IE6中 PNG 背景透明的最佳解决方案

为什么要使用 PNG 图片？

简单来说，使用 PNG 格式比起 GIF 来表现色彩更丰富，特别是表现渐变以及背景透明的渐变要比GIF格式出色很多。目前，最新的浏览器基本上都支持PNG格式。唯独有万恶的 IE6 不支持 PNG 背景透明，在 IE6 中的 PNG 背景图会显示一个灰色的背景。

IE6 中 PNG 背景图片透明的方法

其实方法有很多，但网络上提供的方法也有诸多缺陷。比如不支持 css 中 backgrond-position 与 background-repeat 等，所以园子推荐你使用 DD_belatedPNG，方法如下：

1.首先下载 DD_belatedPNG.js 文件，本文下方有提供下载，将此文件放置到你的网站中的 js 目录下。

2.在你使用了 PNG 背景的页面头部嵌入以下代码，一般是加在 head 区域。

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<!--[if IE 6]> <script src="js/DD_belatedPNG.js" mce_src="DD_belatedPNG.js"></script> <script type="text/javascript"> /* EXAMPLE */ DD_belatedPNG.fix('.png_bg'); /* 将 .png_bg 改成你应用了透明PNG的CSS选择器,例如我例子中的'.trans'*/ </script> <![endif]-->

需要注意的一些问题：

1.如果你放置的 DD_belatedPNG.js 路径不同的话，也需要更改上述代码中的路径。

2.以上代码中的

1	DD_belatedPNG.fix('.png_bg');

括号中的 .png_bg 改成你在 CSS 定义了 PNG 背景图片的选择器，多个选择器请用英文逗号隔开，如下行代码所示：

1	DD_belatedPNG.fix('.png_bg1, .png_bg2');

3.有些朋友可能需要用到透明的 PNG 图片做为鼠标经过时的背景图片，这就需要用到 a:hover 属性，在这种情况下就需要以“a:hover”来做选择器了，你可以参考下面代码的写法：

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<!--[if IE 6]> <script type="text/javascript" src="js/DD_belatedPNG.js" ></script> <script type="text/javascript"> DD_belatedPNG.fix('.trans,.box a:hover'); </script> <![endif]-->

下面是DD_belatedPNG.js修正了一些IE6BUG的代码：

/**
* DD_belatedPNG: Adds IE6 support: PNG images for CSS background-image and HTML <IMG/>.
* Author: Drew Diller
* Email: drew.diller@gmail.com
* URL: http://www.dillerdesign.com/experiment/DD_belatedPNG/
* Version: 0.0.8a
* Licensed under the MIT License: http://dillerdesign.com/experiment/DD_belatedPNG/#license
*
* Example usage:
* DD_belatedPNG.fix('.png_bg'); // argument is a CSS selector
* DD_belatedPNG.fixPng( someNode ); // argument is an HTMLDomElement
**/

/*
PLEASE READ:
Absolutely everything in this script is SILLY.  I know this.  IE's rendering of certain pixels doesn't make sense, so neither does this code!
*/

var DD_belatedPNG = {
    ns: 'DD_belatedPNG',
    imgSize: {},
    delay: 10,
    nodesFixed: 0,
    createVmlNameSpace: function () { /* enable VML */
        if (document.namespaces && !document.namespaces[this.ns]) {
            document.namespaces.add(this.ns, 'urn:schemas-microsoft-com:vml');
        }
    },
    createVmlStyleSheet: function () { /* style VML, enable behaviors */
        /*
            Just in case lots of other developers have added
            lots of other stylesheets using document.createStyleSheet
            and hit the 31-limit mark, let's not use that method!
            further reading: http://msdn.microsoft.com/en-us/library/ms531194(VS.85).aspx
        */
        var screenStyleSheet, printStyleSheet;
        screenStyleSheet = document.createElement('style');
        screenStyleSheet.setAttribute('media', 'screen');
        document.documentElement.firstChild.insertBefore(screenStyleSheet, document.documentElement.firstChild.firstChild);
        if (screenStyleSheet.styleSheet) {
            screenStyleSheet = screenStyleSheet.styleSheet;
            screenStyleSheet.addRule(this.ns + '\:*', '{behavior:url(#default#VML)}');
            screenStyleSheet.addRule(this.ns + '\:shape', 'position:absolute;');
            screenStyleSheet.addRule('img.' + this.ns + '_sizeFinder', 'behavior:none; border:none; position:absolute; z-index:-1; top:-10000px; visibility:hidden;'); /* large negative top value for avoiding vertical scrollbars for large images, suggested by James O'Brien, http://www.thanatopsic.org/hendrik/ */
            this.screenStyleSheet = screenStyleSheet;
            
