Learning Java - Chapter 11 : Java : Supplements

When the image moves, only a fraction of the total display in the red rectangle area below, is changed. No point in redrawing the whole image.

Clipping is the technique to do limit the redrawing to just a chosen part of the image.

The graphics context can be told by the clipRect() method to only accept changes in the clipped area.

The only hard part is calculating the red outlined area above, regardless of which way the mouse moves. Study the clip() method below to see how this is done.

Clipping_Applet7.java
Resources: Apollo16Lander.jpg

import java.awt.*;
import java.awt.event.*;

// Extend the Update_Applet7 class

public class Clipping_Applet7 extends Update_Applet7
{
int newX, newY;
int imageWd, imageHt;

public void mouseDragged( MouseEvent e )
{
    newX=e.getX();
    newY=e.getY();
    repaint();
}

// Here we do the math to get the clip region
void clipToAffectedArea( Graphics g,
      int oldX, int oldY, int nextX, int nextY,
      int width, int height) {
    int x = Math.min( oldX, nextX );
    int y = Math.min( oldy, nextY );
    int w = ( Math.max( oldY, nextX )
                + width ) - x;
    int h = ( Math.max( oldY, nextY )
                + height ) - y;
    g.clipRect( x, y, w, h );
}

  // Override update. Keep track here of
  // position of the image.
public void update( Graphics g )
{
    int lastX = currentX, lastY = currentY;
    currentX = newX;
    currentY = newY;
    clipToAffectedArea( g,
         lastX, lastY,currentX,
         currentY, imageWd, imageHt );
    paint( g );
}
}