元素全屏居中（不变形）

var resize = function(elem,w,h){
    var dw = w,
        dh = h,
        cw = $(window).width(),
        ch = $(window).height();

    var bw = cw > dw ? cw / dw : 1 / (dw / cw),
        bh = ch > dh ? ch / dh : 1 / (dh / ch);

    var w = Math.min(dw*bh,cw),
        h = Math.min(dh*bw,ch);

    elem.css({
        ‘width‘:w,
        ‘height‘:h,
        ‘top‘:ch*0.5 - h*0.5,
        ‘left‘:cw*0.5 - w*0.5,
        ‘position‘:‘absolute‘
    });
};


//调用
resize($(‘.main‘),1008,640);