            /* Add a print-media stylesheet, for preventing VML artifacts from showing up in print (including preview). */
            /* Thanks to R閙i Pr関ost for automating this! */
            printStyleSheet = document.createElement('style');
            printStyleSheet.setAttribute('media', 'print');
            document.documentElement.firstChild.insertBefore(printStyleSheet, document.documentElement.firstChild.firstChild);
            printStyleSheet = printStyleSheet.styleSheet;
            printStyleSheet.addRule(this.ns + '\:*', '{display: none !important;}');
            printStyleSheet.addRule('img.' + this.ns + '_sizeFinder', '{display: none !important;}');
        }
    },
    readPropertyChange: function () {
        var el, display, v;
        el = event.srcElement;
        if (!el.vmlInitiated) {
            return;
        }
        if (event.propertyName.search('background') != -1 || event.propertyName.search('border') != -1) {
            DD_belatedPNG.applyVML(el);
        }
        if (event.propertyName == 'style.display') {
            display = (el.currentStyle.display == 'none') ? 'none' : 'block';
            for (v in el.vml) {
                if (el.vml.hasOwnProperty(v)) {
                    el.vml[v].shape.style.display = display;
                }
            }
        }
        if (event.propertyName.search('filter') != -1) {
            DD_belatedPNG.vmlOpacity(el);
        }
    },
    vmlOpacity: function (el) {
        if (el.currentStyle.filter.search('lpha') != -1) {
            var trans = el.currentStyle.filter;
            trans = parseInt(trans.substring(trans.lastIndexOf('=')+1, trans.lastIndexOf(')')), 10)/100;
            el.vml.color.shape.style.filter = el.currentStyle.filter; /* complete guesswork */
            el.vml.image.fill.opacity = trans; /* complete guesswork */
        }
    },
    handlePseudoHover: function (el) {
        setTimeout(function () { /* wouldn't work as intended without setTimeout */
            DD_belatedPNG.applyVML(el);
        }, 1);
    },
    /**
    * This is the method to use in a document.
    * @param {String} selector - REQUIRED - a CSS selector, such as '#doc .container'
    **/
    fix: function (selector) {
        if (this.screenStyleSheet) {
            var selectors, i;
            selectors = selector.split(','); /* multiple selectors supported, no need for multiple calls to this anymore */
            for (i=0; i<selectors.length; i++) {
                this.screenStyleSheet.addRule(selectors[i], 'behavior:expression(DD_belatedPNG.fixPng(this))'); /* seems to execute the function without adding it to the stylesheet - interesting... */
            }
        }
    },
    applyVML: function (el) {
        el.runtimeStyle.cssText = '';
        this.vmlFill(el);
        this.vmlOffsets(el);
        this.vmlOpacity(el);
        if (el.isImg) {
            this.copyImageBorders(el);
        }
    },
    attachHandlers: function (el) {
        var self, handlers, handler, moreForAs, a, h;
        self = this;
        handlers = {resize: 'vmlOffsets', move: 'vmlOffsets'};
        if (el.nodeName == 'A') {
            moreForAs = {mouseleave: 'handlePseudoHover', mouseenter: 'handlePseudoHover', focus: 'handlePseudoHover', blur: 'handlePseudoHover'};
            for (a in moreForAs) {            
                if (moreForAs.hasOwnProperty(a)) {
                    handlers[a] = moreForAs[a];
                }
            }
        }
        for (h in handlers) {
            if (handlers.hasOwnProperty(h)) {
                handler = function () {
                    self[handlers[h]](el);
                };
                el.attachEvent('on' + h, handler);
            }
        }
        el.attachEvent('onpropertychange', this.readPropertyChange);
    },
    giveLayout: function (el) {
         el.style.zoom = 1;
                if (el.currentStyle.position == 'static') {
                var tags=el.nodeName.toLowerCase();
                if(tags!='html' && tags!='a'&& tags!='img'){
                    el.style.position = 'relative'
                    };
