html canvas压缩图片

<pre name="code" class="javascript">        var initSize = img.src.length;
var width = img.width;
var height = img.height;

//如果图片大于四百万像素，计算压缩比并将大小压至400万以下
var ratio;
if ((ratio = width * height / 4000000)>1) {
ratio = Math.sqrt(ratio);
width /= ratio;
height /= ratio;
}else {
ratio = 1;
}

canvas.width = width;
canvas.height = height;

//        铺底色
ctx.fillStyle = "#fff";
ctx.fillRect(0, 0, canvas.width, canvas.height);

//如果图片像素大于100万则使用瓦片绘制
var count;
if ((count = width * height / 1000000) > 1) {
count = ~~(Math.sqrt(count)+1); //计算要分成多少块瓦片

//            计算每块瓦片的宽和高
var nw = ~~(width / count);
var nh = ~~(height / count);

tCanvas.width = nw;
tCanvas.height = nh;

for (var i = 0; i < count; i++) {
for (var j = 0; j < count; j++) {
tctx.drawImage(img, i * nw * ratio, j * nh * ratio, nw * ratio, nh * ratio, 0, 0, nw, nh);

ctx.drawImage(tCanvas, i * nw, j * nh, nw, nh);
}
}
} else {
ctx.drawImage(img, 0, 0, width, height);
}

//进行最小压缩
var ndata = canvas.toDataURL('image/jpeg', 0.1);

console.log('压缩前：' + initSize);
console.log('压缩后：' + ndata.length);
console.log('压缩率：' + ~~(100 * (initSize - ndata.length) / initSize) + "%");

tCanvas.width = tCanvas.height = canvas.width = canvas.height = 0;

return ndata;
}