Js 等比例縮放圖片

var flag=false;
function DrawImage(ImgD,iwidth,iheight){
  var image=new Image();
  image.src=ImgD.src;
  if(image.width>0 && image.height>0){
  flag=true;
  if(image.width/image.height>= iwidth/iheight){
  if(image.width>iwidth){  
  ImgD.width=iwidth;
  ImgD.height=(image.height*iwidth)/image.width;
  }else{
  ImgD.width=image.width;  
  ImgD.height=image.height;
  }
  ImgD.alt=image.width+"×"+image.height;
  }
  else{
  if(image.height>iheight){  
  ImgD.height=iheight;
  ImgD.width=(image.width*iheight)/image.height;  
  }else{
  ImgD.width=image.width;  
  ImgD.height=image.height;
  }
  ImgD.alt=image.width+"×"+image.height;
  }

  }
}