � Transitions “proven” correct � Link together to make chart � Motivates game grammars
Mechanics Revisited 33
Problem of Discretization
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Summary
� Mechanics are combination actions and interactions � Actions are a direct result of player controls � Interactions triggered by a particular game state
� Interactions require a understanding of game state � Spatial state associated with physics, detection � Resources associated with limitations, unlocking
� Progression design is a process of discretization � Organize state into equivalence classes � Identify mechanics that transition between classes
Mechanics Revisited 34