在svg文间画图过程中放大缩小图片后，坐标偏移问题

  //鼠标坐标：在SVG经过缩放、偏移、ViewBox转换后，鼠标坐标值
      var mouseCoord = {
          x : 0.,
          y : 0.
          };
      //用户坐标：相对于原始SVG,坐标位置   
      var userCoord = {
          x : 0.,   
          y : 0.
          };
      var cRadius     = 4.; //半径
   
    var svgDocument = null;
      var svgRoot     = null;
  
      //获取鼠标事件相关的SVG Document
      function getSVGDocument(evt)
      {
        var target = evt.target;
        var parent = target.parentNode;
        svgDocument = parent.getOwnerDocument();
        svgRoot     = svgDocument.documentElement;
      }
  
      //计算坐标位置:包括用户坐标、鼠标坐标
      function getCoords(evt)
      {
        var x_trans = 0.; //X偏移
        var y_trans = 0.; //Y偏移
        var x_scale = 1.; //ViewBox导致的X缩放比例
        var y_scale = 1.; //ViewBox导致的Y缩放比例
        var scale   = 1.; //缩放比例
        var trans = null;
        var viewbox = null;
       
        getSVGDocument(evt);
      
        scale    = svgRoot.currentScale; //获取当前缩放比例
        trans    = svgRoot.currentTranslate; //获取当前偏移
        //获取SVG的当前viewBox
        viewbox  = svgRoot.getAttributeNS(null, "viewBox"); //获取ViewBox属性
       
        //获取用户坐标：原始SVG的坐标位置
        userCoord.x = evt.getClientX();
        userCoord.y = evt.getClientY();
       
        //计算偏移、缩放等
        x_trans = ( 0.0 - trans.x ) / scale;
        y_trans = ( 0.0 - trans.y ) / scale;
        // Now that we have moved the rectangle's corner to the
        // upper-left position, let's scale the rectangle to fit
        // the current view.  X and Y scales are maintained seperately
        // to handle possible anamorphic scaling from the viewBox
        zoom = scale;
        x_scale = 1.0 / scale;
        y_scale = 1.0 / scale;
        if ( viewbox ) {
            // We have a viewBox so, update our translation and scale
            // to take the viewBox into account
            // Break the viewBox parameters into an array to make life easier
            var params  = viewbox.split(//s+/);
            // Determine the scaling from the viewBox
            // Note that these calculations assume that the outermost
            // SVG element has height and width attributes set to 100%.
            var h_scale = 1.0 / parseFloat(params[2]) * window.innerWidth;
            var v_scale = 1.0 / parseFloat(params[3]) * window.innerHeight;
            // Update our previously calculated transform
            x_trans = x_trans / h_scale + parseFloat(params[0]);
            y_trans = y_trans / v_scale + parseFloat(params[1]);
            x_scale = x_scale / h_scale;
            y_scale = y_scale / v_scale;
        }
       //根据用户坐标、偏移量、缩放等计算鼠标坐标
       mouseCoord.x = userCoord.x * x_scale + x_trans;
       mouseCoord.y = userCoord.y * y_scale + y_trans;
      }