            }
    },
    copyImageBorders: function (el) {
        var styles, s;
        styles = {'borderStyle':true, 'borderWidth':true, 'borderColor':true};
        for (s in styles) {
            if (styles.hasOwnProperty(s)) {
                el.vml.color.shape.style[s] = el.currentStyle[s];
            }
        }
    },
    vmlFill: function (el) {
        if (!el.currentStyle) {
            return;
        } else {
            var elStyle, noImg, lib, v, img, imgLoaded;
            elStyle = el.currentStyle;
        }
        for (v in el.vml) {
            if (el.vml.hasOwnProperty(v)) {
                el.vml[v].shape.style.zIndex = elStyle.zIndex;
            }
        }
        el.runtimeStyle.backgroundColor = '';
        el.runtimeStyle.backgroundImage = '';
        noImg = true;
        if (elStyle.backgroundImage != 'none' || el.isImg) {
            if (!el.isImg) {
                el.vmlBg = elStyle.backgroundImage;
                el.vmlBg = el.vmlBg.substr(5, el.vmlBg.lastIndexOf('")')-5);
            }
            else {
                el.vmlBg = el.src;
            }
            lib = this;
            if (!lib.imgSize[el.vmlBg]) { /* determine size of loaded image */
                img = document.createElement('img');
                lib.imgSize[el.vmlBg] = img;
                img.className = lib.ns + '_sizeFinder';
                img.runtimeStyle.cssText = 'behavior:none; position:absolute; left:-10000px; top:-10000px; border:none; margin:0; padding:0;'; /* make sure to set behavior to none to prevent accidental matching of the helper elements! */
                imgLoaded = function () {
                    this.width = this.offsetWidth; /* weird cache-busting requirement! */
                    this.height = this.offsetHeight;
                    lib.vmlOffsets(el);
                };
                img.attachEvent('onload', imgLoaded);
                img.src = el.vmlBg;
                img.removeAttribute('width');
                img.removeAttribute('height');
                document.body.insertBefore(img, document.body.firstChild);
            }
            el.vml.image.fill.src = el.vmlBg;
            noImg = false;
        }
        el.vml.image.fill.on = !noImg;
        el.vml.image.fill.color = 'none';
        el.vml.color.shape.style.backgroundColor = elStyle.backgroundColor;
        el.runtimeStyle.backgroundImage = 'none';
        el.runtimeStyle.backgroundColor = 'transparent';
    },
    /* IE can't figure out what do when the offsetLeft and the clientLeft add up to 1, and the VML ends up getting fuzzy... so we have to push/enlarge things by 1 pixel and then clip off the excess */
    vmlOffsets: function (el) {
        var thisStyle, size, fudge, makeVisible, bg, bgR, dC, altC, b, c, v;
        thisStyle = el.currentStyle;
        size = {'W':el.clientWidth+1, 'H':el.clientHeight+1, 'w':this.imgSize[el.vmlBg].width, 'h':this.imgSize[el.vmlBg].height, 'L':el.offsetLeft, 'T':el.offsetTop, 'bLW':el.clientLeft, 'bTW':el.clientTop};
        fudge = (size.L + size.bLW == 1) ? 1 : 0;
        /* vml shape, left, top, width, height, origin */
        makeVisible = function (vml, l, t, w, h, o) {
            vml.coordsize = w+','+h;
            vml.coordorigin = o+','+o;
            vml.path = 'm0,0l'+w+',0l'+w+','+h+'l0,'+h+' xe';
            vml.style.width = w + 'px';
            vml.style.height = h + 'px';
            vml.style.left = l + 'px';
            vml.style.top = t + 'px';
        };
        makeVisible(el.vml.color.shape, (size.L + (el.isImg ? 0 : size.bLW)), (size.T + (el.isImg ? 0 : size.bTW)), (size.W-1), (size.H-1), 0);
        makeVisible(el.vml.image.shape, (size.L + size.bLW), (size.T + size.bTW), (size.W), (size.H), 1 );
        bg = {'X':0, 'Y':0};
        if (el.isImg) {
            bg.X = parseInt(thisStyle.paddingLeft, 10) + 1;
            bg.Y = parseInt(thisStyle.paddingTop, 10) + 1;
        }
        else {
            for (b in bg) {
                if (bg.hasOwnProperty(b)) {
                    this.figurePercentage(bg, size, b, thisStyle['backgroundPosition'+b]);
                }
