dom提高分辨率转canvas

dom 内容转成canvas图片

依赖库为html2canvas.js

由于手机的高分辨率，实际dom像素值会很小，解除的图会发虚。

下面的代码演示了如果将当前撑满手机屏幕的dom元素放大2倍的分辨率（ipad为1.2倍）截出图片。

// source dom
var $div = $('#shareImage');
// target canvas
var canvas = document.createElement('canvas');
// 设置大小
(function () {
var scale = document.documentElement.clientWidth > 500 ? 1.2 : 2; // ipad or phone
canvas.width = document.documentElement.clientWidth * scale;
canvas.height = document.documentElement.clientHeight * scale;
$div.width(canvas.width);
$div.height(canvas.height);
canvas.style.width = canvas.width + 'px';
canvas.style.height = canvas.height + 'px';
}());

// 截图
html2canvas($div[0], {useCORS: true, canvas: canvas}).then(function (canvas) {
console.log(canvas);
var b64;
try {
b64 = canvas.toDataURL("image/png");
} catch (err) {
console.log(err);
alert(err);
}

var img = new Image();
img.src = b64;
img.style.transform =  'scale(.5)';
img.style.transformOrigin = 'left top';
// target dom
$('#thePng').prepend(img);
});