Thursday, November 21, 2013

CSS Detecting Collisions (Game Programming)

One of the foundations for any kind of action game is the ability to detect collisions. Did your spaceship crash into the asteroid or not? How about your fancy race car?

There are three ways that I know of to do this, depending on the technology you are using (CSS, Canvas, or SVG).

The first and definitely the most popular is to calculate if one object's boundaries overlap with another object's boundaries. This is called bounding boxes and is simple, but involves some minor calculation. In the example below, I'm making it simple and only comparing the top left corner of two same-sized square objects. Do they match or not?

Maybe not that exciting, but the example also shows how to move a character automatically and also uses three different timing loops. The stars of this show are:

1. A ball. You've seen it before.

2. A box. He's new. And wants to kill the ball. Don't ask why. Boxes just do that.

3. The ground. This isn't a space game, so we need a patch of ground for the battle to take place:

Maybe not the most exciting, but my philosophy about understanding game programming concepts is to keep it as simple as it can be and still show something. So we've got a ball and a box and they're sitting on the ground.

Here's what the game looks like:

The square tries to kill the poor little ball. But the ball has a secret power. Well, probably not too secret if you read earlier posts. The ball can bounce!

Touch the screen anywhere and the ball will jump up in the air for one second. This makes it a game. You must make the ball jump up just before the box would collide with you. If you fall back down again and the box collides with you, you're dead.

If you don't make it and the box collides with you, an alert is displayed that says "Boom".

So here's the code. It has been tested and run on the ZTE Open with 1.0 Firefox OS.

    <meta charset="UTF-8">
      CSS Collision
        width: 20px;
        height: 20px;
        position: absolute;
        top: 150px;
        left: 50px;
        background-image: url(ball.png);
        width: 20px;
        height: 20px;
        position: absolute;
        top: 150px;
        left: 270px;
        background-image: url(box.png);
            border-style: solid;
            border-color: black;
            border-width: 1px;

        width: 320px;
        height: 230px;
        position: absolute;
        top: 170px;
        left: 0px;
        background-color: chocolate;

       <div id="ball"></div>
    <div id="box"></div>
      <div id="ground"></div>

    <script type="text/javascript">

      // Global variables    
      var boardWidth = 320;
      var boardHeight = 460;    
      var ballHor = 50;
      var ballVer = 150;     
      var howFast = 3;
        var boxHor = 270;
      var boxVer = 150;
      var loopID;
        // requestAnimationFrame browser variations
        var requestAnimationFrame =
          window.requestAnimationFrame ||
          window.mozRequestAnimationFrame ||
          window.webkitRequestAnimationFrame ||
      var cancelAnimationFrame =
        window.cancelAnimationFrame ||
        // Load event listener
      window.addEventListener("load", getLoaded, false);
          "mousedown", ballJump, false);

        // Get box and ball information.
        var myBall = document.querySelector("#ball");
        var myBox = document.querySelector("#box");
        var myGround = document.querySelector("#ground");
      // Function called on page load.
      function getLoaded() { 

            // Output to console.
        console.log("The page is loaded.");
          // Start game loop.
        // Game loop
        function gameLoop() {
            // Loop within the loop.
            setTimeout(function() {
          loopID = requestAnimationFrame(gameLoop);
          // Drawing code goes here
              console.log("Moving the box.");
              // Move the box
        }, 1000 / howFast); // Delay / how fast

      // Move the box here.
      function moveBox() {
          // Subtract 10 from box horizontal value.
          boxHor = boxHor - 10;
          // Move the box 10 pixels left.
 = boxHor + "px";
          // If the box hits the left edge, restart.
          if (boxHor < 10) boxHor = 270;   

          // Report position of box.
      // Check for collision.
      if ((boxHor == ballHor) && (boxVer == ballVer)) {
        // Collision took place!

        // Make the ball jump up.
    function ballJump() {
      // Calculate ball jump and move it.
      ballVer = ballVer - 50; = ballVer + "px";
      console.log("Ball up.");
      // Make the ball fall after one second.
      ballTimer = setTimeout(ballFall, 1000);     
    // Make the ball fall down.
    function ballFall() {
      // Calculate the ball fall and move it.
      ballVer = ballVer + 50; = ballVer + "px";
      console.log("Ball down.");  



I'm using the CSS shell that I wrote in this post: The shell just provides the basic HTML5 structure, has a initial function that runs when the page loads, has a game loop, and then other functions that do specific things.


Because this is CSS, I have an inline style that defines the properties of a ball, a box, and the ground. The ball is the same as what you've seen before. The box is just a square of color. I didn't like the way the box looked with just color, so I modified it in the style to have a black outline one pixel thick. The ball and the box were created in an art editor (Paint Shop Pro) and saved as PNG files. For the background, I decided to save room and just had it be a separate div with a background color (chocolate). This shows that there are three ways to make a graphic: import, import with modification, and fill a div with color.

To make these styles work, I had to create a separate div for each one, assign it a style, and then use querySelector to grab it and assign it a variable name. After that, I have three objects I can play with. The ground is just decoration and doesn't serve any programming purpose.

Game Loop

The game loop is called by the page load function. The game loop is interesting and uses two different timers to make sure that the box moves in regular steps as it marches across the screen.

The outer loop uses setTimeout to do something at regular intervals. In this case the interval is 1000 milliseconds (one second) divided by the howFast variable. In this case, howFast is 3, but you could use different values to make a game easier or harder. In this example, the "something" you do every 1/3 of a second is to move the box.

So, every 1/3 of a second, requestAnimationFrame is used to start the main loop over again, but also makes a call to actually draw the box (in a separate function, moveBox). Using requestAnimationFrame here makes sure that the drawing is done right away and helps with battery life. Whenever possible, use this for animations, especially repetitive ones like moving the box one step at a time.

