function prev_image(id)
{
   var offset = images[id][0] - 1;
   if(typeof(images[id][offset]) != "undefined" && offset > 0)
   {
      images[id][0] = offset;
      change_image(id, offset);
   }
}

function next_image(id)
{
   var offset = images[id][0] + 1;
   if(typeof(images[id][offset]) != "undefined")
   {
      images[id][0] = offset;
      change_image(id, offset);
   }
}

function change_image(id, image_id)
{
   document['product_img[' + id + ']'].src =    images[id][image_id].src;
   document['product_img[' + id + ']'].width =  images[id][image_id].width;
   document['product_img[' + id + ']'].height = images[id][image_id].height;

   var link = document.getElementsByName('product_link[' + id + ']');
   if(link.length == 1)
   {
      link[0].href = images[id][image_id].src.replace(/thumb\//, 'large/');
   }
   var magnify = document.getElementsByName('magnify[' + id + ']');
   if(magnify.length == 1)
   {
      magnify[0].href = images[id][image_id].src.replace(/thumb\//, 'large/');
   }
   change_magnify(id);
}

function change_magnify(id)
{
   var magnify;
   magnify = document.getElementsByName('prev_image[' + id + ']');
   if(images[id][0] == 1)
   {
      // hide left
      magnify[0].style.visibility="hidden";
   }
   else
   {
      magnify[0].style.visibility="visible";
   }

   magnify = document.getElementsByName('next_image[' + id + ']');
   if(images[id][0] == images[id].length - 1)
   {
      // hide right
      magnify[0].style.visibility="hidden";
   }
   else
   {
      magnify[0].style.visibility="visible";
   }
}