content: // ===============================================================================
  // Authors: AFRL/RQQD
  // Organization: Air Force Research Laboratory, Aerospace Systems Directorate, Power and Control Division
  // 
  // Copyright (c) 2017 Government of the United State of America, as represented by
  // the Secretary of the Air Force.  No copyright is claimed in the United States under
  // Title 17, U.S. Code.  All Other Rights Reserved.
  // ===============================================================================
  
  package org.flexdock.docking.drag.preview;
  
  import java.awt.BasicStroke;
  import java.awt.Color;
  import java.awt.Graphics2D;
  import java.awt.Point;
  import java.awt.Polygon;
  import java.awt.Stroke;
  import java.util.Map;
  
  import org.flexdock.docking.Dockable;
  import org.flexdock.docking.drag.effects.DefaultPreview;
  
  public class XORPreview extends DefaultPreview {
  
      public void drawPreview(Graphics2D g, Polygon p, Dockable dockable, Map dragInfo) {
          float[] pattern = { 1.0f, 1.0f };
          Stroke stroke = new BasicStroke(1, BasicStroke.CAP_BUTT, BasicStroke.JOIN_MITER, 1f, pattern, 0f);
          g.setStroke(stroke);
  
          g.setColor(Color.BLACK);
          g.setXORMode(Color.WHITE);
          drawPolygon(g, p, 3);
      }
  
      private void drawPolygon(Graphics2D g, Polygon p, int thickness) {
          Point center = getCenterOfGravity(p);
          for(int i=0; i<thickness; i++) {
              g.drawPolygon(p);
              gravitate(p, center, 1);
          }
      }
  
      private void gravitate(Polygon p, Point center, int step) {
          int len = p.npoints;
  
          for(int i=0; i<len; i++) {
              int deltaX = center.x > p.xpoints[i]? step: -step;
              int deltaY = center.y > p.ypoints[i]? step: -step;
              p.xpoints[i] += deltaX;
              p.ypoints[i] += deltaY;
          }
      }
  
      private Point getCenterOfGravity(Polygon p) {
          int x = 0;
          int y = 0;
          int len = p.npoints;
          for(int i=0; i<len; i++) {
              x += p.xpoints[i];
              y += p.ypoints[i];
          }
          return new Point(x/len, y/len);
      }
  }
  