第一種 imageWidth = 600*scale;
var imageHeight = 450*scale;
var sx = imageWidth/2-canvas.width/2
var sy = imageHeight/2-canvas.height/2
var sourceW = 600*3;//原圖像大小
var sourceH = 450*3;//原圖像大小
context.drawImage(image,sx,sy,sourceW-2*sx,sourceH-2*sy,0,0,canvas.width,canvas.height)
