Changing the opacity (transparency) of images using JavaScript

				
Changing the opacity (transparency) of images using
JavaScript
I think this technique is particularly useful and unique- using JavaScript to
change the opacity of an image! The technique works in both IE4+ and NS6+, and
can be used to create some interesting "fading" effects. Let's get
started!
In IE4+, you can adjust the opacity of an image using the STYLE attribute:
<img src="ball.gif" style="filter:alpha(opacity=50)">
I've highlighted the main code. A value of 50 makes this image 50% oblique
(or transparent). You can use a number between 0-100, where 0 would make the
image disappear.
In NS6+, the code needed is a little bit different:
<img src="ball.gif" style="-moz-opacity:0.5">
Here the accepted range of values are 0 to 1, where 0 would make the image
disappear.
You're probably now asking- how can I combine the two HTML above to make
opacity specification work in both IE4+ and NS6+? Just define one STYLE
attribute and put the two definitions inside it, separating the two with a semi
colon:
<img src="ball.gif" style="filter:alpha(opacity=50); -moz-opacity:0.5">
Using JavaScript to alter opacity on the fly
This is where things get interesting and useful- using JavaScript to alter
the value of the image's opacity! By doing so, you can make images fade in or
out, for example.
The JavaScript syntax to change an image's opacity after it's been defined in
the HTML is:
ImageObj.style.filters.alpha.opacity=90 //IE4 syntax
ImageObj.style.MozOpacity=0.9 //NS6 syntax
So for example, here's a simple script that adds a "lighting up"
effect to your images as the mouse hovers over and out:
<script>
function lightup(imageobject, opacity){
if (navigator.appName.indexOf("Netscape")!=-1&&parseInt(navigator.appVersion)>=5)
imageobject.style.MozOpacity=opacity/100
else if (navigator.appName.indexOf("Microsoft")!=-1&&parseInt(navigator.appVersion)>=4)
imageobject.filters.alpha.opacity=opacity
}
</script>
<img src="test.gif" style="filter:alpha(opacity=50); -moz-opacity:0.5" 
onMouseover="lightup(this, 100)" onMouseout="lightup(this, 30)">
If you want to see a more complicated "lighting up" effect, check
out Gradual
highlight script by Dynamic Drive. It uses basically the same technique
as I do, though the opacity is changed incrementally.