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Creating games with entities and components Denis Defreyne / RUG::B / December 4, 2014 1

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I make games*! 2 * Or at least I try.

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Less about games. More about paradigms. 7

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I am not a professional game developer. 8 DISCLAIMER

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Put your data first. 10

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If all you have is the data,
 you know what it’s about.
 If all you have is the behavior,
 you’re still in the dark. 11

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In object orientation, objects encapsulate data. 12

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Objects are defined by their methods, not by their data. 13

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Encapsulation is not data-driven. 14

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Object orientation is not data-driven. 15

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You can be data-driven in an object-oriented language. 16

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Objects combine data with behavior. 17

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Games have data position, velocity, health, … Games have behaviors movement, combat, AI, … 18

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How does the game data get its behavior? 19

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class Spaceship end 20

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class Spaceship def update # TODO: Write me end end 21

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class Spaceship def update # Move @position_x += @velocity_x @position_y += @velocity_y # … end end 22

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class Spaceship def update # Move @position_x += @velocity_x @position_y += @velocity_y # Calculate acceleration @acceleration = 0.0 if Keyboard.key_down?('w') @acceleration = 10.0 # TODO: Play accelerate sound elsif Keyboard.key_down?('s') @acceleration = -3.0 end # … end end 23

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class Spaceship def update # Move @position_x += @velocity_x @position_y += @velocity_y # Calculate acceleration @acceleration = 0.0 if Keyboard.key_down?('w') @acceleration = 10.0 # TODO: Play accelerate sound elsif Keyboard.key_down?('s') @acceleration = -3.0 end # Rotate if Keyboard.key_down?('a') @rotation -= 5 elsif Keyboard.key_down?('d') @rotation += 5 end # TODO: Add rotational velocity # … end end 24

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class Spaceship def update # Move @position_x += @velocity_x @position_y += @velocity_y # Calculate acceleration @acceleration = 0.0 if Keyboard.key_down?('w') @acceleration = 10.0 # TODO: Play accelerate sound elsif Keyboard.key_down?('s') @acceleration = -3.0 end # Rotate if Keyboard.key_down?('a') @rotation -= 5 elsif Keyboard.key_down?('d') @rotation += 5 end # TODO: Add rotational velocity # Accelerate @velocity_x += Math.cos(@rotation) * @acceleration @velocity_y += Math.sin(@rotation) * @acceleration # … end end 25

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class Spaceship def update # Move @position_x += @velocity_x @position_y += @velocity_y # Calculate acceleration @acceleration = 0.0 if Keyboard.key_down?('w') @acceleration = 10.0 # TODO: Play accelerate sound elsif Keyboard.key_down?('s') @acceleration = -3.0 end # Rotate if Keyboard.key_down?('a') @rotation -= 5 elsif Keyboard.key_down?('d') @rotation += 5 end # TODO: Add rotational velocity # Accelerate @velocity_x += Math.cos(@rotation) * @acceleration @velocity_y += Math.sin(@rotation) * @acceleration # Cap speed @velocity_x = [@velocity_x, @max_velocity_x].min @velocity_y = [@velocity_y, @max_velocity_y].min # … end end 26

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class Spaceship def update # Move @position_x += @velocity_x @position_y += @velocity_y # Calculate acceleration @acceleration = 0.0 if Keyboard.key_down?('w') @acceleration = 10.0 # TODO: Play accelerate sound elsif Keyboard.key_down?('s') @acceleration = -3.0 end # Rotate if Keyboard.key_down?('a') @rotation -= 5 elsif Keyboard.key_down?('d') @rotation += 5 end # TODO: Add rotational velocity # Accelerate @velocity_x += Math.cos(@rotation) * @acceleration @velocity_y += Math.sin(@rotation) * @acceleration # Cap speed @velocity_x = [@velocity_x, @max_velocity_x].min @velocity_y = [@velocity_y, @max_velocity_y].min # Render Graphics.translate(@position_x, @position_y) do Graphics.rotate(@rotation) do @sprite.draw end end end end 27

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class Spaceship def update # Move @position_x += @velocity_x @position_y += @velocity_y # Calculate acceleration @acceleration = 0.0 if Keyboard.key_down?('w') @acceleration = 10.0 # TODO: Play accelerate sound elsif Keyboard.key_down?('s') @acceleration = -3.0 end # Rotate if Keyboard.key_down?('a') @rotation -= 5 elsif Keyboard.key_down?('d') @rotation += 5 end # TODO: Add rotational velocity # Accelerate @velocity_x += Math.cos(@rotation) * @acceleration @velocity_y += Math.sin(@rotation) * @acceleration # Cap speed @velocity_x = [@velocity_x, @max_velocity_x].min @velocity_y = [@velocity_y, @max_velocity_y].min # Render Graphics.translate(@position_x, @position_y) do Graphics.rotate(@rotation) do if @acceleration > 0.0 @flame_animation.step @flame_sprite = @flame_animation.sprite @flame_sprite.draw end @sprite.draw end end end end 28

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class Spaceship < Movable … end 29 ?

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class Spaceship include Movable include Decoration … end 30 ?

