【已知】
canvas提供了toDataURL的接口，可以方便的将canvas画布转化成base64编码的image。目前支持的最好的是png格式，jpeg格式的现代浏览器基本也支持，但是支持的不是很好。

【想要的】
往往这么简单直接的接口通常都满足不了需求。我想要的不仅是简单的通过画布生成一个png，我不想新开一个tab，然后还要右键另存为...

我还需要更方便的自由的配置生成的图片的大小，比例等。

另外如果我还要别的图片格式，比如位图bmp，gif等怎么办...

【解决办法】
a)想直接把图片生成后download到本地，其实办法也很简单。直接改图片的mimeType，强制改成steam流类型的。比如‘image/octet-stream’，浏览器就会自动帮我们另存为..

b)图片大小，及比例的可控倒也好办，我们新建一个我们想要大小的canvas，把之前的canvas画布重新按照所要的比例，及大小draw到新的canvas上，然后用新的canvas来toDataURL即可。

c)想要bmp位图会麻烦些... 没有直接的接口，需要我们自己来生成。生成图片的响应头和响应体有一定的规则，略显麻烦。不过还能接受。剩下的就是性能问题，按像素级别来操作，对于一个大图来说计算量很有压力。

【实现】

/*** covert canvas to image* and save the image file*/var Canvas2Image = function () {// check if support sth.var $support = function () {var canvas = document.createElement('canvas'),ctx = canvas.getContext('2d');return {canvas: !!ctx,imageData: !!ctx.getImageData,dataURL: !!canvas.toDataURL,btoa: !!window.btoa};}();var downloadMime = 'image/octet-stream';function scaleCanvas (canvas, width, height) {var w = canvas.width,h = canvas.height;if (width == undefined) {width = w;}if (height == undefined) {height = h;}var retCanvas = document.createElement('canvas');var retCtx = retCanvas.getContext('2d');retCanvas.width = width;retCanvas.height = height;retCtx.drawImage(canvas, 0, 0, w, h, 0, 0, width, height);return retCanvas;}function getDataURL (canvas, type, width, height) {canvas = scaleCanvas(canvas, width, height);return canvas.toDataURL(type);}function saveFile (strData) {document.location.href = strData;}function genImage(strData) {var img = document.createElement('img');img.src = strData;return img;}function fixType (type) {type = type.toLowerCase().replace(/jpg/i, 'jpeg');var r = type.match(/png|jpeg|bmp|gif/)[0];return 'image/' + r;}function encodeData (data) {if (!window.btoa) { throw 'btoa undefined' }var str = '';if (typeof data == 'string') {str = data;} else {for (var i = 0; i < data.length; i ++) {str += String.fromCharCode(data[i]);}}return btoa(str);}function getImageData (canvas) {var w = canvas.width,h = canvas.height;return canvas.getContext('2d').getImageData(0, 0, w, h);}function makeURI (strData, type) {return 'data:' + type + ';base64,' + strData;}/*** create bitmap image* 按照规则生成图片响应头和响应体*/var genBitmapImage = function (data) {var imgHeader = [],imgInfoHeader = [];var width = data.width,height = data.height;imgHeader.push(0x42); // 66 -> BimgHeader.push(0x4d); // 77 -> Mvar fsize = width * height * 3 + 54; // header size:54 bytesimgHeader.push(fsize % 256); // rfsize = Math.floor(fsize / 256);imgHeader.push(fsize % 256); // gfsize = Math.floor(fsize / 256);imgHeader.push(fsize % 256); // bfsize = Math.floor(fsize / 256);imgHeader.push(fsize % 256); // aimgHeader.push(0);imgHeader.push(0);imgHeader.push(0);imgHeader.push(0);imgHeader.push(54); // offset -> 6imgHeader.push(0);imgHeader.push(0);imgHeader.push(0);// info headerimgInfoHeader.push(40); // info header sizeimgInfoHeader.push(0);imgInfoHeader.push(0);imgInfoHeader.push(0);// 横向infovar _width = width;imgInfoHeader.push(_width % 256);_width = Math.floor(_width / 256);imgInfoHeader.push(_width % 256);_width = Math.floor(_width / 256);imgInfoHeader.push(_width % 256);_width = Math.floor(_width / 256);imgInfoHeader.push(_width % 256);// 纵向infovar _height = height;imgInfoHeader.push(_height % 256);_height = Math.floor(_height / 256);imgInfoHeader.push(_height % 256);_height = Math.floor(_height / 256);imgInfoHeader.push(_height % 256);_height = Math.floor(_height / 256);imgInfoHeader.push(_height % 256);imgInfoHeader.push(1);imgInfoHeader.push(0);imgInfoHeader.push(24); // 24位bitmapimgInfoHeader.push(0);// no compressionimgInfoHeader.push(0);imgInfoHeader.push(0);imgInfoHeader.push(0);imgInfoHeader.push(0);// pixel datavar dataSize = width * height * 3;imgInfoHeader.push(dataSize % 256);dataSize = Math.floor(dataSize / 256);imgInfoHeader.push(dataSize % 256);dataSize = Math.floor(dataSize / 256);imgInfoHeader.push(dataSize % 256);dataSize = Math.floor(dataSize / 256);imgInfoHeader.push(dataSize % 256);// blank spacefor (var i = 0; i < 16; i ++) {imgInfoHeader.push(0);}var padding = (4 - ((width * 3) % 4)) % 4;var imgData = data.data;var strPixelData = '';var y = height;do {var offsetY = width * (y - 1) * 4;var strPixelRow = '';for (var x = 0; x < width; x ++) {var offsetX = 4 * x;strPixelRow += String.fromCharCode(imgData[offsetY + offsetX + 2]);strPixelRow += String.fromCharCode(imgData[offsetY + offsetX + 1]);strPixelRow += String.fromCharCode(imgData[offsetY + offsetX]);}for (var n = 0; n < padding; n ++) {strPixelRow += String.fromCharCode(0);}strPixelData += strPixelRow;} while(-- y);return (encodeData(imgHeader.concat(imgInfoHeader)) + encodeData(strPixelData));};/*** saveAsImage* @param canvasElement* @param {String} image type* @param {Number} [optional] png width* @param {Number} [optional] png height*/var saveAsImage = function (canvas, width, height, type) {if ($support.canvas && $support.dataURL) {if (type == undefined) { type = 'png'; }type = fixType(type);if (/bmp/.test(type)) {var data = getImageData(scaleCanvas(canvas, width, height));var strData = genBitmapImage(data);saveFile(makeURI(strData, downloadMime));} else {var strData = getDataURL(canvas, type, width, height);saveFile(strData.replace(type, downloadMime));}}}var convertToImage = function (canvas, width, height, type) {if ($support.canvas && $support.dataURL) {if (type == undefined) { type = 'png'; }type = fixType(type);if (/bmp/.test(type)) {var data = getImageData(scaleCanvas(canvas, width, height));var strData = genBitmapImage(data);return genImage(makeURI(strData, 'image/bmp'));} else {var strData = getDataURL(canvas, type, width, height);return genImage(strData);}}}return {saveAsImage: saveAsImage,saveAsPNG: function (canvas, width, height) {return saveAsImage(canvas, width, height, 'png');},saveAsJPEG: function (canvas, width, height) {return saveAsImage(canvas, width, height, 'jpeg');            },saveAsGIF: function (canvas, width, height) {return saveAsImage(canvas, width, height, 'gif')           },saveAsBMP: function (canvas, width, height) {return saveAsImage(canvas, width, height, 'bmp');           },convertToImage: convertToImage,convertToPNG: function (canvas, width, height) {return convertToImage(canvas, width, height, 'png');},convertToJPEG: function (canvas, width, height) {return convertToImage(canvas, width, height, 'jpeg');               },convertToGIF: function (canvas, width, height) {return convertToImage(canvas, width, height, 'gif');              },convertToBMP: function (canvas, width, height) {return convertToImage(canvas, width, height, 'bmp');              }};}();

