根据屏幕分辩率devicePixelRatio判断canvas缩放

[bojue]

现在是通过mac/window操作系统设置canvas屏幕缩放比例

[bojue]

canvas的dpr兼容原理

var ctx = canvas.getContext('2d');          
var dpr = window.devicePixelRatio || window.webkitDevicePixelRatio || window.mozDevicePixelRatio || 1;
var w = canvas.width;
var h = canvas.height;
canvas.width = Math.round(w * dpr);
canvas.height = Math.round(h * dpr);
canvas.style.width = w + 'px';
canvas.style.height = h + 'px';
ctx.scale(dpr, dpr);