Midpoint Ellipse Algorithm in Java

Question

I am trying to write a program to allow a user to draw an ellipse by using clicks. The user left-clicks at first to select the radius, then right-clicks to select the horizontal radius, then right-clicks again to select the vertical radius. Nothing is drawn after clicking. I don't understand where the error is.

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

public class Ellipse extends JPanel implements MouseListener{

    Graphics P;

    public Ellipse()
    {
        addMouseListener(this);
    }

    static int Radius = 0;
    int CenterX, CenterY, RadiusX, RadiusY;

    public void paintComponent(Graphics g)
    {
        P=g;
        EllipseMidpoint(CenterX, CenterY, RadiusX, RadiusY);
    }

    public void EllipseMidpoint(int Cx, int Cy, int Rx, int Ry)
    {
         int Rx2 = Rx * Rx;
         int Ry2 = Ry * Ry;
         int twoRx2 = 2 * Rx2;
         int twoRy2 = 2 * Ry2;
         int x = 0;
         int y = Ry;
         int p;
         int px= 0;
         int py = twoRx2 * y;
         PlotEllipsePoint(Cx, Cy, x, y);

         //Region 1
         p = (int)(Ry2 - (Rx2 * Ry) + (0.25 + Rx2));
         while (px < py)
         {
             x = x + 1;
             px = twoRy2 + px;
             if (p < 0)
             {
                 p = Ry2 + px + p;
             }
             else 
             {
                 y = y - 1;
                 py = twoRx2 - py;
                 p = Ry2 + px - py + p;
             }
             PlotEllipsePoint(Cx, Cy, x, y);   
         }

         //Region2 
         p = (int)(Ry2 * (x + 0.5) * (x + 0.5) + Rx2 * (y - 1) * (y - 1) - Rx2 * Ry2);
         while (y > 0)
         {
             y = y - 1;
             py = twoRx2 - py;
             if (p > 0)
             {
                 p = Rx2 - py + p;
             }
             else
             {
                 x = x + 1;
                 px = twoRy2 + px;
                 p = Rx2 + px - py + p;
             }
             PlotEllipsePoint(Cx, Cy, x, y);    
         }         
    }   

    public void PlotEllipsePoint(int CX, int CY, int X, int Y)
    {
     drawPixel(CX + X, CY + Y);
     drawPixel(CX - X, CY + Y);
     drawPixel(CX + X, CY - Y);
     drawPixel(CX - X, CY - Y);
    }

    public void drawPixel(int x, int y)
    {
        P.fillOval(x, y, 5, 5);
     } 

    public void mousePressed(MouseEvent e) 
    {
        if (e.getButton() == MouseEvent.BUTTON1)
        {
            CenterX = e.getX();
            CenterY = e.getY();
        }
        else if (e.getButton() == MouseEvent.BUTTON3)
        {
            Radius = Radius + 1;
            if (Radius == 1)
            {
                RadiusX = (int) Math.pow((Math.pow((e.getX() - CenterX), 2) + Math.pow((e.getY() - CenterY), 2)), 0.5);
            }
            else if (Radius == 2)
            {
                RadiusY = (int) Math.pow((Math.pow((e.getX() - CenterX), 2) + Math.pow((e.getY() - CenterY), 2)), 0.5);
            }
            PlotEllipsePoint(CenterX, CenterY, RadiusX, RadiusY);
        }
    }

     public static void main(String[] args) 
        {
            JFrame JF = new JFrame("Ellipse");
            JF.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
            JF.setSize(500,500);
            Ellipse E = new Ellipse();
            JF.getContentPane().add(E);
            JF.setVisible(true);
        }

     public void mouseClicked(MouseEvent e) {}
     public void mouseReleased(MouseEvent e) {}
     public void mouseEntered(MouseEvent e) {}
     public void mouseExited(MouseEvent e) {}
}

have you considered trying to debug it? – Mitch Wheat Oct 21 '14 at 23:31 — Mitch Wheat, Oct 21 '14 at 23:31
step debuggers were created for a reason ... – Oct 21 '14 at 23:33 — , Oct 21 '14 at 23:33

score 0 · Answer 1 · answered Oct 21 '14 at 23:42

Three things immediately jump out...

You should be calling super.paintComponent before doing any custom painting (and there is no need for paintComponent to be public), see Performing Custom Painting and Painting in AWT and Swing for more details
NEVER, EVER maintain a reference to the graphics context, you want something painted, you make a request to the repaint manager and you wait until one of your paint methods is called. Painting is controlled by the repaint manager an repaints may occur at any time, most of the time without yout knowledge or interaction.
You never call repaint to ask the repaint manager to repaint your component...At the end of your mousePressed method, call repaint()...

Well, your x and y radius values are been (as near as I can tell) been updated correctly, the the only thing I can think off is there's something wrong with the way you are rendering it...Try feeding it you own known values and run a debugger over it... — MadProgrammer, Oct 22 '14 at 00:21

Midpoint Ellipse Algorithm in Java

1 Answers1