1.为什么要用到BASE64编码的图片信息?
??? Base64是网络上最常见的用于传输8Bit字节代码的编码方式之一。Base64 主要不是加密，它主要的用途是把一些二进制数转成普通字符用于网络传输。由于一些二进制字符在传输协议中属于控制字符，不能直接传送需要转换一下。最常见的用途是作为电子邮件或WebService附件的传输编码.?
2.base64编码定义?
目前的internet e-mail标准--简单邮件传递协议(smtp)在rfc821中规定了两条重要但不难实现的限制：?
??????? 1)邮件的内容必须全部为7-比特的美国ascii码。?
??????? 2)每一行的长度不能超过1000的字符。?
??? 因此为了通过smtp用e-mail进行传送,内存的序列化对象必须转化为和以上相容的格式。?
??? rfc1521提供了一个可行的方案。它定义了邮件的内容部分,使之能包涵多种形式的数据。这种标准就是目前众所周知的mime。?
按照rfc1521编码过程为:输入是24个比特,输出是4个字节。24个比特输入组从左至右 由3个8比特的输入组形成。这24个比特被看成4个连续的6比特组,而每个6比特输入组被翻译为base64码表中的一个数字。依次反复不断进行，直到全部输入数据转换完成。?
??? 如果最后剩下两个输入数据，在编码结果后加1个“=”；如果最后剩下一个输入数据，编码结果后加2个“=”；如果没有剩下任何数据，就什么都不要加，这样才可以保证资料还原的正确性。?
??? 完整的base64定义可见 RFC1421和 RFC2045。编码后的数据比原始数据略长，为原来的4/3。在电子邮件中，根据RFC822规定，每76个字符，还需要加上一个回车换行。可以估算编码后数据长度大约为原长的135.1%。?
4.显示被存储为Base64编码字符串的图片的例子?
1)使用data: URI直接在网页中嵌入.?
data: URI定义于IETF标准的RFC 2397?
data: URI的基本使用格式如下：?
data:[<MIME-type>][;base64|charset=some_charset],<data>?
mime-type是嵌入数据的mime类型，比如png图片就是image/png。?
如果后面跟base64，说明后面的data是采用base64方式进行编码的?
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? 这种方式,Firfox、Opera、Safari和Konqueror这些浏览器都已经支持，但是IE直到7.0版本都还没有支持.所以,比较好的做法是在服务器端将base64编码的字符串转换成byte流.这里,我提供了java的实现方法.?
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图片转换为base64格式的图片和样式的路径：
http://ippa.se/base64-image-encoder/
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<!DOCTYPE html >
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>电子签名的使用</title>
<style type="text/css">
#canvas {
border: 1px solid #ccc;
}
</style>
</head>
<body>
<div id="canvasDiv"></div>
<button id="btn_clear">Clear</button>
<button id="btn_submit">Submit</button>
<form action="./Base64ToImages" method="post"><input
type="hidden" name="imageData" id="imageData" /></form>
<img id="tempImage" src="" style="display: none;" alt="临时图片文件" />
<script language="javascript">
var canvasDiv = document.getElementById('canvasDiv');
var canvas = document.createElement('canvas');
var canvasWidth = 600, canvasHeight = 400;
var point = {};
point.notFirst = false;
canvas.setAttribute('width', canvasWidth);
canvas.setAttribute('height', canvasHeight);
canvas.setAttribute('id', 'canvas');
canvasDiv.appendChild(canvas);
if (typeof G_vmlCanvasManager != 'undefined') {
canvas = G_vmlCanvasManager.initElement(canvas);
}
var context = canvas.getContext("2d");
canvas.addEventListener("mousedown", function(e) {
var mouseX = e.pageX - this.offsetLeft;
var mouseY = e.pageY - this.offsetTop;
paint = true;
addClick(e.pageX - this.offsetLeft, e.pageY - this.offsetTop);
redraw();
});
canvas.addEventListener("mousemove",
function(e) {
if (paint) {
addClick(e.pageX - this.offsetLeft, e.pageY
- this.offsetTop, true);
redraw();
}
});
canvas.addEventListener("mouseup", function(e) {
paint = false;
});
canvas.addEventListener("mouseleave", function(e) {
paint = false;
});
var clickX = new Array();
var clickY = new Array();
var clickDrag = new Array();
var paint;
function addClick(x, y, dragging) {
clickX.push(x);
clickY.push(y);
clickDrag.push(dragging);
}
function redraw() {
//canvas.width = canvas.width; // Clears the canvas
context.strokeStyle = "#df4b26";
context.lineJoin = "round";
context.lineWidth = 5;
