How to Make a Game Like Space Invaders

1
How to Make a Game LikeSpace Invaders
2
What IS Space Invaders?

• a SHMUP (shoot-em-up)
• Player has one ship, enemy has many
• Player and enemies interact by shooting at each
  other
• Top-down 2D (usually)

3

• Concepts

4
Video Frames/Ticks

• A film is made of 24 still pictures per second
• Motion is achieved by small changes to each
  picture, but 24 fps is still fast

5
Video Frames/Ticks 2

• Video games are usually 30 or 60-80 fps
• Video games achieve movement by moving each
  screen object a little bit every frame

6
Shmups are very object-oriented

• Ships are objects
• Bullets are objects
• The player is an object
• Explosions are objects
• Levels can be objects (certainly made of objects)

7
Question

• How many bullets can be on the screen at once?
• Space Invaders 1 for player, 2 for enemies
• Modern completely arbitrary

8
Lists

• A great way to store and organize objects
• Most beginners get hung up here
• Conceptually harder than arrays
• C and C use linked lists (with pointers)
• BlitzBasic has listTList

9
What is a ship?

• Is-dead flag
• Health value
• X and Y positions
• Path logic and data
• Reference to the art
• Animation state
• Bullet/missile launch state
• Bullets and Explosions are very similar to ships!

10
What is a player ship?

• Not as much, surprisingly
• Path logic is in your fingers, not in code
• So keyboard state checks (for avatar control) go
  here

11
What is a bullet/missile?

• Like a ship, but (usually) simpler movement
• Erased when it goes off screen, not when it
  reaches the end of its path
• State Player shot or Enemy shot
• Each Player-bullet collides against every enemy
• Each Enemy-bullet collides against player

12
So, to make space invaders

• Make a player
• Make a bunch of enemies
• Move them every frame, have them create bullets
• Move the bullets every frame
• Check for enemy-bullet collision every frame
• Keep going, even if all the enemies or the player
  is dead

13

• Programming

14
Main loop in Pseudocode

• Main()
• SetupEverything()
• CreatePlayer()
• CreateAllEnemies()
• done false
• while (done false)
• TickPlayer()
• TickEnemyList()
• TickBulletList()
• DrawPlayer()
• DrawEnemyList()
• DrawBulletList()
• if (EscapeKeyPressed() TRUE)

15
Timer callback version

• TimerFunction()
• TickPlayer()
• TickEnemyList()
• TickBulletList()
• // some systems, like Flash and Torque, do the
  drawing for you
• DrawPlayer()
• DrawEnemyList()
• DrawBulletList()
• if (EscapeKeyPressed() TRUE)
• done TRUE

16
TickBulletList()

• TickBulletList()
• ForEach( bullet)
• x x dx
• y y dy
• if (BulletOffScreen())
• isDead TRUE
• ForEach(enemy)
• if (Collides(enemy, bullet))
• isDead TRUE
• DamageEnemy(enemy)

17

• Basic Math

18
Vectors and Offsets

• Where is your Ship? X and Y

19
Vectors and Offsets 2

• Where is your bullet? Also X and Y
• Where is your bullet in relation to your ship?
  bulletX shipX and bulletY shipY

20
Vectors and Offsets 3

• How far apart are they? Pythagorean theorem
  (sqr(a) sqr(b) sqr(c))
• This requires a slow square root function

21
Vectors and Offsets 2

• What direction from the ship to the bullet?
  Arctangent
• Atan2(bulletX shipX, bulletY shipY)

22
Arctangent

• Usually gives a direction in radians, from
  0-(2PI)
• PI is 3.1415927 ( 180 degrees)
• Radian to degrees dir / (PI2) 360

23
Vectors and Offsets 3

• So you can describe the relationship between two
  objects on the screen in two different ways

24
Offsets

• Offset x and y coordinates (or differences)

25
Vectors

• Vector direction and distance

26
Translate Offsets to Vectors

• Get distance with Pythagoras
• Get direction with Atan2 (Arctangent)

27
Translate Vectors to Offsets

• X sin(direction) distance
• Y cos(direction) distance

28
Vectors Offsets important

• Shooting bullet directly at the player
• Homing missiles
• Collision detection (is bullet close enough?)
• Enemy follows path

29
What is TurnTowardsPoint()

• If you want the homing missile to turn slowly
  towards the enemy (instead of instantly) what do
  you do?
• The answer is the TurnTowardPoint() algorithm.

30

• Designing Your Game

31
Bosses

• Traditional part of shmups
• Each one is a love letter to the player
• Multiple weapons
• Multiple parts
• Multiple Modes
• Its a boss, not just an extended enemy

32
Powerups

• Functionally just like Bullets
• Give expanded powers to the player
• Key to one of the basic metagames

33
The Game Modes

• Play mode
• Start mode
• Results mode (you are dead, howd you do)
• Pause mode
• Credits mode
• Options mode

34
Game Modes 2

• Recognize that game modes are just states
• Completely different states than game states

35
Scrolling Background

• Space Invaders background was black
• Galaga and others had winking, scrolling stars
• Zaxxon and others started making the background
  complex and interactive
• Treasure games are famous for complex,
  puzzle-like environments

36
What are Shmup levels?

• Hand-crafted definitions of when each enemy shows
  up
• The things that happen before a boss shows up
• Divisions of art
• Scoring opportunities

37
How are levels made?

• Make the editors yourself, for yourself
• Ship path editor (mirror-able)
• Level editor
• Place art tiles
• Place ship spawn points
• Place camera path
• Boss editor

38
(No Transcript)
39
(No Transcript)
40
(No Transcript)
41
(No Transcript)
42
(No Transcript)
43
(No Transcript)