How to Make a Game Like

Space Invaders

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)

Concepts

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

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

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)

Question

• How many bullets can be on the screen at once?

• Space Invaders = 1 for player, 2 for enemies

• Modern = completely arbitrary

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 list:TList

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!

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

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

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

Programming

Main loop in PseudocodeMain(){

SetupEverything();

CreatePlayer();CreateAllEnemies();

done = false;while (done == false){

TickPlayer();TickEnemyList();TickBulletList();

DrawPlayer();DrawEnemyList();DrawBulletList();

if (EscapeKeyPressed() == TRUE)done = TRUE;

WaitForNextFrame();}

ShutDownEverything();}

Timer callback versionTimerFunction()

{

TickPlayer();

TickEnemyList();

TickBulletList();

// some systems, like Flash and Torque, do the drawing for you

DrawPlayer();

DrawEnemyList();

DrawBulletList();

if (EscapeKeyPressed() == TRUE)

done = TRUE;

}

TickBulletList()TickBulletList(){

ForEach( bullet){

x = x + dx;y = y + dy;

if (BulletOffScreen()){

isDead = TRUE;}ForEach(enemy){

if (Collides(enemy, bullet)){

isDead = TRUE;DamageEnemy(enemy);

}}

}

RemoveDeadBulletsFromList();}

Basic Math

Vectors and Offsets

• Where is your Ship? X and Y

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

Vectors and Offsets 3• How far apart are they? Pythagorean

theorem (sqr(a) + sqr(b) = sqr(c))

• This requires a slow square root function

Vectors and Offsets 2

• What direction from the ship to the bullet? Arctangent

• Atan2(bulletX – shipX, bulletY – shipY)

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

Vectors and Offsets 3

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

Offsets

• Offset = x and y coordinates (or differences)

Vectors

• Vector = direction and distance

Translate Offsets to Vectors

• Get distance with Pythagoras

• Get direction with Atan2 (Arctangent)

Translate Vectors to Offsets

• X = sin(direction) * distance

• Y = cos(direction) * distance

Vectors & Offsets = important

• Shooting bullet directly at the player

• Homing missiles

• Collision detection (is bullet close enough?)

• Enemy follows path

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.

Designing Your Game

Bosses

• Traditional part of shmups

• Each one is a love letter to the player

• Multiple weapons• Multiple parts• Multiple Modes• It’s a boss, not just an

extended enemy

Powerups

• Functionally just like Bullets

• Give expanded powers to the player

• Key to one of the basic metagames

The Game Modes

• Play mode

• Start mode

• Results mode (you are dead, how’d you do)

• Pause mode

• Credits mode

• Options mode

Game Modes 2

• Recognize that game modes are just states

• Completely different states than game states

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

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

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