另外如果我还要别的图片格式，比如位图bmp，gif等怎么办...
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。
/**
* 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');
}
};
}();