code: function initPaperBackground () {
  
          var brush = new PatternBrush(),
              rect = null,
              radius = 10*100,
              bgRect = new Rect( 600 * ZOOM, 300 * ZOOM );
  
          brush.setWidth( 2*radius ).setHeight( 2*radius );
  
          for ( var i = 0, len = 2; i < len; i++ ) {
  
              rect = new Rect( radius, radius, i*radius, i*radius );
  
              rect.fill( new Color( "lightgray" ).set( 'a', 0.2 ) );
  
              brush.addShape( rect );
  
          }
  
          bgRect.fill( brush );
          paper.addResource( brush );
          paper.addShape( bgRect );
  
      }
docs: 初始化画布背景