/**
 * CLASS: ImageScrollWorld
 *
 * DESCRIPTION: creates a limited center screen actor scrolling world
 */

import greenfoot.*;

 

public class ImageScrollWorld extends World
{
    public static final int WIDE = 800; // world width (viewport)
    public static final int HIGH = 600; // world height (viewport)
   
    Scroller scroller; // the object that performs the scrolling
    Actor scrollActor; // an actor to stay in view
   
    public ImageScrollWorld()
    {
        super(WIDE, HIGH, 1, false); // creates an unbounded world
        GreenfootImage bg = new GreenfootImage("bg.png"); // the limiting image
        int bgWide = bg.getWidth(); // scrolling image width
        int bgHigh = bg.getHeight(); // scrolling image height
        scroller = new Scroller(this, bg, bgWide, bgHigh); // the Scroller object
        scrollActor = new Player(); // creates the actor to maintain view on
        addObject(scrollActor, bgWide/2, bgHigh/2); // add actor to world (wherever)
        scroll(); // sets initial background image and scrolls if needed
    }
   
    public void act()
    {
        if (scrollActor != null) scroll();
    }
   
    // attempts scrolling when actor is not in center of visible world
    private void scroll()
    {
        // determine scrolling offsets and scroll
        int dsx = scrollActor.getX()-WIDE/2; // horizontal offset from center screen
        int dsy = scrollActor.getY()-HIGH/2; // vertical offset from center screen
        scroller.scroll(dsx, dsy);
    }
}

​

​

The following can be used to allow some actor movement before scrolling begins:

 

// attempts scrolling when actor is outside roaming limits
private void scroll()
{
    // roaming limits of actor
    int loX = 100;
    int hiX = WIDE-100;
    int loY = 50;
    int hiY = HIGH-50;
    // determine offsets and scroll
    int dsx = 0, dsy = 0;
    if (scrollActor.getX() < loX) dsx = scrollActor.getX()-loX;
    if (scrollActor.getX() > hiX) dsx = scrollActor.getX()-hiX;
    if (scrollActor.getY() < loY) dsy = scrollActor.getY()-loY;
    if (scrollActor.getY() > hiY) dsy = scrollActor.getY()-hiY;
    scroller.scroll(dsx, dsy);
}