请问这句话的意思-tdiv.style.pixelTop>=tdiv.offsetHeight*1

请教这句话的意思--tdiv.style.pixelTop>=tdiv.offsetHeight*-1

是一个图片滚动的代码。其中的这一句怎么也不明白，尤其是一个*-1,给指导一下啊，。各位大佬！

JScript code

function move1(whichdiv){tdiv=eval(whichdiv)if (tdiv.style.pixelTop>0&&tdiv.style.pixelTop<=car_num)  //如果顶部的距离大于零并且小于每次移动的距离的话，就直接使顶部距离为0. {  tdiv.style.pixelTop=0  setTimeout("move1(tdiv)",pausebetweenimages)  setTimeout("move2(second2)",pausebetweenimages)  return  }if (tdiv.style.pixelTop>=tdiv.offsetHeight*-1) //不明白这句话的意思。 {  tdiv.style.pixelTop-=car_num  setTimeout("move1(tdiv)",50) }else {  tdiv.style.pixelTop=scrollheight  tdiv.innerHTML=slideimages[0]  if (i==slideimages.length-1)  i=0  else i++ }}function move2(whichdiv){  tdiv2=eval(whichdiv)  if (tdiv2.style.pixelTop>0&&tdiv2.style.pixelTop<=car_num)   {    tdiv2.style.pixelTop=0    setTimeout("move2(tdiv2)",pausebetweenimages)    setTimeout("move1(first2)",pausebetweenimages)    return   } if (tdiv2.style.pixelTop>=tdiv2.offsetHeight*-1)  {   tdiv2.style.pixelTop-=car_num   setTimeout("move2(second2)",50)  } else  {   tdiv2.style.pixelTop=scrollheight   tdiv2.innerHTML=slideimages[1]    if (i==slideimages.length-1)    i=0    else     i++  }}function conmouse(){car_num=0;} //鼠标经过时图片停止function cmouseout(){car_num=5;}  //鼠标离开时继续按原来的速度上移。function startscroll(){    move1(first2)  second2.style.top=scrollheight}</SCRIPT>