1. Getting Started with Unity and Entity/Component Model
Dr. Robert Zubek, SomaSim LLC
EECS-395/495: Game Development Studio Winter Quarter Northwestern University

2. What’s Unity?
Started as a simple game development toolkit back in ~2005
Grew to a *huge* multi-platform engine-and-editor tool
Multiplatform development: Win + OSX
Multiplatform targeting: Win, OSX, iOS, Android, Xbox, Playstation, and more!
Handles mixed mobile + PC targeting better than competition
These days Unity is
An engine
A built-in level editor
A marketplace of assets

3. What’s a game engine?
Provides functionality expected of all sorts of games
Some engines are genre-specific, like (old) Unreal Engine
Some engines are genre-agnostic, like Unity
Lets you dive right into content authoring and gameplay coding
Content authoring: adding levels, character models, animations, etc.
Gameplay coding: adding gameplay systems (like weapons or upgrades), as opposed to generic infrastructure (like savefile management or network I/O)

4. What’s a game engine?
Diagram from “Game Engine Architecture” by Jason Gregory
OK so that’s not actually very readable…

5. What’s a game engine? 3rd party SDKs OS + drivers
(not part of game engine :D )
Hardware

6. What’s a game engine?
Memory allocation, math libraries, RNG, data structures and algorithms, etc.
Core libraries
3rd party SDKs
OS + drivers
Hardware

7. Async I/O, networking, serialization, localization, debugging, etc
What’s a game engine?
Async I/O, networking, serialization, localization, debugging, etc
Core systems
Core libraries
3rd party SDKs
OS + drivers
Hardware

8. What’s a game engine?
Loading/managing textures, 3d models, fonts, animations, collision, scenes, etc.
Resource management
Core systems
Core libraries
3rd party SDKs
OS + drivers
Hardware

9. What’s a game engine?
Main engine systems
Rendering system, skeletal animation, VFX, particles, collision / physics, cinematics, UI, input management, etc.
Resource management
Core systems
Core libraries
3rd party SDKs
OS + drivers
Hardware

10. Game objects and reusable systems
What’s a game engine?
Game objects and reusable systems
Game objects, async events, action scripts, state machines, AI, cameras, components
Main engine systems
Resource management
Core systems
Core libraries
3rd party SDKs
OS + drivers
Hardware

11. What’s a game engine? Title-specific gameplay systems
Resources, buffs, powerups, weapons, loot drops, levels, enemies, etc.
Game objects and reusable systems
Main engine systems
Resource management
Core systems
Core libraries
3rd party SDKs
OS + drivers
Hardware

12. What’s a game engine? Title-specific gameplay systems
Game objects and reusable systems
Main engine systems
Typical C/C++ engine
Unity
Resource management
Core systems
.NET (Mono)
Core libraries
3rd party SDKs
OS + drivers
Hardware

13. Game Loop Initialize()
while(KeepPlaying()) { ReadInputs(); UpdateGameState(); Draw(); }
Release();

14. Game Loop Initialize()
while(KeepPlaying()) { ReadInputs(); UpdateGameState(); Draw(); }
Release();
Physics really wants to run at a fixed framerate
Visuals can run on variable framerate but as often as possible
Game logic can run at a lower framerate. Also, it can be “spikey”
60 FPS =
max 16 ms per frame

15. Game Loop Initialize()
while(KeepPlaying()) { ReadInputs(); UpdateGameState(); Draw(); }
Release();
Read controller inputs
Read data over the network
GAMEPLAY CODE: update player – update AI – run game-specific logic – manage entity lifetime – call engine for anything more complicated
ENGINE CODE: physics – animation – vfx / sfx – networking – other specialized systems
Prepare meshes for display – viewport culling and other optimizations – convert to raw data and send to the GPU

