corner detection

import java.awt.*;
import java.awt.event.*;
import java.util.*;
import javax.swing.*;



public class Main{
    public static final double displace=25,tolerance=.7;

    public static JJPanel panel;
    public static ArrayList<Point> list;
    public static double px,py,x,y;

    public static void main(String[] args) throws Exception{
        list=new ArrayList<Point>();
        JFrame frame=new JFrame("corner detection");
        frame.setContentPane(panel=new JJPanel());
        Mouse m=new Mouse();
        panel.addMouseListener(m);
        panel.addMouseMotionListener(m);
        frame.setVisible(true);
        frame.setSize(800,600);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    }

    public static double d(double x1,double y1,double x2,double y2){
        double dx=x2-x1,dy=y2-y1;
        return Math.sqrt(dx*dx+dy*dy);
    }

    public static void click(int xx,int yy){
        list=new ArrayList<Point>();
        list.add(new Point(x=px=xx,y=py=yy));
        panel.repaint();
    }

    public static void drag(int xx,int yy){
        double dist;
        while((dist=d(x,y,xx,yy))>=displace){
            double dx=px-xx,dy=py-yy;
            double dd=Math.sqrt(dx*dx+dy*dy);
            if(dd==0) break;
            dx/=dd;
            dy/=dd;
            dd=(x-xx)*dx+(y-yy)*dy;
            double ddx=dx*dd,ddy=dy*dd;
            double backup=Math.sqrt(displace*displace-dist*dist+ddx*ddx+ddy*ddy);
            list.add(new Point(x=xx+ddx-backup*dx,y=yy+ddy-backup*dy));
        }
        px=xx;
        py=yy;
        panel.repaint();
    }

    public static void paint(Graphics g){
        g.setColor(Color.black);
        g.fillRect(0,0,800,600);
        g.setColor(Color.gray);
        boolean[] corner=new boolean[list.size()];
        Point prev=null;
        double pdx=0,pdy=0;
        int i=0;
        for(Point p:list){
            if(prev!=null){
                g.drawLine((int)p.x,(int)p.y,(int)prev.x,(int)prev.y);
                double dx=p.x-prev.x,dy=p.y-prev.y;
                corner[i-1]=i>1 && d(dx,dy,pdx,pdy)>displace*tolerance;
                pdx=dx;
                pdy=dy;
            }
            i++;
            prev=p;
        }
        i=0;
        for(Point p:list){
            g.setColor(corner[i++]? Color.red:Color.white);
            int x=(int)p.x,y=(int)p.y;
            g.fillRect(x-2,y-2,5,5);
        }
    }
}



class Point{
    public double x,y;
    public Point(double x,double y){this.x=x;this.y=y;}
}



class JJPanel extends JPanel{
    public void paint(Graphics g){
        Main.paint(g);
    }
}



class Mouse implements MouseListener,MouseMotionListener{
    @Override
    public void mousePressed(MouseEvent arg0){Main.click(arg0.getX(),arg0.getY());}
    @Override
    public void mouseDragged(MouseEvent arg0){Main.drag(arg0.getX(),arg0.getY());}

    @Override
    public void mouseMoved(MouseEvent arg0){}
    @Override
    public void mouseClicked(MouseEvent arg0){}
    @Override
    public void mouseReleased(MouseEvent arg0){}
    @Override
    public void mouseEntered(MouseEvent arg0){}
    @Override
    public void mouseExited(MouseEvent arg0){}
}