这篇文章主要介绍了js对图片base64编码字符串进行解码并输出图像的具体实现，大家可以参考下面的示例
代码如下:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=gb2312" />
<style>
body{padding-left:75px;background-color:beige}
</style>
<script>
///////////////////////////
//base64编码的GIF图像解码
//By Mozart0
//2005/10/29
////////////////////
//建立GIF类的对象
//类GIF在此函数内部定义
//str64：gif文件的Base64编码字符串
//成功返回创建的GIF对象
//失败返回null
function getGif(str64){
var bytes=decodeBase64(str64);
if(!bytes){
alert("错误：无效的Base64编码");
return null;
}
var gif=new GIF();
for(var i=0;i<6;i++)
gif.version+=String.fromCharCode(bytes[i]);
if(gif.version.slice(0,3)!="GIF"){
alert("错误：非Gif图像格式");
return null;
}
gif.width=bytes[i]|(bytes[i+1]<<8);
gif.height=bytes[i+2]|(bytes[i+3]<<8);
var f=bytes[i+4];
gif.colorResolution=(f>>4&0x7)+1;
gif.sorted=(f&0x8)?true:false;
gif.backgroundIndex=bytes[i+5];
gif.pixelAspectRadio=bytes[i+6];
if(f&0x80){
gif.globalPalette=[];
i+=getPalette(i+7,bytes,gif.globalPalette,2<<(f&0x7));
}
i+=7;
for(var j=i;j<bytes.length;j++)
if(bytes[j]==0x21&&bytes[j+1]==0xf9)
break;
if(j==bytes.length){
for(;i<bytes.length;i++)
if(bytes[i]==0x2c)
break;
if(i==bytes.length){
alert("错误：找不到图像数据");
return null;
}
var f=new GIF_Frame();
if(!getSingleFrame(i,f))
return null;
else
gif.frames.push(f);
}
else{
i=j;
do{
var f=new GIF_Frame();
var t=getSingleFrame(i,f);
if(!t)
return null;
gif.frames.push(f);
for(i+=t;i<bytes.length;i++)
if(bytes[i]==0x21&&bytes[i+1]==0xf9)
break;
}
while(i<bytes.length);
}
return gif;
//内部过程，生成色表
function getPalette(pos,s,d,len){
len*=3;
for(var i=pos;i<pos+len;i+=3)
d.push('#'+(s[i]<=0xf?"0":"")+s[i].toString(16)
+(s[i+1]<=0xf?"0":"")+s[i+1].toString(16)
+(s[i+2]<=0xf?"0":"")+s[i+2].toString(16));
return len;
}
//内部过程，整合数据段
function getBlock(pos,s,d){
var p=pos;
while(len=s[p++]){
for(var i=0;i<len;i++)
d.push(s[p+i]);
p+=len;
}
return p-pos;
}
//内部过程，获取一帧数据
function getSingleFrame(pos,frame){
var i=pos;
if(bytes[i]==0x21){
i+=3;
if(bytes[i]&1)
frame.transparentIndex=bytes[i+3];
frame.delay=bytes[i+1]|(bytes[i+2]<<8);
for(i+=5;i<bytes.length&&bytes[i]!=0x2c;i++);
if(i==bytes.length){
alert("错误:找不到图像标志符");
return 0;
}
}
frame.offsetX=bytes[i+1]|(bytes[i+2]<<8);
frame.offsetY=bytes[i+3]|(bytes[i+4]<<8);
frame.width=bytes[i+5]|(bytes[i+6]<<8);
frame.height=bytes[i+7]|(bytes[i+8]<<8);
var f=bytes[i+9];
i+=10;
if(f&0x40)
frame.interlace=true;
if(f&0x20)
frame.sorted=true;
if(f&0x80){
frame.colorResolution=(f&0x7)+1;
frame.localPalette=[];
i+=getPalette(i,bytes,frame.localPalette,1<<frame.colorResolution);
}
else{
frame.colorResolution=gif.colorResolution;
frame.localPalette=gif.globalPalette;
}
var lzwLen=bytes[i++]+1;
i+=getBlock(i,bytes,frame.data);
frame.data=decodeLzw(frame.data,lzwLen);
return frame.data?i-pos:0;
}
//定义存储GIF文件的数据结构
//提供方法showInfo,返回图片信息
function GIF(){
this.version=""; //版本号
this.width=0; //逻辑屏幕宽度
this.height=0; //逻辑屏幕高度
this.colorResolution=0; //颜色深度
this.sorted=false; //全局色表分类标志
this.globalPalette=null; //全局色表
this.backgroundIndex=-1; //背景色索引
this.pixelAspectRadio=0; //像素宽高比
this.frames=[]; //图像各帧，见GIF_Frame
this.showInfo=function(sep){ //显示图片信息,sep为行分隔符
if(!sep)
sep="n";
var s="Gif infomation:"+sep+"-------------------";
s+=subInfo(this)+sep;
for(var i=0;i<this.frames.length;i++)
s+=sep+"frames "+i+"----------"+subInfo(this.frames[i]);
return s;
function subInfo(o){
var s="";
for(var i in o){
if(i=="showInfo"||i=="draw")
continue;
s+=sep+i+":";
if(typeof(o[i])=="object")
s+=(o[i]?o[i].length:"null");
else
s+=o[i];
}
return s;
}
}
}
//定义存储一帧图象的数据结构
//提供方法draw,绘图
function GIF_Frame(){
this.offsetX=0; //X方向偏移量
this.offsetY=0; //Y方向偏移量
this.width=0; //图象宽度
this.height=0; //图象高度
this.localPalette=null; //局部色表
this.colorResolution=0; //颜色深度
this.interlace=false; //交错标志
this.sorted=false; //局部色表分类标志
this.data=[]; //图像数据，存储各像素颜色的整数索引
this.transparentIndex=-1; //透明色索引
this.delay=0; //帧延时
