Space Invaders Overview

Overview of the game Space Invaders.

In this video I’m going to give you an overview of the game Space Invaders.

We’ll go over how Space Invaders work, what entities make up the game and the winning and losing condition.

The game deconstruction is an important step when trying to recreate it. So, even if you know the game, this is useful to get an in depth view of how it actually works.

Space Invaders

The game consists in a bunch of aliens that are trying to invade the earth. The goal is to destroy all invaders before the invasion.

Winning Condition

But, the player never actually wins the game. Each time it destroys a wave of aliens, a new one appears and the game continues. It only ends when the player loses all lives. So the end goal is to achieve the highest score possible.

Losing Condition

The player starts with 3 lives and it’s game over and the lives reach zero.

Entities

The player controls a Laser Cannon that shoots Lasers. And the Invaders are also able to shoot Missiles at the cannon. But, the player can take take cover below Bunkers positioned between them.

From time to time a Mystery Ship appears on screen that gives extra points.

And these are the main entities of the game:

  • Laser Cannon
  • Lasers
  • Invaders
  • Missiles
  • Mystery Ship
  • Bunkers

Let’s see each one in more detail.

Laser Cannon

The Laser Cannon is at the bottom of the screen and can only move left and right.

When it’s hit by a missile, it gets destroyed. The destroy animation uses two frames and plays an explosion sound.

The player loses 1 life when the cannon is destroyed.

Lasers

The cannon shoots lasers, that start at the cannon position and move upwards. These play a sound effect when shot.

If a laser hits an invader, a bunker or the mystery ship it gets destroyed. Otherwise, it’s destroyed at the top edge of the screen.

Note: A laser is also destroyed when it hits a missile. But this was not implemented in this version.

The cannon can only shoot a single laser at a time. Only when it gest destroyed, it can shoot again.

Note: This is an important mechanic to force the player to time his shots perfectly.

Invaders

The invaders start in the middle of the screen organized in a grid of 5 rows by 11 columns.

There are three different types of invaders. Each type looks different and has a different score value associated with it.

The top row is the most valuable one, it’s worth 30 points. The second and third row from the top are worth 20 points. And the last two rows from the top are worth 10 points.

Each invader shows an animation made up of two frames.

The block of invaders starts moving to the right. When the first invader reaches the edge of the screen, the entire block shifts down and inverts direction. This patterns repeats until reaching the bottom of the screen.

When it happens, the player loses a life and a new wave starts.

Note: When the invaders reach the bunkers, they erase them. This was not implemented in this version.

If an invader is hit by a laser, it gets destroyed. It shows an explosion and plays a sound effect.

The movement speed increases as invaders are being destroyed.

Also, the invaders play a rhythmic music made up of four beats. The speed of the music also increases as invaders are destroyed.

Missiles

The invaders shoot missiles down at the cannon at random intervals. There is a maximum of 3 missiles on screen at a single time.

And there are three different types of missiles: slow straight; fast straight; wiggly. These differ in both their aspect and their speed.

Note: They also differ in their power and damage inflicted. This was not implemented in this version.

Each missile shows an animation made up of 4 frames.

When a missile hits a bunker, the cannon or the ground floor it gets destroyed and shows an explosion effect.

Mystery Ship

The Mystery Ship appears on the top of the screen moving from left to right at regular intervals of 25 seconds.

It gets destroyed when hit by a laser and shows an explosion effect.

In the original game the score of the Mystery Ship is dependent on the number of lasers shot until hitting it. But in our implementation I just went with a high score value of 100 points.

Bunkers

There are 4 stationary defense bunkers positioned between the invaders and the cannon.

These are made up of a grid of tiles that take damage from both the laser and the missiles. Each tile can take up to 4 hits before being completely destroyed.

Note: Not sure about the number of hits in the original game.


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