Stem 2015 JavaScript Animation Activity

  • Open Firefox browser and go to http://cs.mvnu.edu
  • Click Link to CS Wiki.
  • Under Events, Projects, and Belize stuff, select STEM DAY 2013.

Introduction to Animation with HTML5 and JavaScript

  • HTML5 is the latest language for building web pages.
  • HTML5 has a feature called the <canvas> element that allows graphics.
  • Using a programming language called JavaScript, web developers can create graphics, static and moving.
  • Here is an example of a simple animation using HTML5: Bouncing Ball Example
  • Here is another fun one! MVNU Bouncing Balls
  • And a few more: Gravity Simulation, Ball Equilibrium
  • More Fun (Bouncing balls different sizes)

Step 1 - Draw a ball

  • First we need to create a canvas to draw on.
  • The following code creates a area to "draw" on that is 50 pixels wide, and 400 pixels high:
<body>
  <canvas id="myCanvas"* width="500" height="400">
  </canvas>
</body>
  • Now we can write code to draw something on this canvas:
<script>
  var context;
  function init()
  {
    context= myCanvas.getContext('2d');
    context.beginPath();
    // fill objects with blue color
    context.fillStyle="#0000ff";
    // Draws a circle of radius 20 at the coordinates 100,100 on the canvas
    context.arc(100,100,20,0,Math.PI*2,true); context.closePath();
    context.fill();
  }
</script>

<body onLoad="init();">
  <canvas id="myCanvas" width="300" height="300" >
  </canvas>
</body>
  • How can we test this out?
  • We need an editor. Go to the Start menu, find Assessories, and then the NotePad editor.
  • Copy the code above and paste it into the editor.
  • Save as "Ball1.html" on the desktop.
  • Go to the desktop, right click on the new file, and open with FireFox.

Simple static ball:
11-02_blue_circle.png

Step 2 - Move the ball

  • Now that we have the circle, let’s try to move it. We’ll replace the hardcoded values of the coordinates in the .arc method (100, 100 — the first two arguments) with variables x and y, which we will then increment by an amount of dx and dy.
  • Also since we need to redraw the circle at the new positions, we’ll move the code into a function called draw() and call it every 10ms using JavaScript’s setInterval() function.
  • Try the code below the same way, naming it "=ball2.html=". Simply add the new code in blue.
<script>
var context;
var x=100; var y=200; var dx=5; var dy=5;

function init()
{
  context= myCanvas.getContext('2d');
  setInterval(draw,10);
}

function draw()
{
  context.beginPath();
  context.fillStyle="#0000ff";
  // Draws a circle of radius 20 at the coordinates 100,100 on the canvas
  context.arc(x,y,20,0,Math.PI*2,true);
  context.closePath();
  context.fill();
 x+=dx;   y+=dy;
}

</script>
<body onLoad="init();">
  <canvas id="myCanvas" width="300" height="300" >
  </canvas>
</body>

We have a problem! The circle is actually forming a line (see the image below, click image for an actual demo).

11-03_line_bug.png

Step 3 - Erase the old circles!

  • The balls merged into a line because each time the draw() function is called, it draws a circle at the new coordinates without removing the old ones.
  • To erase the old circles, we’ll need to call the clearRect method right at the start of our draw()function so that it clears out the previous circle before it draws the new one.
<script>
var context;
var x=100;
var y=100;
var dx=5;
var dy=5;

function init()
{
  context= myCanvas.getContext('2d');
  setInterval(draw,10);
}

function draw()
{
  context.clearRect(0,0, 300,300);
  context.beginPath();
  context.fillStyle="#0000ff";
  // Draws a circle of radius 20 at the coordinates 100,100 on the canvas
  context.arc(x,y,20,0,Math.PI*2,true);
  context.closePath();
  context.fill();
  x+=dx;
  y+=dy;
}

</script>
<body onLoad="init();">
  <canvas id="myCanvas" width="300" height="300" >
  </canvas>
</body>

  • Try it out as before, adding the new code in blue. .... the balls moves across the screen and disappears.
  • Click image below for a demo:

11-02_blue_circle.png

Step 4 - bounce the ball inside imaginary walls

  • All you need to do is check if the values of x and y are beyond the canvas dimensions, and if so, we need to reverse the direction by setting values of dx and dy to the negative values.
<script>
var context;
var x=100;
var y=200;
var dx=5;
var dy=5;

function init()
{
  context= myCanvas.getContext('2d');
  setInterval(draw,10);
}

function draw()
{
  context.clearRect(0,0, 300,300);
  context.beginPath();
  context.fillStyle="#0000ff";
  // Draws a circle of radius 20 at the coordinates 100,100 on the canvas
  context.arc(x,y,20,0,Math.PI*2,true);
  context.closePath();
  context.fill();
  // Boundary Logic
  if( x<0 || x>300) dx=-dx;
  if( y<0 || y>300) dy=-dy;
  x+=dx;
  y+=dy;
}

</script>
<body onLoad="init();">
  <canvas id="myCanvas" width="300" height="300" >
  </canvas>
</body>

  • Edit and try it out as before!
  • Click this link: Bouncing ball for an example.
  • This bouncing ball illustrates some fundamental methods of game development logic and it can be easily extended to build a ping-pong game or a breaker-type of game.

Step 5

Can you try to add some more features?

  • Make the box bigger.
  • Change the color of the ball whenever it hits the wall.
  • More than one ball.
  • Simulate the effects of gravity and friction (the ball's height decreases with each bounce, and the side to side motion slows down)
  • Whatever you can think of!
  • See the links below for some more ideas.

Links

Topic revision: r1 - 2015-10-03 - JimSkon
 
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