Andromeda Invaders: A 1980s-arcade-style game using HTML5, Canvas, Web Audio
Andromeda Invaders is a 1980s-arcade-style game runs in a
modern web browser. This game is inspired by Space Invaders, the 1978
arcade game developed by Tomohiro Nishikado. However, the game
characters, gameplay, and some technical aspects of this game are very
different from those of Space Invaders.
Contents
Play
The current stable version of the game is available at the following
links:
A testing version of the game with recent bug fixes is available here:
There is rudimentary support for playing this game on small screens
and touchscreens using the buttons provided below the game canvas.
However, this game is best enjoyed on a laptop/desktop device with a
physical keyboard.
Since it is a single-file game, you can also save this game to your
system by right clicking one of the links mentioned above and
selecting the option to save/download the HTML file.
Keys
While playing on a keyboard-enabled device, the following keys are
supported:
ato move player leftdto move player rightpto pause/unpause gameeto restart gamemto mute/unmute audiofto enter/exit to fullscreen mode
The last key to enter/exit to fullscreen mode may not work on all web
browsers.
Technical Details
All of the graphics is done by drawing rectangles and text on an HTML5
element using the Canvas API.
All of the audio is done by playing sine waves generated using
OscillatorNode of the Web Audio API. The sine waves used for the
game audio correspond to actual musical notes from the C major scale.
Multiple notes are played together to form chords. The background
music is a chord progression consisting of four chords repeating over
and over again as long as the game is being played. When the game
characters get hit, the hit sounds are made of a single chord that
plays for a very short duration.
Gameplay
You are encouraged to play the game without reading this section. I
believe there is a certain joy in figuring out the game on your own
without referring to any hints or existing documentation. For this
reason, I suggest that you skip this section and go play the
game first. If you really must understand the gameplay right now, the
following subsections contain notes about various details of this
game.
Game Characters
Not so long time ago in a galaxy near, near away …
Bright orange ships visit the player’s planet and begins dropping
canons to hit the player. The player defends its planet by hitting the
ships and the falling canons with laser pulses. When a canon is hit
successfully, it vanishes instantly. When a ship is hit successfully,
it loses its health. The health is indicated by the colour of the
ship. Bright orange indicates perfect health. When a bright orange is
hit, it turns dark orange. When a dark orange ship is hit, it turns
dark red. A dark red ship is in critical condition. If a dark red ship
is hit again, it dies.
When a canon hits the player it loses health. The player is initially
bright green that indicates that it is in perfect health. When a
perfectly healthy player is hit, it turns dark green. If the player is
hit again while it is dark green, it turns dull yellow. If the player
is hit when it is dull yellow, it dies. The game ends when the player
dies.
To summarize, each ship has three grades of health and the player too
has three grades of health. After being hit and losing health, the
ships and canons can regain health as the game continues. A ship gains
health by one grade after it drops ten canons since it was last hit.
The player gains health by one grade after it gains 100 points since
it was last hit.
Game Levels
The game can be played indefinitely long. There are multiple levels in
the game that are numbered 1, 2, 3, and so on. After level 1000, the
level is reset to 1, however, the score remains intact, so one can
keep playing levels 1 to 1000 repeatedly in a loop if one has the
patience and skill to do so.
At level 1, there are only three ships that visit the player’s planet.
At level 2, there are six ships that visit the planet. At level 3 and
above, there are ten ships that visit the planet.
The canons drop at various random speeds. The range of possible speeds
for the canons is decided by the game level. The speed of the ships
too depends on the level. Further, the tempo of the background music
too depends on the level.
The following table shows the various game parameters for each level.
| Level | Ships | Ship Speed | Canon Speed | Music Tempo |
|---|---|---|---|---|
| 1 | 3 | 2 | [1, 4] | 2 |
| 2 | 6 | 2 | [1, 4] | 2 |
| 3 | 10 | 3 | [1, 4] | 3 |
| 4 | 10 | 4 | [2, 5] | 4 |
| 5 | 10 | 5 | [3, 6] | 5 |
| 6 | 10 | 6 | [4, 7] | 6 |
| 7 | 10 | 7 | [5, 8] | 7 |
| 8 | 10 | 8 | [6, 9] | 8 |
| 9 | 10 | 9 | [7, 10] | 9 |
| 10 | 10 | 10 | [8, 11] | 10 |
| 11 | 10 | 11 | [9, 12] | 11 |
| 12 | 10 | 12 | [10, 13] | 12 |
| 13 | 10 | 13 | [10, 14] | 12 |
| 14 | 10 | 14 | [10, 15] | 12 |
| 15 | 10 | 15 | [10, 15] | 12 |
| 16 | 10 | 16 | [10, 15] | 12 |
| 17 | 10 | 17 | [10, 15] | 12 |
| 18 | 10 | 18 | [10, 15] | 12 |
| 19 | 10 | 19 | [10, 15] | 12 |
| 20 | 10 | 20 | [10, 15] | 12 |
| … | “ | “ | “ | “ |
| 1000 | “ | “ | “ | “ |
In the table above, the unit of speed is pixels per frame (PPF), i.e.,
the number of pixels an object appears to move from one game frame to
another. Here, the term game frame refers to a single instance of
rendering the game state to the HTML5 . The game state is
rendered 50 times a second. The speed of the canons is chosen randomly
from the closed interval shown in the Canon Speed column. The unit
of music tempo is beats per second. Each chord is played in a beat.
To summarize the table above, the game becomes progressively more
difficult to play in each level until level 20. The game parameters do
not change between levels 20 to 1000. After level 1000, the game
resets to level 1 but the player’s score remains intact. Thus the game
can be played indefinitely long in theory. However, in practice you
might find that it is quite difficult to reach even level 10 or so.
Game Points
The player gets 10 points for hitting a bright orange ship, 20 points
for hitting a dark orange ship, and 30 points for hitting a dark red
ship. The player gets 1 point for hitting a canon.
Why?
I first came across Space Invaders in the 1990s in the computer
laboratory of my lower secondary school. Soon after playing the game a
few times, I wanted to develop a similar game of my own. However, the
little GW-BASIC programming I knew then was insufficient to write
anything more sophisticated than simple text-based input/output
programs. I did write several simple text-based quiz and adventure
games back then but a more sophisticated game with graphics and audio
remained elusive. As years went by, I gradually forgot about it,
learnt more mainstream languages like C, Python, Lisp, etc. and got
into programming as a career.
Although it is 25 years too late, I decided to spend a weekend to
fulfill my childhood desire to write my own Invaders-like game. This
project fulfills a childhood dream of mine!
License
This is free and open source software. You can use, copy, modify,
merge, publish, distribute, sublicense, and/or sell copies of it,
under the terms of the MIT License. See LICENSE.md for details.
This software is provided “AS IS”, WITHOUT WARRANTY OF ANY KIND,
express or implied. See LICENSE.md for details.
Support
To report bugs or ask questions, create issues.
