function drawImage(sx,sy,imgW,imgH)
    {
        canvas = document.querySelector("#big_img .swiper-slide-active canvas");
        canvas.width= bigSwiper.width;
        canvas.height = bigSwiper.width;
        var context =canvas.getContext('2d');
        img.src = canvas.dataset.src;
        img.onload = function(){
            context.clearRect(0, 0, canvas.width, canvas.height);
            context.drawImage(img,sx,sy,img.naturalWidth,img.naturalHeight,0,0,imgW,imgH);
        }
    }
$(function(){
drawImage(0,0,100,100)//当sx和sy为0时画布能正常显示图片
drawImage(10,10,100,100)//当sx和sy为10时画布就不能显示图片了，这是为什么？PS:#big_img .swiper-slide-active canvas的这个画布是jQuery动态创建的，并且能成功querySelector到该画布
})