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Javascript Animation


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You can use JavaScript to create a complex animation which includes but not limited to:

  • Fireworks
  • Fade Effect
  • Roll-in or Roll-out
  • Page-in or Page-out
  • Object movements

You might be interested in existing JavaScript based animation library : Script.Aculo.us.

This tutorial will give you basic understanding on how to use JavaScript to create an animation.

JavaScript can be used to move a number of DOM elements (<img />, <div> or any other HTML element) around the page according to some sort of pattern determined by a logical equation or function.

JavaScript provides following two functions to be frequently used in animation programs.

  • setTimeout( function, duration) - This function calls function after duration milliseconds from now.

  • setInterval(function, duration) - This function calls function after every duration milliseconds.

  • clearTimeout(setTimeout_variable) - This function calls clears any timer set by the setTimeout() functions.

JavaScript can also set a number of attributes of a DOM object including its position on the screen. You can set top and left attribute of an object to position it anywhere on the screen. Here is the simple syntax:

// Set distance from left edge of the screen.
object.style.left = distance in pixels or points; 

or
// Set distance from top edge of the screen.
object.style.top = distance in pixels or points; 

Manual Animation:

So let's implement one simple animation using DOM object properties and JavaScript functions as follows:

<html>
<head>
<title>JavaScript Animation</title>
<script type="text/javascript">
<!--
var imgObj = null;
function init(){
   imgObj = document.getElementById('myImage');
   imgObj.style.position= 'relative'; 
   imgObj.style.left = '0px'; 
}
function moveRight(){
   imgObj.style.left = parseInt(imgObj.style.left) + 10 + 'px';
}
window.onload =init;
//-->
</script>
</head>
<body>
<form>
<img id="myImage" src="/images/html.gif" />
<p>Click button below to move the image to right</p>
<input type="button" value="Click Me" onclick="moveRight();" />
</form>
</body>
</html>

Here is the explanation of the above example:

  • We are using JavaScript function getElementById() to get a DOM object and then assigning it to a global variable imgObj.

  • We have defined an initialization function init() to initialize imgObj where we have set its position and left attributes.

  • We are calling initialization function at the time of window load.

  • Finally, we are calling moveRight() function to increase left distance by 10 pixels. You could also set it to a negative value to move it to the left side.

To understand it in better way you can Try it yourself.

Automated Animation:

In the above example we have seen , how an image moves to right with every click. We can automate this process by using JavaScript function setTimeout() as follows:

<html>
<head>
<title>JavaScript Animation</title>
<script type="text/javascript">
<!--
var imgObj = null;
var animate ;
function init(){
   imgObj = document.getElementById('myImage');
   imgObj.style.position= 'relative'; 
   imgObj.style.left = '0px'; 
}
function moveRight(){
   imgObj.style.left = parseInt(imgObj.style.left) + 10 + 'px';
   animate = setTimeout(moveRight,20); // call moveRight in 20msec
}
function stop(){
   clearTimeout(animate);
   imgObj.style.left = '0px'; 
}
window.onload =init;
//-->
</script>
</head>
<body>
<form>
<img id="myImage" src="/images/html.gif" />
<p>Click the buttons below to handle animation</p>
<input type="button" value="Start" onclick="moveRight();" />
<input type="button" value="Stop" onclick="stop();" />
</form>
</body>
</html>

Here we have add more spice. So let's see what is new here:

  • The moveRight() function is calling setTimeout() function to set the position of imgObj.

  • We have added a new function stop() to clear the timer set by setTimeout() function and to set the object at its initial position.

To understand it in better way you can Try it yourself.

Rollover with a Mouse Event:

Here is a simple example showing image rollover with a mouse events:

<html>
<head>
<title>Rollover with a Mouse Events</title>
<script type="text/javascript">
<!--
if(document.images){
    var image1 = new Image();      // Preload an image
    image1.src = "/images/html.gif";
    var image2 = new Image();      // Preload second image
    image2.src = "/images/http.gif";
}
//-->
</script>
</head>
<body>
<p>Move your mouse over the image to see the result</p>
<a href="#" onMouseOver="document.myImage.src=image2.src;"
            onMouseOut="document.myImage.src=image1.src;">
<img name="myImage" src="/images/html.gif" />
</a>
</body>
</html>

Let's see what is different here:

  • At the time of loading this page, the if statement checks for the existence of the image object. If the image object is unavailable, this block will not be executed.

  • The Image() constructor creates and preloads a new image object called image1.

  • The src property is assigned the name of the external image file called /images/html.gif.

  • Similar way we have created image2 object and assigned /images/http.gif in this object.

  • The # (hash mark) disables the link so that the browser does not try to go to a URL when clicked. This link is an image.

  • The onMouseOver event handler is triggered when the user's mouse moves onto the link, and the onMouseOut event handler is triggered when the user's mouse moves away from the link (image).

  • When the mouse moves over the image, the HTTP image changes from the first image to the second one. When the mouse is moved away from the image, the original image is displayed.

  • When the mouse is moved away from the link, the initial image html.gif will reappear on the screen.

To understand it in better way you can Try it yourself.



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