Click on a link above to make the image disappear and re-appear by gradually fading in/out. It uses CSS transparency, in CSS you can set the transparency in different ways. To ensure that it works on most browsers we use all three.
opacity: 0.5; This one is the official CSS3 method, at the moment it works in newer Mozilla versions.
-moz-opacity: 0.5;
This one works in older versions of Mozilla and Phoenix/FireBird/FireFox.
-khtml-opacity: 0.5;
This is used by browsers that use teh KHTML rendering engine, namely Konquerer on Linux and Safari on MacOS.
filter: alpha(opacity=50); This one works only in MSIE.
There is actually another one: -khtml-opacity: 0.5; works for the browsers Konquerer on Linux and Safari on MacOS. You could add it too if you want to support these users. Somewhere in the near future most browsers will support CSS3 and opacity: 0.5; should work everywhere.
function opacity(id, opacStart, opacEnd, millisec) {
//speed for each frame
var speed = Math.round(millisec / 100);
var timer = 0;
//determine the direction for the blending, if start and end are the same nothing happens
if(opacStart > opacEnd) {
for(i = opacStart; i >= opacEnd; i--) {
setTimeout("changeOpac(" + i + ",'" + id + "')",(timer * speed));
timer++;
}
} else if(opacStart < opacEnd) {
for(i = opacStart; i <= opacEnd; i++)
{
setTimeout("changeOpac(" + i + ",'" + id + "')",(timer * speed));
timer++;
}
}
}
//change the opacity for different browsers
function changeOpac(opacity, id) {
var object = document.getElementById(id).style;
object.opacity = (opacity / 100);
object.MozOpacity = (opacity / 100);
object.KhtmlOpacity = (opacity / 100);
object.filter = "alpha(opacity=" + opacity + ")";
}
function shiftOpacity(id, millisec) {
//if an element is invisible, make it visible, else make it ivisible
if(document.getElementById(id).style.opacity == 0) {
opacity(id, 0, 100, millisec);
} else {
opacity(id, 100, 0, millisec);
}
}
function blendimage(divid, imageid, imagefile, millisec) {
var speed = Math.round(millisec / 100);
var timer = 0;
//set the current image as background
document.getElementById(divid).style.backgroundImage = "url(" + document.getElementById(imageid).src + ")";
//make image transparent
changeOpac(0, imageid);
//make new image
document.getElementById(imageid).src = imagefile;
//fade in image
for(i = 0; i <= 100; i++) {
setTimeout("changeOpac(" + i + ",'" + imageid + "')",(timer * speed));
timer++;
}
}
function currentOpac(id, opacEnd, millisec) {
//standard opacity is 100
var currentOpac = 100;
//if the element has an opacity set, get it
if(document.getElementById(id).style.opacity < 100) {
currentOpac = document.getElementById(id).style.opacity * 100;
}
//call for the function that changes the opacity
opacity(id, currentOpac, opacEnd, millisec)
}
更多参考
http://www.brainerror.net/scripts_js_blendtrans.phphttp://realazy.org/blog/2006/03/21/ie-firefox-opera-alpha-transparency/ http://alistapart.com/stories/pngopacity/ 
