Creating the random enemy spawner, improving performance

When building my Win8 port of Super Rawr-Type, I ran into all sorts of performance issues, due to lack of optimization. This was to be expected, as I on gave myself 1 month to complete the base game, then I would take the second and final month to port it to different platforms and worry about optimization.

The design of my levels is where most of my issues came from. I explain it pretty well in this post here. The gist of it is this: When the map loads, it is filled with ~150 enemies, which are constantly being updated. Obviously this comes with quite a bit of overhead. The browser version played well, but on a dedicated OS it chugs.

Removing all enemies

I went back to the drawing board and redesigned the way I spawn enemies now. The first thing I did was remove all entities (enemies) from the level.

I looked at a number of different ways for spawning enemies for shmup and bullet hell. Matt and Jason Doucette at Xona games do it in an interesting way for their upcoming title Duality-ZF: They store all of the enemies in an excel spreadsheet, which is then parsed by their game engine and appropriately draws the entities on screen.

Then again, they are using C# and running on the Xbox, whereas I’m running JavaScript on a PC, so JSON may be a more appropriate format here, if I were to take this approach. I don’t even think the Xbox has a way of parsing JSON; I know that Visual Studio just adopted JSON.NET, so it’s doubtful that the 8 year old Xbox has the ability to do the same.

Creating the Random Enemy Spawner

I found that my best approach was to create an entity, Random Enemy Spawner which sits just off the right side of the screen, and is always aligned on the Y-axis with the player. For debug purposes, I threw in placeholder art which is a copy of the player’s ship, and had it sit on the edge of the screen so that I could see not only where my enemies spawn, but also how frequently.


As you can see here, the enemies and spawner are all aligned on the X-axis. This may appear as though the enemies come in flat waves, but to counter that I have them all spawning on a timer. Three timers, actually.

I broke the enemy ships into three groups, and one example is illustrated below:

 * randomFromTo
 * Random number generator
 * Courtesy of Liza Shulyayeva's flea project
 randomFromTo: function (from, to) {
     return Math.floor(Math.random() * (to - from + 1) + from);

 * spawnEnemyGrpA
 * Spawns 1 of 2 types of enemies at random intervals
 spawnEnemyGrpA: function () {
     // Resets random number
     var rndNum = null;
         // Rolls a random number
         rndNum = this.randomFromTo(1, 10);

     // Spawns enemies within the Y bounds of the screen
     this.randomSpawnLocY = this.randomFromTo(ig.system.height - 20, ig.system.height / 20);

     // determines which enemy type will spawn
     if (rndNum > 5){, this.pos.x, this.randomSpawnLocY);
     if (rndNum < 5){, this.pos.x, this.randomSpawnLocY);
     // Resets timer

I use spawnEnemyGrpB and C to spawn different types of enemies. They are set on timers like this:

 * init
 * Handles initialization
init: function (x, y, settings) {

     // Spawning timers for enemy ships
     this.spawnTimerGrpA = new ig.Timer(this.randomFromTo(3, 9));
     this.spawnTimerGrpB = new ig.Timer(this.randomFromTo(3, 8));
     this.spawnTimerGrpC = new ig.Timer(this.randomFromTo(4, 9));

These timers are responsible for triggering enemy spawns at random times, which is what allows for a staggered appearance on screen.

I’ve noticed an incredible performance improvement from this, although it has been tricky to balance the difficulty. What I really need to do is create a variable that affects how frequently enemies are spawned, and that variable will adjust based on the chosen difficulty level of the game. A higher difficulty will obviously spawn more enemies at once.

The EnemySpawner.js class can be found in its entirety on my GitHub.



One thought on “Creating the random enemy spawner, improving performance

  1. Pingback: Tutorial Links – AIN Online Journal

Leave a Reply