16. Game Loop Initialize()
while(KeepPlaying()) { ReadInputs(); UpdateGameState(); Draw(); }
Release();
Read controller inputs
Read data over the network
GAMEPLAY CODE: update player – update AI – run game-specific logic – manage entity lifetime – call engine for anything more complicated
ENGINE CODE: physics – animation – vfx / sfx – networking – other specialized systems
Prepare meshes for display – viewport culling and other optimizations – convert to raw data and send to the GPU

17. Entities Everything is a separate entity!
Entity has no logic inside itself
Instead it relies on components for actual gameplay code
entity
entity
entity
entity

18. Inheritance vs Composition
Classic way to do OOP via inheritance:
Object
Agent inherits from Object
NPC inherits from Agent
Car inherits from Agent
Building inherits from Object
House inherits from Vehicle
Business inherits from Vehicle
Decoration inherits from Object
Tree inherits from Decoration
Object
Agent
Building
Deco
NPC
House
Tree
Car
Business

19. Cautionary Tale About Inheritance
Sketch of the entity class hierarchy:
DECEMBER 2010
Cautionary Tale About Inheritance
Zubek, “Engineering CityVille”, GDC Online 2011

20. Sketch of the entity class hierarchy:
DECEMBER 2010
MARCH 2011
Zubek, “Engineering CityVille”, GDC Online 2011

21. Sketch of the entity class hierarchy:
DECEMBER 2010
MARCH 2011
JUNE 2011
Zubek, “Engineering CityVille”, GDC Online 2011

22.  ??? Sketch of the entity class hierarchy: DECEMBER 2010 MARCH 2011
JUNE 2011
 ???
Zubek, “Engineering CityVille”, GDC Online 2011

23. Relentless Evolution “The only constant is change”
Everything will change given enough time
Need to keep “future compatibility” in mind
Prepare to keep growing and iterating
“Construction set” approach to keep building the game out over time
Zubek, “Engineering CityVille”, GDC Online 2011

24. Inheritance vs Composition
Using composition instead of inheritance:
1. Game logic stored in “components”
Each component responsible for one thing
Use as many as needed
2. Entities are just bags of components
3. Each component gets called by the engine
Start()
Update()
etc.
House Entity
Sprite
House Logic -
Business Entity
Business Logic
NPC Entity AI
Pathfinding
Tree Entity

25. Unity-specific class hierarchy
Object
contains
one or more of
GameObject
Component
Transform
(position, rotation, scale, …)
Renderer
(how to draw this thing…)
Rigidbody
(how it moves as a physics object…) …
MonoBehaviour
User-defined components

26. Unity-specific class hierarchy
Each Unity component combines data and code:
Component
Transform
Renderer
Rigidbody …
MonoBehaviour
User-defined components

27. Composition benefits House Entity Business Entity NPC Entity
Composition presents a “has-a” relationship instead of an “is-a” relationship
NPC contains a pluggable AI module, instead of “being a subclass of smart entity”
Benefits over inheritance:
Easier to mix and match functionality
Easier to make data-driven
Easier for a large team to work together
House Entity
Sprite
House Logic -
Business Entity
Business Logic
NPC Entity AI
Pathfinding
Tree Entity

28. Game Loop, Unity-style
On Each Frame { … foreach game object in the scene if this game object is active foreach component in this game object component.update()
House #52
House #53
NPC #356
Tree #975
sprite comp
sprite comp
sprite comp
sprite comp
house comp
house comp
AI comp

29. But there’s another way
Something that an entity/component engine could do (but Unity doesn’t):
1. Lay out all component data for various entities in one big array
2. An entity just contains array indices to those big arrays
3. Component update: just iterate over everything in the big array

30. Data-centric composition
“Processors” contain arrays of each component’s data.
Entities are just indices to those arrays.
Preparation: pack all component data into flat arrays, one for each comp.
On each frame:
For each component type Get its array of data Iterate and process each entry
House #52
House #53
NPC #356
Tree #975
Sprite Processor
sprite data
etc
Housing Processor
house data -
AI Processor
AI data

