STEM Day 2019 JavaScript Web Animation Activity

  • Open Chrome or Firefox web browser and go to https://cs.mvnu.edu
  • Click Link to CS Wiki.
  • Under "Special Events", select STEM Day on November 11, 2019.

Introduction to Animation with HTML5 and JavaScript

In the steps that follow, you will learn how simple animations can be programmed to run in a web browser.

Step 1 - Draw a ball

  • First we need to create a canvas to draw on.
  • The following HTML code creates an area to "draw" on that is 500 pixels wide, and 400 pixels high:
<body>
  <canvas id="myCanvas"* width="500" height="400">
  </canvas>
</body>
  • Now we can write Javascript code to draw something on this canvas:
<script>
  var context;
  function init()
  {
    context= myCanvas.getContext('2d');
    context.beginPath();
    // fill objects with blue color, using RGB value in hexadecimal code
    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="500" height="400" >
  </canvas>
</body>
  • How can we test this out?
  • We need an editor to write and save a code file. Go to the Start menu, find Assessories, and then the Notepad editor. Notepad is a simple text editor available on any Windows computer. Programmers often use other text editors, such as Notepad++, with more useful features for editing code.
  • Copy the code shown in blue above and paste it into the editor.
  • Save the file as "Ball1.html" on the desktop. Note: When using Notepad, change "Save as type" from (.txt) to "All Files".
  • Go to the desktop, right click on the new file named Ball1.html, and open with Google Chrome.
  • A web page should appear showing a simple blue circle.

Simple static ball:
11-02_blue_circle.png

Step 2 - Move the ball

  • Now that we have the circle, lets try to move it. We'll do this by redrawing the circle many times, changing its position a small amount each time.
  • Well replace the hardcoded values of the coordinates in the .arc method (100, 100 the first two parameters) with variables x and y.
  • Each time we redraw the circle, we will increment the x coordinate by an amount of dx, and increment y by an amount of dy.
  • Also since we need to redraw the circle at the new positions, well move the code into a function called draw() and invoke it every 20 ms (milliseconds) using JavaScript s setInterval() function.
  • Try the code below the same way, naming it "Ball2.html". Simply add the new code shown in blue.
<script>
var context;
var x=100; var y=100; var dx=5; var dy=5;

function init()
{
  context= myCanvas.getContext('2d');
  setInterval(draw,20);
}

function draw()
{
  context.beginPath();
  context.fillStyle="#0000ff";
  // Draws a circle of radius 20 at the coordinates x,y 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="500" height="400" >
</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, well need to call a 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,20);
}

function draw()
{
  context.clearRect(0,0, 500,400);
  context.beginPath();
  context.fillStyle="#0000ff";
  // Draws a circle of radius 20 at the coordinates x,y 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="500" height="400" >
  </canvas>
</body>

  • Edit your code from Step 2, and add the new code in blue.
  • Save the code file, and try it out as before. The ball should move across the screen and disappear.
  • 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=100;
var dx=5;
var dy=5;

function init()
{
  context= myCanvas.getContext('2d');
  setInterval(draw,20);
}

function draw()
{
  context.clearRect(0,0, 500,400);
  context.beginPath();
  context.fillStyle="#0000ff";
  // Draws a circle of radius 20 at the coordinates x,y on the canvas
  context.arc(x,y,20,0,Math.PI*2,true);
  context.closePath();
  context.fill();
  // Boundary Logic
  if( x<0 || x>500) dx=-dx;
  if( y<0 || y>400) dy=-dy;
  x+=dx;
  y+=dy;
}

</script>
<body onLoad="init();">
  <canvas id="myCanvas" width="500" height="400" >
  </canvas>
</body>

  • Edit your code to add the boundary conditions (shown in blue above), and try it out as before!
  • This link shows a working example: Bouncing ball
  • This bouncing ball animation illustrates some fundamental methods used in programs to simulate moving objects. Similar animations are used in movies and game development, and with a little effort, this program could be extended to build a ping-pong game or a breaker-type of game.

Step 5

Can you try to add some more features?

  • Change the size of the ball, or the dimensions of the area.
  • Change the color of the ball whenever it hits the wall.
  • Adjust the boundary conditions so that the ball changes direction as soon as its edge hits a wall, instead of compressing to the center of the ball.
  • Draw more than one ball.
  • Simulate the effects of gravity and friction (make the ball's height decrease with each bounce, and make the side to side motion slow down).
  • Be creative, and try out other changes and see what happens! See the links below for some more ideas.

Links

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Topic revision: r1 - 2019-11-10 - RobertKasper
 
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