css样式里面   在 i:affter 里面加上 opacity: 0; 然后再 i_current:affter 里面加上  opacity: 1; 应该就可以了

// 5. 排序海报

function rsort(n){

_photo = g('.photo');

var photos = [];

for(s=0;s<_photo.length;s++){

_photo[s].className = _photo[s].className.replace(/\s*photo_center\s*/,' ');

//要加上

_photo[s].className = _photo[s].className.replace(/\s*photo_front\s*/,' ');

_photo[s].className = _photo[s].className.replace(/\s*photo_back\s*/,' ');

_photo[s].className += ' photo_front ';

_photo[s].style.left = '';

_photo[s].style.top = '';

_photo[s].style['-webkit-transform'] = 'rotate(360deg)';

//要加上

photos.push(_photo[s]);

}

var photo_center = g('#photo_' +　n);

photo_center.className += ' photo_center ';

photo_center = photos.splice(n, 1)[0];

// 把海报分为左、 右区域两个部分

var photos_left = photos.splice(0, Math.ceil(photos.length/2));

var photos_right = photos;

var ranges = range();

for(s in photos_left){

var photo = photos_left[s];

photo.style.left = random(ranges.left.x) + 'px';

photo.style.top = random(ranges.left.y) + 'px';

photo.style['-webkit-transform'] = 'rotate('+random([-150,150])+'deg)';

}

for(s in photos_right){

var photo = photos_right[s];

photo.style.left = random(ranges.right.x) + 'px';

photo.style.top = random(ranges.right.y) + 'px';

photo.style['-webkit-transform'] = 'rotate('+random([-150,150])+'deg)';

}

var navs = g('.i');

for(var s=0;s<navs.length;s++){

navs[s].className = navs[s].className.replace(/\s*i_current\s*/, ' ');

navs[s].className = navs[s].className.replace(/\s*i_back\s*/, ' ');

}

g( '#nav_'+n ).className += ' i_current '; 

console.log(photos_left.length, photos_right.length);

}