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Java scrolling background

Posted on 2011-02-24
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Last Modified: 2012-05-11
I am creating Road Rash in a java midlet. I have come to the part where I have to create the track that scrolls towards the player. Does anyone have any idea about how to go about doing this? Could I just have the trees move towards the player to give the effect of the track and if so how would I put bends in the track.
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Question by:tango2009
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Expert Comment

by:dpearson
ID: 34975984
Yes you're thinking along the right lines.  You're going to want to draw the trees closer and closer to the player (and getting larger) to give the sense of motion.

In order to draw the road you want to think in terms of scan lines.  Say the horizon is half way up the screen.  To draw the road you need to keep track of where the left and right side of the road is for each line from the midpoint of the screen down.  With each frame repaint of the game, you'd draw a line in the color of the road (or copied from a bitmap if you want more beauty at the cost of less speed) from left-to-right for that line of the screen.

On a straight road your left and right values would all be the same.  For a bend, the values would vary.

For example, say the lower half of the screen is 200 pixels and the screen is 400 pixels wide.  You'd have:

 
int[] left = new int[200] ;
int[] right = new int[200] ;

For a straight road:
for (int i =  0; i < 200 ; i++) { left[i] = 100 ; right[i] = 300 ; }

For a road with a bend:
for (int i =  0 ; i < 200 ; i++) { left[i] = i/2 ; right[i] = 200+i/2 ; }

Then to create the next frame you'd shift the road down one (or more) lines:
void nextFrame(int moveLines) {
    for (int i = 0 ; i < 200-moveLines ; i++) { left[i] = left[i+moveLines] ; right[i] = right[i+moveLines] ; }
    // Now fill in new values for left[200-movesLines] to left[200], these define the new road appearing
   // over the horizon.
}

void drawFrame() {
   for (int i = 0 ; i < 200 ; i++) {
       // Draw the road
       int y = 400-i ;
       drawLine(left[i],y, right[i],y) ;
   }
}

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Hope that makes sense.

Doug
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Author Comment

by:tango2009
ID: 34982984
What should drawLine be defined as? and should I just call the drawFrame method in the part of the code that draws the objects?
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Accepted Solution

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dpearson earned 2000 total points
ID: 34985813
That was just pseudo code.

If you want details on how to use the actual drawLine method in Java there's an example here: http://www.java2s.com/Code/Java/2D-Graphics-GUI/Drawline.htm

And yes you should call the drawFrame() method in the part of the code that draws the objects.  You should draw the background first and then draw the trees etc. afterwards, so they appear to be in front of the background.

Doug
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