# Box2D: Adjusting character position for side-scrolling

In my side-scroller I would like to give the illusion of the character walking forward while his sprite is actually being held in place. If he walks near the edge of the screen, I would like to adjust his position at each step so that he is remaining still while the background moves backwards at the same rate he would have been moving. I think you get what I mean.

I figured that a simple case of "position = position - velocity" would do the trick, but then I realized it is not quite that easy with box2d. Velocity is expressed not at distance per frame, but distance per second. So instead I am essentially using position = position - velocity/60 since my game refreshes (or attempts to refresh at) 60 frames per second. But I can still see some lilting in the character's movement, and if I monitor his position, it shows that it indeed changes, most likely due to variations in frame rate.

Is velocity/60 the best I can hope for, save for hard coding the stop positions?

I am using the newest release of Box2Dweb. Yes, the javascript one. Here's a sample of the code:

var posX = this.body.GetPosition().x;
var posY = this.body.GetPosition().y;
var velX = this.body.GetLinearVelocity().x;

if(posX > 21 || posX < 12){
screen.pos -= velX;
this.body.SetPosition(new b2Vec2(posX - (velX/60),posY));
}


I realize I can hardcode the positions like this:

    if(posX > 21) {
screen.pos -= velX;
this.body.SetPosition(new b2Vec2(21,posY));
} else if(pos < 12){
screen.pos -= velX;
this.body.SetPosition(new b2Vec2(12,posY));
}


But I'm not super excited about how decidedly undynamic that option is.