How would I implement a level for a vertical shooter game

Question

I've been working in my free time on a vertical shooter similar to Xenon 2 Megablast. It's a tribute of sorts to the game that destroyed a considerable part of my childhood.

I've got the basic stuff in place - weapons, enemies, collision detection and a scrolling background. Now, I want to design the levels.

I want to create some kind of an engine which can render a level based on some kind of (say) textual description so that I can easily create levels. Are there any articles or links to how platformer/vertical shooter levels can be stored as files and loaded by the game engine? I'm not looking for completely solutions but ideas and starting points so that I can try to do it myself.

Specifically, how would you store the level of a platformer game like Prince of Persia or a shooter like Xenon 2?

Answer 1

I don't know if you can find any valuable resources or articles about the topic, but I think that the idea of the textual description is good. Personally, I have some experience with vertical-scroller shmups, and I have always followed this approach.

Roughly speaking, you can have a series of text files (or other kind of file...); each file corresponds to a level of your game. Every row of a file contains the essential informations that you need for generating the enemies (or other objects, such as background elements - like planets, stars,...). These informations, obviously, vary accordingly with your specific game but, in general, they could be:

• The spawn time: this is the time that you have to wait between a "row" and another. Basically, you control the frequency of spawning, with this "parameter";

• The enemy type: this could be an index, or a string, that represents the specific enemy type. As an example, if you have 20 types of enemies in your game, this can be a numerical index, from 1 to 20, that represents the type of object to be instantiated: a particular function in your code will bind the index with the specific asset of the enemy. As I said, you could also use the string that represents the asset's name in your project: It will just be a unique identifier for the object to be instantiated.

• The x and y coordinates that define the spawn point (y-coordinate could be avoided, and defined as a constant, if all the enemies come from the top of the screen).

So, your txt file can be structured in a similar fashion:

4.0 little_enemy_1 200 150
0.0 little enemy_1 240 150
0.0 little_enemy_1 280 150
3.5 mothership 140 170

In Unity (sorry, but I'm familiar with that...) these lines correspond to:

yield WaitForSeconds(4);
Instantiate (little_enemy_1, Vector3.Position(200, 150, 0), Quaternion.identity); 
Instantiate (little_enemy_1, Vector3.Position(240, 150, 0), Quaternion.identity); 
Instantiate (little_enemy_1, Vector3.Position(280, 150, 0), Quaternion.identity); 
yield WaitForSeconds(3.5);
Instantiate (mothership, Vector3.Position(140, 170, 0), Quaternion.identity); 

If you're not familiar with Unity, those lines say: "wait 4 seconds, generate 3 little enemies in a row, wait 3.5 seconds, and generate a mothership (all in the proper coordinates)". In my example I have not considered the particular rotation of the objects that have to be spawned, but, obviously, you can also add that.

I know that it was just a quick tip, but I hope that it could represent a good starting point for you, as you asked. Any feedback will be appreciated, of course.

Answer 2

Similar to @Goblin's answer here, I am using a random endless wave generator for my game, which is an HTML5 side-scrolling space shooter. The code below is plain Javascript. I am not posting the entire code here. Some functions are left out for your implementation. I just wanted to depict the logic.

var shipTypes = ['warbird','crosswing','raider','drone','tinydrone'];
var levelTimer = 0; // This variable increments by 1 per frame
var wave = {};
var screenHeight = canvas.innerHeight; // Dont bother about this step
function generateNextWave () {

    var numberOfShips = getRandomNumberBetween(1,5); // This function is to pick a integer between the two numbers (inclusive)
    wave = {};

    var len = shipTypes.length - 1,ships = [];
    for(i=0;i<numberOfShips;i++) {
      var pickAShipType = shipTypes[getRandomNumberBetween(0,len-1)];
      ships.push(createShip(pickAShipType)); // Assume I have a function to create a ship according to its type
    }
    wave[levelTimer + getRandomNumberBetween(100,200)] = ships;

}


function gameLoop() {

    .... 

    // check if there is a wave this time
    if(wave[levelTimer]) {

      // add the ships to the list of ships on screen
      addShipsToScreen(wave[levelTimer]);

      // now generate the next wave
      generateNextWave();

    }

   ....
   levelTimer+=1;

}

Every frame in our game loop we check for wave[levelTimer] to see if its empty. If its not we add all the ships in that wave to the game. Please let me know if you need more clarification.