Moving the Box

The box moves every 1/3 of a second. As it moves, it checks for two things:

1. Did I hit the left wall? If I did, move back to my initial position and try again.

2. Did I collide with the ball? If I did, then display an alert.

Step 2 is a bit tricky. You want to test for matching the top left corner of each object, and see if they both match. If they do, you've got a collision. The test is for this:

          ((boxHor == ballHor) && (boxVer == ballVer))

This line tests for three things:

  1. Does the horizontal (left) point of both box and ball match?
  2. Does the vertical (top) point of both box and ball match?
  3. Are both of these true?
The && is a symbol that means both of these things are true, and the placement of parentheses is important so you know what you're comparing and that they fit into the if statement.

Collision Test

If the top left of both match, we have a collision. Where things are moving in different directions or are of different sizes, you might figure out what the bounding box would be of each object and then compare them. A bounding box is an imaginary square that encloses the part of the object you want to test for. Maybe you only care about the center of your spaceship and so you have a small bounding box. The point is that you care and then by subtraction, you see if the two boxes overlap. Here because I kept the movement and size simple, you're only caring about the top left corner of each object, and the moving object is moving at a regular pace that will always match the position of the ball in its steady march of 10 pixels left every 1/3 of a second.

Bouncing Balls

You knew we had to have a bouncing ball somewhere. What makes a game is the choice the player makes to avoid or capture or destroy the enemy. Because this is a phone game, you make the ball jump up by tapping the screen anywhere. The easy way to do this is by using the mousedown event, which gets translated to a touch by Firefox OS. Don't use click, which requires the mouse to go down and then up.

If you touch the screen and fire the mousedown event, the ballJump function is called. This will draw the ball 50 pixels above where it was (don't forget that you're measuring pixels from the top left corner of the screen, so up is smaller numbers and down is larger numbers).

setTimeout (a different timer than the setTimeout one in the game loop) is called that will run for one second (1000 milliseconds). After the second has passed, the ballFall function is called. This simply changes the ball position 50 pixels below where it was (by adding 50 to it -- up is down, down is up).


This example uses three timers:

1. The first is the 1/3 second delay every time the game loop is processed. Slow.

2. The second is the call using requestAnimationFrame, which draws the box as fast as the browser can, but is more efficient than setTimeout. Fast.

3. The third is the one-second delay that the ball spends hanging in air before it falls back down. Slow.

The first and third use setTimeout, which is okay because you don't need precision timing or power efficiency. The second uses requestAnimationFrame because anything that uses repetitive drawing (like marching across the screen) will do well here. Of course this example is pretty simple, but as you move on to more complex animations, this kind of looping will be very useful. Slow timing is fine for setTimeout, but requestAnimationFrame is better for fast and efficient motion.

There is a great article about using both of these timing methods (and more) at
requestAnimationFrame and close

I thought it would be nice to stop the game when the collision took place. But I found that there is some kind of problem with using cancelAnimationFrame. I coujldn't get it to work. This function is supposed to stop the animation frame loop (which is used to reset the game loop). It works just fine in Firefox browser, but does NOT work on the ZTE.

The above code that I used for this was straight from the MDN page.  I might be doing something wrong or I left out a semicolon. All I can say is that I copy and pasted this from this page, which should allow either cancelAnimationFrame or mozCancelAnimationFrame, and does work in the big browser (go Aurora!) but doesn't work on my poor old ZTE Open stuck at Firefox OS 1.0. If someone wants to tell me what is supposed to work when, I'll be happy to post the changes needed to make this work.

I also thought that well, if I can't kill the animation timer, maybe I can just close the window. It turns out that Firefox doesn't support window.close() unless you opened that window yourself. Probably a good idea. But I would like to find a way to shut down an app. Where's that task manager when you need it?

But the principles of the rest of the code are just fine and having the marching repeat endlessly can be considered a feature, not a bug. (Ha ha!)

You can apply this type of collision testing to any of the three technologies (CSS, Canvas, and SVG). Decide on your bounding box coordinates and test for boundary overlaps.

But there is ... another. In fact, two others. One for Canvas and the other for SVG. I'll do the Canvas thing next and then SVG.


Note: when I started this series, the three technologies of CSS, Canvas, and SVG were all that seemed currently useful. It now looks like some serious WebGL technologies are ready for prime time. In the last few days there have been two cool advances in WebGL.

WebGL is a technology that takes the core of OpenGL and applies it to web programming to create cool visual effects. OpenGL is used by most 3D game programmers but was claimed by Microsoft to be evil (because DirectX was better). Then, in a move that surprised a lot of folks, Microsoft included WebGL in Internet Explorer 11. Maybe not as good as Firefox's WebGL, but at least they're not saying it is evil any longer.  So, like SVG, once Microsoft says it is okay, the rest of the world can jump in and play.

The first news about WebGL is that Firefox now has a cool way to debug and edit WebGL in the browser itself. Check out this post: WebGL (or any 3D graphics) isn't easy to understand, but I'll be curious to see people using it on Firefox OS.

The second bit of news about WebGL is that Sony is using WebGL for their user interface for the Playstation 4 for the store, music player, and other junk. See this post by one of the developers (a very smart guy) I can't wait to see it (oh, wait, I don't have a Playstation 4. But I'm too busy playing Firefox OS games to care).

WebGL sits on top of Canvas, so maybe when I finish writing about all the useful aspects of 2D games written in CSS, Canvas, or SVG, I'll start tackling WebGL. I have done some programming in DirectX and OpenGL (and even a little WebGL), but I like 2D programming better.

Stay tuned, but not iTuned!

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