GAMES LESSON THIRTEEN: Moving Background
Imagine a game in which you look down upon a street map of a city. The map
is bigger than the screen area and so it is necessary to adjust the map to
allow for game movement which goes beyond the portion of the map which is
currently visible on the screen.
In the sample applet an image is displayed which is larger than the applet
area and the mouse can be used to change the portion of the image which is
visible. The applet area is made small on purpose so that the user must use
the mouse to explore the picture.
The portion of the code responsible for moving the picture is in the
mouseMoved method:
<![CDATA[
public void mouseMoved(MouseEvent m) {
newx = m.getX();
newy = m.getY();
if(newx>oldx) x+=3;
else if(newx<oldx) x-=3;
if(newy>oldy) y+=3;
else if(newy<oldy) y-=3;
if(x<-150) x=-150;
if(x>0) x=0;
if(y<-150) y=-150;
if(y>0) y=0;
oldx=newx;
oldy=newy;
repaint();
}
]]>
HUCK.java
<![CDATA[
import java.awt.*;
import java.applet.*;
import java.awt.event.*;
public class HUCK extends Applet {
Dimension d;
Image offI;
Image image;
int x, y, oldx, oldy, newx, newy;
public void init(){
d = getSize();
oldx=(int)d.getWidth()/2;
oldy=(int)d.getHeight()/2;
offI=createImage(d.width,d.height);
image = getImage(getDocumentBase(), "images/huckleberry.jpg");
this.addMouseMotionListener(new MouseMotionAdapter(){
public void mouseMoved(MouseEvent m) {
newx = m.getX();
newy = m.getY();
if(newx>oldx) x+=3;
else if(newx<oldx) x-=3;
if(newy>oldy) y+=3;
else if(newy<oldy) y-=3;
if(x<-150) x=-150;
if(x>0) x=0;
if(y<-150) y=-150;
if(y>0) y=0;
oldx=newx;
oldy=newy;
repaint();
}
});
}
public void update(Graphics g){
paint(g);
}
public void paint(Graphics g){
Graphics offG = offI.getGraphics();
offG.drawImage(image,x,y,this);
g.drawImage(offI,0,0,this);
}
}
]]>
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