【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 -> B
imgHeader.push(0x4d); // 77 -> M
var fsize = width * height * 3 + 54; // header size:54 bytes
imgHeader.push(fsize % 256); // r
fsize = Math.floor(fsize / 256);
imgHeader.push(fsize % 256); // g
fsize = Math.floor(fsize / 256);
imgHeader.push(fsize % 256); // b
fsize = Math.floor(fsize / 256);
imgHeader.push(fsize % 256); // a
imgHeader.push(0);
imgHeader.push(0);
imgHeader.push(0);
imgHeader.push(0);
imgHeader.push(54); // offset -> 6
imgHeader.push(0);
imgHeader.push(0);
imgHeader.push(0);
// info header
imgInfoHeader.push(40); // info header size
imgInfoHeader.push(0);
imgInfoHeader.push(0);
imgInfoHeader.push(0);
// 横向info
var _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info
var _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bitmap
imgInfoHeader.push(0);
// no compression
imgInfoHeader.push(0);
imgInfoHeader.push(0);
imgInfoHeader.push(0);
imgInfoHeader.push(0);
// pixel data
var 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 space
for (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>