Rounding errors when plotting rectangles (simple code snippet included)

Question

hi,
I'm having a problem with rounding errors, and am unsure what the best thing to do is..
If you run the snippet of code below, you'll see some rectangles with black lines between them. The range of values covered in the model behind the plot is contiguous, and its only after scaling the trouble begins..
does anyone have suggestions for what to do here? I need this resize behaviour.. I'm unable to change the interval class :)
thanks,
asjf
import java.awt.Color;
import java.awt.Graphics;
import java.util.ArrayList;
import java.util.List;
import java.util.Random;
import javax.swing.JFrame;
import javax.swing.JPanel;
@SuppressWarnings("serial")
public class RoundingError extends JPanel {
     List<Interval> intervals;
     private static final int WIDTH= 25000000;
     private static final int INC= 1000000;
     public RoundingError() {
          setBackground(Color.BLACK);
          intervals= new ArrayList<Interval>();
          for(int i=0; i<WIDTH; i+=INC) {
               intervals.add(new Interval(i, i+INC));
     @Override
     protected void paintComponent(Graphics g) {
          super.paintComponent(g);
          double scale= getWidth()/(double)WIDTH;
          for(Interval i : intervals) {
               Random r= new Random(System.identityHashCode(i));
               g.setColor(new Color(r.nextInt(256), r.nextInt(256), r.nextInt(256)));
               int sx= (int) (i.start * scale);
               int sy= getHeight()>>2;
               int sw= (int) ((i.end-i.start) * scale);
               int sh= getHeight()>>1;
               g.fillRect(sx,sy,sw,sh);
     public static void main(String[] args) {
          JFrame frame= new JFrame();
          frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
          frame.getContentPane().add(new RoundingError());
          frame.setSize(800,600);
          frame.setVisible(true);
class Interval {
     final int start, end;
      * @param start inclusive bound
      * @param end exclusive bound
     Interval(int start, int end) {
          this.start= start;
          this.end= end;
}

Answer 1

import java.awt.Color;
import java.awt.Graphics;
import java.util.ArrayList;
import java.util.List;
import java.util.Random;
import javax.swing.JFrame;
import javax.swing.JPanel;
@SuppressWarnings("serial")
public class RoundingError extends JPanel {
    List<Interval> intervals;
    private static final int WIDTH= 25000000;
    private static final int INC= 1000000;
    boolean firstTime = true;
    public RoundingError() {
        setBackground(Color.BLACK);
        intervals= new ArrayList<Interval>();
        for(int i=0; i<WIDTH; i+=INC) {
            intervals.add(new Interval(i, i+INC));
    @Override
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        double scale= getWidth()/(double)WIDTH;
        int count = 0;
        int errorX = 0;
        int totalW = 0;
        int lastX = 0;
        int[] errors = new int[intervals.size()];
        for(Interval i : intervals) {
            Random r= new Random(System.identityHashCode(i));
            g.setColor(new Color(r.nextInt(256),
                                 r.nextInt(256), r.nextInt(256)));
            int sx= (int) (i.start * scale);
            int sy= getHeight()>>2;
            int sw= (int) ((i.end-i.start) * scale);
            int sh= getHeight()>>1;
            if(firstTime) {
                totalW += sw;
                if(count > 0) {
                    int error = sx - lastX;
                    if(error != 0) {
                        errors[count] = error;
                    errorX += error;
                lastX = sx + sw;
                System.out.printf("count = %2d  sx = %3d  sy = %3d  " +
                                  "sw = %3d  dh = %3d  totalW = %d%n",
                                   count++, sx, sy, sw, sh, totalW);
            g.fillRect(sx,sy,sw,sh);
        if(firstTime) {
            System.out.printf("errorX = %d%n" +
                              "lastX - totalW = %d%n",
                               errorX, lastX - totalW);
            for(int i = 0; i < errors.length; i++) {
                if(errors[i] != 0) {
                    System.out.printf("errors[%2d] = %d%n", i, errors);
firstTime = false;
public static void main(String[] args) {
JFrame frame= new JFrame();
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.getContentPane().add(new RoundingError());
frame.setSize(800,600);
frame.setVisible(true);
class Interval {
final int start, end;
* @param start inclusive bound
* @param end exclusive bound
Interval(int start, int end) {
this.start= start;
this.end= end;
This seems to work okay.
import java.awt.Color;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.util.ArrayList;
import java.util.List;
import java.util.Random;
import javax.swing.JFrame;
import javax.swing.JPanel;
@SuppressWarnings("serial")
public class RoundingErrorRx extends JPanel {
    List<Interval> intervals;
    private static final int WIDTH= 25000000;
    private static final int INC= 1000000;
    boolean firstTime = true;
    public RoundingErrorRx() {
        setBackground(Color.BLACK);
        intervals= new ArrayList<Interval>();
        for(int i=0; i<WIDTH; i+=INC) {
            intervals.add(new Interval(i, i+INC));
    @Override
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        Graphics2D g2 = (Graphics2D)g;
        double scale= getWidth()/(double)WIDTH;
        int count = 0;
        double errorX = 0;
        double totalW = 0;
        double lastX = 0;
        double[] errors = new double[intervals.size()];
        for(Interval i : intervals) {
            Random r= new Random(System.identityHashCode(i));
            g.setColor(new Color(r.nextInt(256),
                                 r.nextInt(256), r.nextInt(256)));
            double sx= i.start * scale;
            double sy= getHeight()>>2;
            double sw= (i.end-i.start) * scale;
            double sh= getHeight()>>1;
            if(firstTime) {
                totalW += sw;
                if(count > 0) {
                    double error = sx - lastX;
                    if(error != 0) {
                        errors[count] = error;
                    errorX += error;
                lastX = sx + sw;
                System.out.printf("count = %2d  sx = %5.1f  sy = %5.1f  " +
                                  "sw = %5.1f  dh = %5.1f  totalW = %5.1f%n",
                                   count++, sx, sy, sw, sh, totalW);
            g2.fill(new java.awt.geom.Rectangle2D.Double(sx,sy,sw,sh));
        if(firstTime) {
            System.out.printf("errorX = %5.1f  lastX - totalW = %5.1f%n",
                               errorX, lastX - totalW);
            for(int i = 0; i < errors.length; i++) {
                if(errors[i] != 0) {
                    System.out.printf("errors[%2d] = %5.1f%n", i, errors);
firstTime = false;
public static void main(String[] args) {
JFrame frame= new JFrame();
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.getContentPane().add(new RoundingErrorRx());
frame.setSize(800,600);
frame.setVisible(true);
class Interval {
final int start, end;
* @param start inclusive bound
* @param end exclusive bound
Interval(int start, int end) {
this.start= start;
this.end= end;