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How can you be data-driven in an object oriented language? 32

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A better approach: entities, components, systems. 33

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Objects can have logic. That doesn’t mean they should. 34

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class Spaceship attr_reader :position_x, :position_y attr_reader :velocity_x, :velocity_y attr_reader :acceleration_x, :acceleration_y attr_reader :rotation attr_reader :shield_cur, :shield_max, :shield_rate attr_reader :armor_cur, :armor_max def initialize(params = {}) … end end 35

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Entities are stupid objects; they’re just structs. 36

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How do these entities get their behaviors? 37

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Behaviors are handled by systems. 38

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A system is essentially a procedure* that is called thirty times per second. 39 * A function with only side effects

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If you mutate state, you might as well be explicit about it. 40

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spaceship = Spaceship.new(
 position_x: 200,
 position_y: 150,
 velocity_x: 10,
 velocity_y: -5,
 armor_cur: 100,
 armor_max: 100)
 movement_system = MovementSystem.new movement_system.update(spaceship)
 p [spaceship.position_x, spaceship.position_y] # => [210, 145] 41

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class MovementSystem
 def update(entity)
 entity.position_x += entity.velocity_x entity.position_y += entity.velocity_y end end 42

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class MovementSystem class HealthSystem class InputSystem class AISteeringSystem class RenderingSystem class … 43

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Systems are decoupled. 44

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Data is stored in components. 45

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class Spaceship attr_reader :position_x, :position_y attr_reader :velocity_x, :velocity_y attr_reader :acceleration_x, :acceleration_y attr_reader :rotation attr_reader :shield_cur, :shield_max, :shield_rate attr_reader :armor_cur, :armor_max def initialize(params = {}) … end end 46

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class Spaceship attr_reader :position attr_reader :velocity attr_reader :acceleration attr_reader :rotation attr_reader :shield attr_reader :armor def initialize(params = {}) … end end 47

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Position = Struct.new(:x, :y) Velocity = Struct.new(:x, :y) Acceleration = Struct.new(:x, :y) Rotation = Struct.new(:rad) Shield = Struct.new(:cur, :max, :rate) Armor = Struct.new(:cur, :max) 48

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Components have no behavior. 49

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Entities are essentially just collections of components. 50

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class Spaceship attr_reader :position attr_reader :velocity attr_reader :acceleration attr_reader :rotation attr_reader :shield attr_reader :armor def initialize(params = {}) … end end 51

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spaceship = Entity.new 52

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spaceship = Entity.new spaceship.add(Position.new(400, 200)) spaceship.add(Velocity.new(0, 0)) spaceship.add(Acceleration.new(0, 0)) spaceship.add(Rotation.new(Math::PI / 4)) spaceship.add(Armor.new(100, 100)) spaceship.add(Shield.new(100, 100, 10)) 53

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class MovementSystem def update(entity) entity[Position] += entity[Velocity] end end 54

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Entities group components. Components contain data. Systems update components. 55

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What are the advantages being data-driven? 56

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It gives us the power to change behavior at runtime! 57 WIN #1

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# Become invisible spaceship.remove(Sprite) # Become mortal angel.add(Health.new(100)) # Become a ghost asteroid.remove(CollisionShape) # Mind control enemy enemy.remove(AISteering) enemy.add(PlayerSteering) 58

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It gives us data locality*! 59 * Not so much in Ruby. WIN #2

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60 HD RAM Cache ~ 1 000 000 ns ~ 1-10 ns ~ 500 ns

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To avoid cache misses, keep similar data together in memory. 63

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64 position velocity rotation armor shield

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65 position velocity 14 used bytes / 32 total bytes = 44% efficiency (for movement system) rotation armor shield

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Store components in contiguous arrays. 68

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positions[47] = Position.new(400, 200) velocities[47] = Velocity.new(0, 0) shields[47] = Shield.new(cur: 100, max: 100, rate: 2) armors[47] = Armor.new(cur: 50, max: 50) 69

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Smart use of the CPU cache can lead to a 50x speedup! 70 * Source: Game Programming Patterns by Robert Nystrom

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Saving and loading becomes trivial! 71 WIN #3

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# Save File.write('save.json', JSON.dump(@world)) # Load @world = JSON.parse(File.read('save.json')) 72

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Level data becomes plaintext! 73 WIN #4

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{ "position": [400, 200], "velocity": [0, 0], "shield": {"cur": 100, "max": 100, "rate": 2}, "armor": {"cur": 50, "max": 50} } 74

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In-game editors become easy to write! 75 WIN #5

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Entities group components. Components contain data. Systems update components. 77

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DEMO 78

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79 @ddfreyne denis@stoneship.org Denis Defreyne, expert game dev newbie.


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80 This talk would not have been the same without some great assets that I could use, either for free or for a well-deserved donation. The fonts in this presentation are Clear Sans by Intel (01.org/clear-sans) and Ubuntu Mono by Canonical Ltd (font.ubuntu.com). Most of the sprites are by Kenney Vleugels (kenney.nl) and are part of the Kenney Donation Pack (kenney.itch.io/kenney-donation). The planet sprite in the 2D space shooter example is by Justin Nichol. Assets in the initial screenshot are by Sven Ahlgrimm.