Problem with drawing in a scrollpane using clipbounds

Question

Hello
Setup is a JFrame containing a splitpane with on the left just some config buttons and on the right a scrollpane.
Inside the scrollpane I want to draw big waveforms.
This is represented by a long array of 1's and 0's for the Y coordinates.
On drawing in paintComponents method I allways reconstruct a GeneralPath, based on the Clipbouns returned by the Graphics variable.
I have to reconstruct the GeneralPath polygon because my data can be millions long. And therefor I have to limit the points that are drawn.
When I allways reconstruct this GeneralPath, upon scrolling the polygon is shown scrappy, as in the line connecting the points will only be half shown or almost not.
Resizing the window or just minimize maximize and it is shown ok.
I do not have this problem when the GeneralPath I construct is only once made upfront and never remade in the paintComponent method.
But as I need the clipbounds to know which part to construct I need to rebuild this polygon over and over again.
I have tried to draw it with Line2D.Double but I get almost the same effect, parts of the lines not being drawn ok.
Below is the code of the Panel inside the ScrollPane responsible for this drawing.
public class Usr_Test_L extends JPanel
  final static Color bg   = Color.black;
  final static Color fg   = Color.white;
  final static Color wave = Color.red;
  final static Color txt  = Color.green;
  final static Color grid = Color.lightGray;
  private static final BasicStroke SOLID = new BasicStroke(1.0f);
  private static final BasicStroke BOLD  = new BasicStroke(3.0f);
  private double maxX = 0.0;
  private double maxY = 0.0;
  private double gridWidth;
  private double gridHeight;
  private int panelWidth;
  private int panelHeight;
  boolean firstTime = true;
  private double[] xPoints = null;
  private double[] yPoints = null;
  GeneralPath polygon;
  boolean polyOk = false;
  private double waveHeight = 0;
  private boolean gridOn = true;
  private Usr_Test_R scrollPane = null;
  Usr_Test_L(Usr_Test_R scrollPane)
    super(true);
    this.scrollPane = scrollPane;
    buildGui();
  void buildGui()
    setBackground(bg);
    setForeground(fg);
    //Let the user scroll by dragging to outside the window.
    setAutoscrolls(true); //enable synthetic drag events
//    addMouseMotionListener(this); //handle mouse drags
    repaint();
  private void generateWave(int cycles, double height)
    waveHeight = height;
    xPoints = new double[cycles];
    yPoints = new double[cycles];
    Random r = new Random();
    for ( int i = 0; i < cycles; i++ )
      xPoints[i] = i;
      yPoints[i] = (int)( Math.pow(-1.0,i) ) * ( r.nextInt(2) );
//      if ( ( i % 10 ) == 0 ) yPoints[i] = height;
//      else                   yPoints[i] = height + gridHeight;
//      System.err.println("P"+i+"("+xPoints[i]+","+yPoints[i]+")");
  private void generateSineWave(int cycles, double height)
    waveHeight = height;
    xPoints = new double[cycles];
    yPoints = new double[cycles];
    double j, xval, yval;
    j = 0;
    int i = 0;
    while (i < cycles)
      xval = j;
      yval = Math.sin(j) * 10.0 + 20;
      xPoints[i] = xval;
      yPoints[i] = yval;
      i++;
      j += 0.01;
  public Dimension getPreferredSize()  // <== used because of revalidate, parent scrollpane asks preferredsize ..., so we overruled the method
    return new Dimension(panelWidth, panelHeight);
  public void drawLine(int cycles, int height)
    generateWave(cycles,(int)(gridHeight*height));
    maxX = gridWidth  * cycles;
    maxY = gridHeight * cycles;
    while ( maxX > panelWidth  ) panelWidth  *= 2;
    while ( maxY > panelHeight ) panelHeight *= 2;
//    polyOk = buildPolygon();
    revalidate();
    repaint();
  public void drawSine(int cycles, int height)
    generateSineWave(cycles,(int)(gridHeight*height));
    maxX = gridWidth  * cycles;
    maxY = gridHeight * cycles;
    while ( maxX > panelWidth  ) panelWidth  *= 2;
    while ( maxY > panelHeight ) panelHeight *= 2;
//    polyOk = buildPolygon();
    revalidate();
    repaint();
  public void zoomIn()
    gridWidth  *= 2;
    gridHeight *= 2;
    repaint();
  public void zoomOut()
    gridWidth  /= 2;
    gridHeight /= 2;
    repaint();
  private void initPanel()
    Dimension d = getSize();
    panelWidth = d.width;
    panelHeight = d.height;
    maxX = panelWidth;
    maxY = panelHeight;
    gridWidth  = panelWidth / 8;
    gridHeight = panelHeight / 8;
    scrollPane.getHorBar().setUnitIncrement((int)(gridWidth*3));           // times 2 otherwise no clipbounds are allways too small
    scrollPane.getHorBar().setBlockIncrement((int)(gridWidth*3));
    scrollPane.getVerBar().setUnitIncrement((int)(gridHeight*3));
    scrollPane.getVerBar().setBlockIncrement((int)(gridHeight*3));
  public void grid()
    if ( gridOn ) gridOn = false;
    else          gridOn = true;
  private void drawGrid(Graphics2D g2, Rectangle clip)
    if ( ! gridOn ) return;
    // draw grid
    g2.setPaint(grid);
    // new good grid
    double startX = clip.getX() + ( gridWidth - ( clip.getX() % gridWidth ) );
    double endX   = clip.getX() + clip.getWidth();
    double startY = clip.getY() + ( gridHeight - ( clip.getY() % gridHeight ) );
    double endY   = clip.getY() + clip.getHeight();
    double y = startY;
    while ( startX < endX )
      while ( y < endY )
        g2.draw(new Rectangle2D.Double( startX - 0.5, y - 0.5, 1.0, 1.0 ));
        y += gridHeight;
      y = startY;
      startX += gridWidth;
  private boolean buildPolygon(Rectangle clip)
    if ( xPoints != null && yPoints != null )
      polygon = new GeneralPath(GeneralPath.WIND_NON_ZERO,xPoints.length);
    else
      return false;
    double x = xPoints[0] * gridWidth;
    double y = waveHeight;
    polygon.moveTo((float)xPoints[0], (float)yPoints[0]);
    for ( int index = 0; index < xPoints.length; index++ )
      x = xPoints[index] * gridWidth;
      y = waveHeight + ( yPoints[index] * gridHeight );
      if ( x > ( clip.getX() + clip.getWidth() ) ) break;
      if ( y < clip.getY() || y > ( clip.getY() + clip.getHeight() ) ) continue;
      if ( x < clip.getX() || x > ( clip.getX() + clip.getWidth()  ) ) continue;
      polygon.lineTo((float)x, (float)y);
    return true;
  private boolean drawLines(Graphics2D g2, Rectangle clip)
    if ( xPoints == null || yPoints == null ) return false;
    double x1 = xPoints[0] * gridWidth;
    double y1 = waveHeight;
    double x2, y2;
    boolean cont = false;
    for ( int index = 0; index < xPoints.length; index++ )
      x2 = xPoints[index] * gridWidth;
      y2 = waveHeight + ( yPoints[index] * gridHeight );
      if ( x1 > ( clip.getX() + clip.getWidth() ) )
        break;
      if ( x2 > ( clip.getX() + clip.getWidth() ) )
        break;
      if ( y2 < clip.getY() || y2 > ( clip.getY() + clip.getHeight() ) )
        cont = true;
      if ( x2 < clip.getX() || x2 > ( clip.getX() + clip.getWidth()  ) )
        cont = true;
      if ( ! cont ) g2.draw(new Line2D.Double(x1,y1,x2,y2));
      x1 = x2;
      y1 = y2;
      cont = false;
    return true;
  public void paintComponent(Graphics g)
    super.paintComponent(g);
    Graphics2D g2 = (Graphics2D) g;
    if ( firstTime )
      initPanel();
      firstTime = false;
    // get clip bounds
    Rectangle clip = new Rectangle();
    clip = g2.getClipBounds(clip);
    // draw grid
    drawGrid(g2,clip);
    // draw waveform
    g2.setPaint(wave);
    g2.setStroke(SOLID);
    if ( buildPolygon(clip) ) g2.draw(polygon);
//    if ( polyOk ) g2.draw(polygon);
//    drawLines(g2,clip);
  }

