STEM Day 2019 JavaScript Web Animation Activity

  • Open Chrome or Firefox web browser and go to https://cs.mvnu.edu
  • Under "Special Events", select the link for 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 standard 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 search box and enter "Notepad" to open either Notepad or Notepad++. 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.
  • Use the File menu Save As ... command, and save the file on the Desktop with the filename "Ball1.html". 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 select 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 multiple times, 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 in the same way, saving the file as "Ball2.html". The new code is 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 runs, it draws a circle at the new coordinates without removing the old ones.
  • To erase the old circles, well need to invoke 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.
  • Edit your code from Step 2, and add just the one new line of code shown in blue below.
  • Save the code file, and try it out as before. Now the ball should move across the screen and disappear.
<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>

Step 4 - bounce the ball inside imaginary walls

  • Instead of letting the ball disappear, let's see how to make it bounce and change direction when it reaches the edge of the canvas.
  • All we 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 of movement by setting the value of dx or dy to the opposite (negative) value.
  • Edit your code to add the boundary conditions (shown in blue below), and try it out as before!
<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 Conditions
  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>
  • 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 or breaker-type of game.

Step 5

Now try some experiments by making small changes to the code in your file to see if you can produce some of these effects:

  • Make the ball move faster (by changing the number of milliseconds when using the setInterval() function).
  • 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!

Reference Links

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Topic revision: r2 - 2019-11-10 - RobertKasper
 
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