while (clickX.length > 0) {
point.bx = point.x;
point.by = point.y;
point.x = clickX.pop();
point.y = clickY.pop();
point.drag = clickDrag.pop();
context.beginPath();
if (point.drag && point.notFirst) {
context.moveTo(point.bx, point.by);
} else {
point.notFirst = true;
context.moveTo(point.x - 1, point.y);
}
context.lineTo(point.x, point.y);
context.closePath();
context.stroke();
}
/*
for(var i=0; i < clickX.length; i++)
{
context.beginPath();
if(clickDrag[i] && i){
context.moveTo(clickX[i-1], clickY[i-1]);
}else{
context.moveTo(clickX[i]-1, clickY[i]);
}
context.lineTo(clickX[i], clickY[i]);
context.closePath();
context.stroke();
}
*/
}
var clear = document.getElementById("btn_clear");
var submit = document.getElementById("btn_submit");
clear.addEventListener("click", function() {
canvas.width = canvas.width;
});
submit.addEventListener("click", function() {
//获取当前页面的信息，在当前页面的img下展示
var image = document.getElementById("tempImage");
image.src = canvas.toDataURL("image/png");
document.getElementById("imageData").value = canvas
.toDataURL("image/png");
image.style = "display:block;";
//获取canvas的数据格式如下
alert(canvas.toDataURL("image/png"));
//提交表单数据信息
document.forms[0].submit();
});
</script>
</body>
</html>
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Servlert代码如下：
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package com.easyway.html5.canvas;
import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import java.util.Date;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import sun.misc.BASE64Decoder;
/**
*
* Servlet implementation class Base64ToImages
*/
public class Base64ToImages extends HttpServlet {
private static final long serialVersionUID = 1L;
/**
* @see HttpServlet#HttpServlet()
*/
public Base64ToImages() {
super();
}
/**
* @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse
* response)
*/
protected void doGet(HttpServletRequest request,
HttpServletResponse response) throws ServletException, IOException {
doPost(request,response);
}
/**
* @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse
* response)
*/
protected void doPost(HttpServletRequest request,
HttpServletResponse response) throws ServletException, IOException {
base64toImage(request);
}
/**
* 处理base64转化image的方法
* @param request
*/
public void base64toImage(HttpServletRequest request) {
try {
String base64data = request.getParameter("imageData");
String imageData=base64data.split(",")[1];
BASE64Decoder decoder = new BASE64Decoder();
byte[] imgBytes = decoder.decodeBuffer(imageData);
for (int i = 0; i < imgBytes.length; ++i) {
if (imgBytes[i] < 0) {// 调整异常数据
imgBytes[i] += 256;
}
}
//保存特定的目录下面
String imagepath =getServletContext().getRealPath("/images");
System.out.println("imagepath="+imagepath);
File dir=new File(imagepath);
if(!dir.exists()){
dir.mkdirs();
}
String filename=new Date().getTime()+".png";
File decFile = new File(imagepath+File.separator+filename);
String dd=decFile.getAbsolutePath();
if(decFile.exists()){
decFile.delete();
}
decFile.createNewFile();
FileOutputStream ops = new FileOutputStream(decFile);
ops.write(imgBytes, 0, imgBytes.length);
ops.flush();
ops.close();
} catch (IOException e) {
e.printStackTrace();
}
}
}