Answer 1

I have added a standalone testcase for ease.
import java.awt.*;
import java.awt.geom.*;
import javax.swing.*;
import java.util.*;
public class Scrolling {
    public static void main(String[] args) {
        JFrame f = new JFrame();
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.getContentPane().add(new JScrollPane(new DrawPanel()));
        f.setSize(400,300);
        f.setLocationRelativeTo(null);
        f.setVisible(true);
class DrawPanel extends JPanel {
    private GeneralPath polygon = new GeneralPath(GeneralPath.WIND_NON_ZERO,1000);
    private int[] coords;
    public DrawPanel() {
        setBackground(Color.white);
        buildPolygon();
    private void buildPolygon() {
     coords = new int[2000];
        Random r = new Random();
        int height = 80;
        int gridHeight = 10;
        int gridWidth = 10;
     int j = 0;       
        for (int i=0, dir=-1; i<1000; i++, dir=-dir) {
            coords[j++] = i * gridWidth;
            coords[j++] = height + dir*gridHeight*r.nextInt(2);
    private void drawPolygonClipped(Rectangle clip)
polygon = new GeneralPath(GeneralPath.WIND_NON_ZERO,1000);
       polygon.moveTo(0,80);
       for ( int i = 0; i < 2000; i++ )
      if ( coords[i] > ( clip.getX() + clip.getWidth() ) ) break;
      if ( coords[i+1] < clip.getY() || coords[i+1] > ( clip.getY() + clip.getHeight() ) ) continue;
      if ( coords[i] < clip.getX() || coords[i] > ( clip.getX() + clip.getWidth()  ) ) continue;
            polygon.lineTo(coords[i++],coords);
private void drawPolygon(Rectangle clip)
polygon = new GeneralPath(GeneralPath.WIND_NON_ZERO,1000);
     polygon.moveTo(0,80);
     for ( int i = 0; i < 2000; i++ )
          polygon.lineTo(coords[i++],coords[i]);
protected void paintComponent(Graphics g) {
super.paintComponent(g);
Graphics2D g2 = (Graphics2D)g;
     Rectangle clip = new Rectangle();
     g.getClipBounds(clip);
     drawPolygonClipped(clip);
     //drawPolygon(clip);
g2.draw(polygon);
public Dimension getPreferredSize() {
return new Dimension(1500,500);