小知识，大挑战！本文正在参与“程序员必备小知识”创作活动。
先安装html2canvas
npm install html2canvas --save
引入Canvas2Image.js
/**
* 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 (oData) {
//
// BITMAPFILEHEADER: http://msdn.microsoft.com/en-us/library/windows/desktop/dd183374(v=vs.85).aspx
// BITMAPINFOHEADER: http://msdn.microsoft.com/en-us/library/dd183376.aspx
//
var biWidth = oData.width;
var biHeight = oData.height;
var biSizeImage = biWidth * biHeight * 3;
var bfSize = biSizeImage + 54; // total header size = 54 bytes
//
// typedef struct tagBITMAPFILEHEADER {
// WORD bfType;
// DWORD bfSize;
// WORD bfReserved1;
// WORD bfReserved2;
// DWORD bfOffBits;
// } BITMAPFILEHEADER;
//
var BITMAPFILEHEADER = [
// WORD bfType -- The file type signature; must be "BM"
0x42, 0x4D,
// DWORD bfSize -- The size, in bytes, of the bitmap file
bfSize & 0xff, bfSize >> 8 & 0xff, bfSize >> 16 & 0xff, bfSize >> 24 & 0xff,
// WORD bfReserved1 -- Reserved; must be zero
0, 0,
// WORD bfReserved2 -- Reserved; must be zero
0, 0,
// DWORD bfOffBits -- The offset, in bytes, from the beginning of the BITMAPFILEHEADER structure to the bitmap bits.
54, 0, 0, 0
];
//
// typedef struct tagBITMAPINFOHEADER {
// DWORD biSize;
// LONG biWidth;
// LONG biHeight;
// WORD biPlanes;
// WORD biBitCount;
// DWORD biCompression;
// DWORD biSizeImage;
// LONG biXPelsPerMeter;
// LONG biYPelsPerMeter;
// DWORD biClrUsed;
// DWORD biClrImportant;
// } BITMAPINFOHEADER, *PBITMAPINFOHEADER;
//
var BITMAPINFOHEADER = [
// DWORD biSize -- The number of bytes required by the structure
40, 0, 0, 0,
// LONG biWidth -- The width of the bitmap, in pixels
biWidth & 0xff, biWidth >> 8 & 0xff, biWidth >> 16 & 0xff, biWidth >> 24 & 0xff,
// LONG biHeight -- The height of the bitmap, in pixels
biHeight & 0xff, biHeight >> 8 & 0xff, biHeight >> 16 & 0xff, biHeight >> 24 & 0xff,
// WORD biPlanes -- The number of planes for the target device. This value must be set to 1
1, 0,
// WORD biBitCount -- The number of bits-per-pixel, 24 bits-per-pixel -- the bitmap
// has a maximum of 2^24 colors (16777216, Truecolor)
24, 0,
// DWORD biCompression -- The type of compression, BI_RGB (code 0) -- uncompressed
0, 0, 0, 0,
// DWORD biSizeImage -- The size, in bytes, of the image. This may be set to zero for BI_RGB bitmaps
biSizeImage & 0xff, biSizeImage >> 8 & 0xff, biSizeImage >> 16 & 0xff, biSizeImage >> 24 & 0xff,
// LONG biXPelsPerMeter, unused
0,0,0,0,
// LONG biYPelsPerMeter, unused
0,0,0,0,
// DWORD biClrUsed, the number of color indexes of palette, unused
0,0,0,0,
// DWORD biClrImportant, unused
0,0,0,0
];
var iPadding = (4 - ((biWidth * 3) % 4)) % 4;
var aImgData = oData.data;
var strPixelData = '';
var biWidth4 = biWidth<<2;
var y = biHeight;
var fromCharCode = String.fromCharCode;
do {
var iOffsetY = biWidth4*(y-1);
var strPixelRow = '';
for (var x = 0; x < biWidth; x++) {
var iOffsetX = x<<2;
strPixelRow += fromCharCode(aImgData[iOffsetY+iOffsetX+2]) +
fromCharCode(aImgData[iOffsetY+iOffsetX+1]) +
fromCharCode(aImgData[iOffsetY+iOffsetX]);
}
for (var c = 0; c < iPadding; c++) {
strPixelRow += String.fromCharCode(0);
}
strPixelData += strPixelRow;
} while (--y);
var strEncoded = encodeData(BITMAPFILEHEADER.concat(BITMAPINFOHEADER)) + encodeData(strPixelData);
return strEncoded;
};
/**
* 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 && $support.dataURL) {
if (typeof canvas == "string") { canvas = document.getElementById(canvas); }
if (type == undefined) { type = 'png'; }
type = fixType(type);
if (/bmp/.test(type)) {
var data = getImageData(scaleCanvas(canvas, width, height));
var strData = genBitmapImage(data);
saveFile(makeURI(strData, downloadMime));
} else {
var strData = getDataURL(canvas, type, width, height);
saveFile(strData.replace(type, downloadMime));
}
}
};
var convertToImage = function (canvas, width, height, type) {
if ($support.canvas && $support.dataURL) {
if (typeof canvas == "string") { canvas = document.getElementById(canvas); }
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');
}
};
}();
export default Canvas2Image;
然后使用组件引入
<script>
import html2canvas from 'html2canvas';
import Canvas2Image from '../assets/js/canvas2image'
</script>
然后页面代码
<div>
<el-dialog title="查看证书" top="0" :visible.sync="dialogkaifa" width="600px" center :destroy-on-close='true'>
<div class="center" style="margin-top:10px;">
<el-button size="mini" type="primary" @click="createImg" >下载证书</el-button>
</div>
<div id="about-kaifa" ref="contain" @contextmenu.prevent>
<div style="position:relative;">
<img src="../../../static/img/kaifa.jpg" style="width:100%;" alt="" srcset="">
<p class="roleinfo" v-if="comSta===1">{{crumbs.user_info.com_name}}</p>
<p class="role-num" v-if="comSta===1">{{crumbs.user_info.developer_no}}</p>
<p class="role-time" v-if="comSta===1">{{crumbs.user_info.com_time | formatDate}}</p>
</div>
</div>
</el-dialog>
</div>
方法
createImg(){
var opts = {
useCORS: true,//支持图片跨域
allowTaint: false,//这个属性和useCORS不能同时为true哦
logging: false,
letterRendering: true,
};
html2canvas(this.$refs.contain,opts)//传入你想生成图片的dom
.then(
function(canvas) {
var img = Canvas2Image.convertToImage(
canvas,
canvas.width,
canvas.height
);
img.style.width="500px";
img.style.height="100%";
var url = canvas.toDataURL("image/png"); //得到图片的base64编码数据
var a = document.createElement("a"); // 生成一个a元素
var event = new MouseEvent("click"); // 创建一个单击事件
a.download = name || "认证证书"; // 设置图片名称
a.href = url; // 将生成的URL设置为a.href属性
a.dispatchEvent(event); // 触发a的单击事件
}
);
},
