source: josm/src/org/openstreetmap/josm/actions/mapmode/DebugAction.java@ 8

package org.openstreetmap.josm.actions.mapmode;

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Graphics;
import java.awt.Point;
import java.awt.Rectangle;
import java.awt.event.KeyEvent;
import java.awt.event.MouseEvent;

import javax.swing.JLabel;

import org.openstreetmap.josm.data.osm.LineSegment;
import org.openstreetmap.josm.data.osm.Track;
import org.openstreetmap.josm.gui.MapFrame;

/**
 * To debug zone information
 * 
 * @author imi
 */
public class DebugAction extends MapMode {

        private JLabel label = new JLabel();

        public DebugAction(MapFrame mapFrame) {
                super("Debug Zones", "debug", "Debug only. Just ignore.", KeyEvent.VK_D, mapFrame);
        }
        
        @Override
        public void registerListener() {
                super.registerListener();
                mv.addMouseMotionListener(this);
                mv.addMouseListener(this);
                mapFrame.add(label, BorderLayout.SOUTH);
        }

        @Override
        public void unregisterListener() {
                super.unregisterListener();
                mv.removeMouseMotionListener(this);
                mv.removeMouseListener(this);
                mapFrame.remove(label);
        }

        @Override
        public void mouseClicked(MouseEvent e) {
                Graphics g = mapFrame.mapView.getGraphics();
                g.setColor(Color.WHITE);
                for (Track t :mapFrame.mapView.dataSet.tracks())
                        for (LineSegment ls : t.segments()) {
                                Point A = mapFrame.mapView.getScreenPoint(ls.getStart().coor);
                                Point B = mapFrame.mapView.getScreenPoint(ls.getEnd().coor);
                                Point C = e.getPoint();
                                Rectangle r = new Rectangle(A.x, A.y, B.x-A.x, B.y-A.y);
                                double dist = perpendicularDistSq(B.distanceSq(C), A.distanceSq(C), A.distanceSq(B));
                                g.drawString(""+dist, (int)r.getCenterX(), (int)r.getCenterY());
                        }
        }

        private double perpendicularDistSq(double a, double b, double c) {
                return a-(a-b+c)*(a-b+c)/4/c;
        }
}