【Demo】
http://hongru.github.com/proj/canvas2image/index.html
可以试着在canvas上涂涂画画，然后保存看看。如果用bmp格式的话，需要支持 btoa 的base64编码，关于base64编码规则可看上一篇博文

【不完美的地方】
1）jpeg接口本身就不完善，当canvas没有填充颜色或图片时，保存的jpeg由于是直接由png的alpha通道强制转换过来的，所以在png的透明部分在jpeg里面就是黑色的。

2）gif的限制太多。且可用性不大，有png就够了

3）bmp位图生成，计算量稍显大了。

4）由于是强制改mimeType来实现的自动下载，所以下载的时候文件类型不会自动识别。

原文：http://www.cnblogs.com/hongru/archive/2012/01/14/2322540.html

index.html

<!doctype html>
<html>
<meta charset="utf-8" />
<script src="canvas2image.js"></script>
<style>.doc {width: 604px;margin: 0 auto;}canvas {display: block;border: 2px solid #888;}
</style>
<body>
<div class="doc"><canvas width="600" height="400" id="cvs"></canvas><div><p><button id="save">save</button> or <button id="convert">convert to</button> as: <select id="sel"><option value="png">png</option><option value="jpeg">jpeg</option><option value="bmp">bmp</option></select><br/>width : <input type="number" value="300" id="imgW" /><br/>height : <input type="number" value="200" id="imgH" /></p></div><div id="imgs"></div>
</div>
<script>var canvas, ctx, bMouseIsDown = false, iLastX, iLastY,$save, $imgs,$convert, $imgW, $imgH,$sel;function init () {canvas = document.getElementById('cvs');ctx = canvas.getContext('2d');$save = document.getElementById('save');$convert = document.getElementById('convert');$sel = document.getElementById('sel');$imgs = document.getElementById('imgs');$imgW = document.getElementById('imgW');$imgH = document.getElementById('imgH');bind();draw();}function bind () {canvas.onmousedown = function(e) {bMouseIsDown = true;iLastX = e.clientX - canvas.offsetLeft + (window.pageXOffset||document.body.scrollLeft||document.documentElement.scrollLeft);iLastY = e.clientY - canvas.offsetTop + (window.pageYOffset||document.body.scrollTop||document.documentElement.scrollTop);}canvas.onmouseup = function() {bMouseIsDown = false;iLastX = -1;iLastY = -1;}canvas.onmousemove = function(e) {if (bMouseIsDown) {var iX = e.clientX - canvas.offsetLeft + (window.pageXOffset||document.body.scrollLeft||document.documentElement.scrollLeft);var iY = e.clientY - canvas.offsetTop + (window.pageYOffset||document.body.scrollTop||document.documentElement.scrollTop);ctx.moveTo(iLastX, iLastY);ctx.lineTo(iX, iY);ctx.stroke();iLastX = iX;iLastY = iY;}};$save.onclick = function (e) {var type = $sel.value,w = $imgW.value,h = $imgH.value;Canvas2Image.saveAsImage(canvas, w, h, type);}$convert.onclick = function (e) {var type = $sel.value,w = $imgW.value,h = $imgH.value;$imgs.appendChild(Canvas2Image.convertToImage(canvas, w, h, type))}}function draw () {ctx.fillStyle = '#ffffff';ctx.fillRect(0, 0, 600, 400);ctx.fillStyle = 'red';ctx.fillRect(100, 100, 50, 50);}onload = init;
</script>
</body>
</html>