31. Memory access patterns
For each entity For each component Call Component.Update()
For each component type Get its array of data Iterate and process each entry
(Talk about how entities implemented as dictionaries of components allocated on the heap can have pretty random memory locations)

32. Why is cache friendliness important?
To understand why this is so, let’s take a little detour down the computer architecture lane.
RAM
HD / SSD

33. Why is cache friendliness important?
Intel i3-2120
Size
Random Access
Register
100s of bytes
0.3 ns
L1 cache
10s of KB
1.5 ns
L2 cache
100s of KB
4 ns
L3 cache
1s of MB
9 ns
RAM
1s of GB
70 ns
SSD
100s of GB
15,000 ns HD
1000s of GB
4,000,000 ns L1 L2 L1 L2 L1 L2 L1 L2
To understand why this is so, let’s take a little detour down the computer architecture lane.
1 frame at 60 FPS ,000,000 ns
More info:
RAM
HD / SSD

34. Does this matter? Indie games 100s or 1000s of entities
CPU + memory generally under-utilized
 Doesn’t matter
AAA games
10,000s of entities or more
CPU + memory heavily contested by all game systems
 Yes, definitely matters
THIS CLASS :)

35. Unity-specific class hierarchy
Object
contains
one or more of
GameObject
Component
Transform
(position, rotation, scale, …)
Renderer
(how to draw this thing…)
Rigidbody
(how it moves as a physics object…) …
Accessed in code via
GameObject.AddComponent<>()
GameObject.GetComponent<>()
etc
MonoBehaviour
User-defined components

36. What’s in a component?
A very basic Unity component is a subclass of MonoBehaviour class
This one implements two functions:
Start()
Update()
You can attach this component
in the visual editor, or
in code by calling AddComponent()

37. Component functionality
Implemented as “magic functions” for legacy reasons
See list at
Some highlights:
Awake() – when an object is created
Start() – after Awake(), when an object is enabled and ready to run
Update() – once per frame (at variable frame rate)
FixedUpdate() – once per fixed framerate (when interacting with physics subsystem)
… and many more, for physics, collisions, mouse interactions, rendering, etc.

38. Entity in the editor Transform Mesh GameObject Collider Renderer
click to inspect

39. Unity the Engine / Unity the Editor
In addition to the engine and programming support, Unity also contains a built-in editor
(Some engines don’t.)
Good for
Laying out levels, adding lights, etc.
Designing the UI
Doing visual work in general
… or you can ignore it and just code :)

40. 2D games
Here’s an example of a game done entirely with 2d assets

41. 2D sprites Unity has a lot of built in functionality for 2D:
Baking spritesheets from loose images
Animating sprites
2D physics and collision

42. 2D resources
Unity’s own tutorials on everything 2D:
Variety of free graphical assets:
_sites.php
Also free (and paid) assets in the Unity Asset Store

43. 3D games
More complicated
Much more complicated

44. 3D asset creation 3d mesh texture map animated 3d model skeleton
animation

45. 3D resources
Tons of different skills go into making 3d assets from scratch
You don’t have to do anything like that for your final project!
But if you feel like experimenting with 3D, check out free assets in the Unity Asset Store

46. How to learn? Unity evolves quickly Online resources are best
Official:
Recommended:

47. Homework preparation
1. Install Unity (personal edition) from
2. Bookmark sections of the Unity Manual for reference:
Unity Overview:
Scripting Overview:

48. Homework 1. Follow along the two PDF docs:
“First Steps in Unity for Programmers”
“Simple Endless Runner”
2. Pick some other online tutorial and also complete it
Recommended: Catlike Coding Unity Tutorials. Pick any one. “Maze” is recommended.
You can also do other tutorials.
3. Implement some kind of an interesting variant on one of the tutorials.
Deliberately vague. Do what you want! Will it be simple, or ambitious?
4. Submit everything by EOD Sun 1/10 – instructions on Canvas