canvas导出为图片并用JS下载

最新推荐文章于 2025-06-07 10:24:25 发布

夜之雨

最新推荐文章于 2025-06-07 10:24:25 发布

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本文介绍如何使用HTML5 Canvas实现绘图功能，并通过修改MIME类型将绘制的内容导出为图片，最后实现下载功能。

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1、从canvas中直接提取图片元数据

//
 图片导出为 png 格式

var type
 = 'png';

var imgData
 = canvas.toDataURL(type);

上面的代码得到的数据格式为：data:image/png;base64,.....

2、将mime-type改为image/octet-stream，强制让浏览器直接download

/**

 *
 获取mimeType

 *
 @param  {String} type the old mime-type

 *
 @return the new mime-type

 */

var _fixType
 = function(type)
 {

    type
 = type.toLowerCase().replace(/jpg/i, 'jpeg');

    var r
 = type.match(/png|jpeg|bmp|gif/)[0];

    return 'image/' +
 r;

};

   

//
 加工image data，替换mime type

imgData
 = imgData.replace(_fixType(type),'image/octet-stream');

上面这个代码得到的数据格式为：data:image/octet-stream;base64,.....

3、图片download到本地

/**

 *
 在本地进行文件保存

 *
 @param  {String} data     要保存到本地的图片数据

 *
 @param  {String} filename 文件名

 */

var saveFile
 = function(data,
 filename){

    var save_link
 = document.createElementNS('http://www.w3.org/1999/xhtml', 'a');

    save_link.href
 = data;

    save_link.download
 = filename;

   

    var event
 = document.createEvent('MouseEvents');

    event.initMouseEvent('click', true, false,
 window, 0, 0, 0, 0, 0, false, false, false, false,
 0, null);

    save_link.dispatchEvent(event);

};

   

//
 下载后的问题名

var filename
 = 'baidufe_' +
 (new Date()).getTime()
 + '.' +
 type;

//
 download

saveFile(imgData,filename);

测试页面：

<%@ page language="java" contentType="text/html; charset=UTF-8"
pageEncoding="UTF-8"%>
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>DEMO6：自定义画板</title>
</head>
<body>
<canvas id="canvas" width="600" height="300">
浏览器不支持canvas 
</canvas>
<br/>
<button style="width:78px;background-color:yellow;" οnclick='linecolor="yellow";'>黄</button>
<button style="width:78px ;background-color:red;" οnclick='linecolor="red";'>红</button>
<button style="width:78px ;background-color:blue;" οnclick='linecolor="blue";'>蓝</button>
<button style="width:78px ;background-color:green;" οnclick='linecolor="green";'>绿</button>
<button style="width:78px ;background-color:white;" οnclick='linecolor="white";'>白</button>
<button style="width:78px ;background-color:gray;" οnclick='linecolor="gray";'>灰</button>
<button style="width:78px ;background-color:pink;" οnclick='linecolor="pink";'>粉</button>
<br/>
<br/>
<button style="width: 80px;background-color: white;" οnclick="linw=4;">细</button>
<button style="width: 80px;background-color: white;" οnclick="linw=8;">中</button>
<button style="width: 80px;background-color: white;" οnclick="linw=16;">粗</button>
    
<button style="width: 80px;background-color: white;" οnclick="copyimage();">预览</button>
<button style="width: 80px;background-color: white;" οnclick="downloadimage();">下载</button>
<br/>

<br/>
<img src="" id="image_png" width="600px" height="300px">
<br/>

<script type="text/javascript">
var canvas = document.getElementById('canvas'); //获取标签
var ctx = canvas.getContext("2d");

var fillStyle = "black";
ctx.fillRect(0,0,600,300);
var onoff = false; //按下标记
var oldx = -10;
var oldy = -10;
//设置颜色
var linecolor = "white";
var linw = 4;
canvas.addEventListener("mousemove",draw,true); //鼠标移动事件
canvas.addEventListener("mousedown",down,false); //鼠标按下事件
canvas.addEventListener("mouseup",up,false); //鼠标弹起事件
function down(event){
onoff = true;
oldx = event.pageX - 10;
oldy = event.pageY - 10;
}
function up(){
onoff = false;
}
function draw(event){
if (onoff==true) {
var newx = event.pageX - 10;
var newy = event.pageY - 10
ctx.beginPath();
ctx.moveTo(oldx,oldy);
ctx.lineTo(newx,newy);
ctx.strokeStyle = linecolor;
ctx.lineWidth = linw;
ctx.lineCap = "round";
ctx.stroke();

oldx = newx;
oldy = newy;
}
}
function copyimage(event)
{
var img_png_src = canvas.toDataURL("image/png"); //将画板保存为图片格式的函数
document.getElementById("image_png").src = img_png_src;
}

function downloadimage(event)
{
// 图片导出为 png 格式
var type = 'png';
// 返回一个包含JPG图片的<img>元素
var img_png_src = canvas.toDataURL("image/png"); //将画板保存为图片格式的函数
// 加工image data，替换mime type
imgData = img_png_src.replace(_fixType(type),'image/octet-stream');
// 下载后的问题名
var filename = '个人画板_' + (new Date()).getTime() + '.' + type;
// download
saveFile(imgData,filename);
}

/**
* 在本地进行文件保存
* @param {String} data 要保存到本地的图片数据
* @param {String} filename 文件名
*/
var saveFile = function(data, filename){
var save_link = document.createElementNS('http://www.w3.org/1999/xhtml', 'a');
save_link.href = data;
save_link.download = filename;

var event = document.createEvent('MouseEvents');
event.initMouseEvent('click', true, false, window, 0, 0, 0, 0, 0, false, false, false, false, 0, null);
save_link.dispatchEvent(event);
};

/**
* 获取mimeType
* @param {String} type the old mime-type
* @return the new mime-type
*/
var _fixType = function(type) {
type = type.toLowerCase().replace(/jpg/i, 'jpeg');
var r = type.match(/png|jpeg|bmp|gif/)[0];
return 'image/' + r;
};
</script>
</body>
</html>