<a id="thumb" href="#">
<img src="http://img.alibaba.com/photo/291909368/Free-Shipping-Popular-ladies-shoes-Tote-shoes-paypal-accept-201001.jpg" alt="Thumbnail" />
<span id="glass"></span>
</a>

/**
* 獲取鏡片在放大目標(biāo)元素上的位置
* @param ev 觸發(fā)的事件
* @param thumb 縮略圖對象
* @param glass 鏡片對象
* @return x:鏡片在放大目標(biāo)元素上的橫向位置, y:鏡片在放大目標(biāo)元素上的縱向位置
*/
function getGlassOffset(ev, thumb, glass) {
var offset = {
left:0,
top:0
};

// 偏移量
var thumbOffset = getCumulativeOffset(thumb);
// 鼠標(biāo)在頁面上的位置
var mousePoint = getMousePoint(ev);
// 鏡片實際尺寸
var glassSize = getSize(glass);
// 簡縮圖實際尺寸
var thumbSize = getSize(thumb);

// 光標(biāo)橫向位置
var cursorX = mousePoint.x - thumbOffset.left;
// 鏡片橫向偏移量
offset.left = cursorX - glassSize.width / 2;
if(offset.left < 0) {
offset.left = 0;
} else if(offset.left > thumbSize.width - glassSize.width) {
offset.left = thumbSize.width - glassSize.width;
}

// 光標(biāo)縱向位置
var cursorY = mousePoint.y - thumbOffset.top;
// 鏡片縱向偏移量
offset.top = cursorY - glassSize.height / 2;
if(offset.top < 0) {
offset.top = 0;
} else if(offset.top > thumbSize.height - glassSize.height) {
offset.top = thumbSize.height - glassSize.height;
}

return offset;
}