this.draw=function(parent,zoom){
if(!this.data.length)
return;
if(!parent)
parent=document.body;
if(!zoom)
zoom=1;
if(parent.clientWidth<this.width*zoom)
parent.style.width=this.width*zoom;
if(parent.clientHeight<this.height*zoom)
parent.style.height=this.height*zoom;
var id="ImgDefaultDraw";
var img=document.getElementById(id);
if(img)
delete parent.removeChild(img);
img=document.createElement("DIV");
img.id=id;
parent.appendChild(img);
img.style.position="absolute";
var t=document.createElement("DIV");
t.style.overflow="hidden";
t.style.position="absolute";
defLayout(this.data,this.localPalette,this.width,this.height,img,t,zoom);
delete t;
}
}
}
//Base64解码
//strIn，输入字符串
//成功返回一个数组，每一个元素包含一字节信息
//失败返回null
function decodeBase64(strIn){
if(!strIn.length||strIn.length%4)
return null;
var str64=
"ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=";
var index64=[];
for(var i=0;i<str64.length;i++)
index64[str64.charAt(i)]=i;
var c0,c1,c2,c3,b0,b1,b2;
var len=strIn.length;
var len1=len;
if(strIn.charAt(len-1)=='=')
len1-=4;
var result=[];
for(var i=0,j=0;i<len1;i+=4){
c0=index64[strIn.charAt(i)];
c1=index64[strIn.charAt(i+1)];
c2=index64[strIn.charAt(i+2)];
c3=index64[strIn.charAt(i+3)];
b0=(c0<<2)|(c1>>4);
b1=(c1<<4)|(c2>>2);
b2=(c2<<6)|c3;
result.push(b0&0xff);
result.push(b1&0xff);
result.push(b2&0xff);
}
if(len1!=len){
c0=index64[strIn.charAt(i)];
c1=index64[strIn.charAt(i+1)];
c2=strIn.charAt(i+2);
b0=(c0<<2)|(c1>>4);
result.push(b0&0xff);
if(c2!='='){
c2=index64[c2];
b1=(c1<<4)|(c2>>2);
result.push(b1&0xff);
}
}
return result;
}
//用于GIF的LZW解码函数
//arrBytes为源数据,nBits为初始编码位数
//成功返回数组，每个元素包括一个颜色索引
//失败返回null
function decodeLzw(arrBytes,nBits){
var cc=1<<(nBits-1);
var eoi=cc+1;
var table=[],mask=[],result=[];
for(var i=0;i<cc;i++)
table[i]=(i>>8&0xf).toString(16)
+(i>>4&0xf).toString(16)+(i&0xf).toString(16);
for(i=2,mask[1]=1;i<13;i++)
mask[i]=mask[i-1]<<1|1;
var bc=nBits;
var pos=0,temp=0,tleft=0,code=0,old=0;
while(true){
while(tleft<bc){
temp=temp|(arrBytes[pos++]<<tleft);
tleft+=8;
}
code=temp&mask[bc];
tleft-=bc;
temp>>=bc;
if(code==eoi)
break;
if(code==cc){
table.length=cc+2;
bc=nBits;
old=code;
continue;
}
var t="";
if(code<table.length){
t=table[code];
if(old!=cc)
table.push(table[old]+t.slice(0,3));
}
else if(old<table.length){
t=table[old]+table[old].slice(0,3);
table.push(t);
}
else{
alert("错误：图像数据无效");
return null;
}
old=code;
for(var i=0;i<t.length;i+=3)
result.push(parseInt(t.substr(i,3),16))
if(table.length==1<<bc&&bc<12)
bc++;
}
return result;
}
//根据字节数组data布局，以最少的div完成绘图
function defLayout(data,palette,width,height,image,block,zoom){
var map=new Array(height);
for(var i=0;i<height;i++){
map[i]=new Array(width);
for(var j=0;j<width;j++)
map[i][j]=data[i*width+j];
}
var i,j,i1,i2,j1,j2,c;
for(i=0;i<height;i++)
for(j=0;j<width;){
if(map[i][j]==0x100){
j++;
continue;
}
c=map[i][j];
for(i1=i+1;i1<height&&map[i1][j]==c;i1++);
for(j1=j+1;j1<width;j1++){
for(i2=i;i2<i1&&map[i2][j1]==c;i2++);
if(i2<i1)
break;
}
for(i2=i;i2<i1;i2++)
for(j2=j;j2<j1;j2++)
map[i2][j2]=0x100;
var x=block.cloneNode(true);
x.style.left=j*zoom;
x.style.top=i*zoom;
x.style.width=(j1-j)*zoom;
x.style.height=(i1-i)*zoom;
x.style.backgroundColor=palette[c];
image.appendChild(x);
j=j1;
}
}
</SCRIPT>
<script>
function main(){
var t=new Date().getTime();
var xmldom=document.getElementById("imgData");
var gif=getGif("R0lGODlhDwAPAKECAAAAzMzM/////wAAACwAAAAADwAPAAACIISPeQHsrZ5ModrLlN48CXF8m2iQ3Ymm
KqVlRtW4MLwWACH+H09wdGltaXplZCBieSBVbGVhZCBTbWFydFNhdmVyIQAAOw==");
var info=document.getElementById("info");
info.innerHTML=gif.showInfo("<br>");
t=new Date().getTime();
gif.frames[0].draw(document.getElementById("canvas"),1);
info.innerHTML+="<br>绘图耗时"+(new Date().getTime()-t)+"ms";
}
</SCRIPT>
<body onload="main()">
<div id="canvas"></div>
<hr>
<div id="info">页面载入中，请稍候...</div>
</body>
</html>