            }
        }
        el.vml.image.fill.position = (bg.X/size.W) + ',' + (bg.Y/size.H);
        bgR = thisStyle.backgroundRepeat;
        dC = {'T':1, 'R':size.W+fudge, 'B':size.H, 'L':1+fudge}; /* these are defaults for repeat of any kind */
        altC = { 'X': {'b1': 'L', 'b2': 'R', 'd': 'W'}, 'Y': {'b1': 'T', 'b2': 'B', 'd': 'H'} };
        if (bgR != 'repeat' || el.isImg) {
            c = {'T':(bg.Y), 'R':(bg.X+size.w), 'B':(bg.Y+size.h), 'L':(bg.X)}; /* these are defaults for no-repeat - clips down to the image location */
            if (bgR.search('repeat-') != -1) { /* now let's revert to dC for repeat-x or repeat-y */
                v = bgR.split('repeat-')[1].toUpperCase();
                c[altC[v].b1] = 1;
                c[altC[v].b2] = size[altC[v].d];
            }
            if (c.B > size.H) {
                c.B = size.H;
            }
            el.vml.image.shape.style.clip = 'rect('+c.T+'px '+(c.R+fudge)+'px '+c.B+'px '+(c.L+fudge)+'px)';
        }
        else {
            el.vml.image.shape.style.clip = 'rect('+dC.T+'px '+dC.R+'px '+dC.B+'px '+dC.L+'px)';
        }
    },
    figurePercentage: function (bg, size, axis, position) {
        var horizontal, fraction;
        fraction = true;
        horizontal = (axis == 'X');
        switch(position) {
            case 'left':
            case 'top':
                bg[axis] = 0;
                break;
            case 'center':
                bg[axis] = 0.5;
                break;
            case 'right':
            case 'bottom':
                bg[axis] = 1;
                break;
            default:
                if (position.search('%') != -1) {
                    bg[axis] = parseInt(position, 10) / 100;
                }
                else {
                    fraction = false;
                }
        }
        bg[axis] = Math.ceil(  fraction ? ( (size[horizontal?'W': 'H'] * bg[axis]) - (size[horizontal?'w': 'h'] * bg[axis]) ) : parseInt(position, 10)  );
        if (bg[axis] % 2 === 0) {
            bg[axis]++;
        }
        return bg[axis];
    },
    fixPng: function (el) {
        el.style.behavior = 'none';
        var lib, els, nodeStr, v, e;
        if (el.nodeName == 'BODY' || el.nodeName == 'TD' || el.nodeName == 'TR') { /* elements not supported yet */
            return;
        }
        el.isImg = false;
        if (el.nodeName == 'IMG') {
            if(/.png(?:W|$)/.test(el.src.toLowerCase())) {
                                el.isImg = true;
                                el.style.visibility = 'hidden';
            }
            else {
                return;
            }
        }
        else if (el.currentStyle.backgroundImage.toLowerCase().search('.png') == -1) {
            return;
        }
        lib = DD_belatedPNG;
        el.vml = {color: {}, image: {}};
        els = {shape: {}, fill: {}};
        for (v in el.vml) {
            if (el.vml.hasOwnProperty(v)) {
                for (e in els) {
                    if (els.hasOwnProperty(e)) {
                        nodeStr = lib.ns + ':' + e;
                        el.vml[v][e] = document.createElement(nodeStr);
                    }
                }
                el.vml[v].shape.stroked = false;
                el.vml[v].shape.appendChild(el.vml[v].fill);
                el.parentNode.insertBefore(el.vml[v].shape, el);
            }
        }
        el.vml.image.shape.fillcolor = 'none'; /* Don't show blank white shapeangle when waiting for image to load. */
        el.vml.image.fill.type = 'tile'; /* Makes image show up. */
        el.vml.color.fill.on = false; /* Actually going to apply vml element's style.backgroundColor, so hide the whiteness. */
        lib.attachHandlers(el);
        lib.giveLayout(el);
        lib.giveLayout(el.offsetParent);
        el.vmlInitiated = true;
        lib.applyVML(el); /* Render! */
    }
};
try {
    document.execCommand("BackgroundImageCache", false, true); /* TredoSoft Multiple IE doesn't like this, so try{} it */
} catch(r) {}
DD_belatedPNG.createVmlNameSpace();
DD_belatedPNG.createVmlStyleSheet();