Limiting a player's movement to the screen's dimensions

Question

I have a player, that I can control using WASD. And I don't want him to leave the screen.

So I've tried the following:

if (player1.getX()+5 < 974 && player1.getX()-5 > 0) {
    if(Keyboard.isKeyDown(Input.KEY_D)) {
        player1.addForce(5, "d");
    }
    if(Keyboard.isKeyDown(Input.KEY_A)) {
        player1.addForce(5, "a");
    }
    if(Keyboard.isKeyDown(Input.KEY_W)) {
        player1.addForce(5, "w");
    }
    if(Keyboard.isKeyDown(Input.KEY_S)) {
        player1.addForce(5, "s");
    }
} else if(player1.getX() >= 974) {
    player1.addForce(5, "a");
} else if(player1.getX() <= 0) {
    player1.addForce(5, "d");
}

It actually works, but the player gets stuck at the edge of screen, and can't move anymore. Any ideas why? Or rather, how do I make a proper collision detection?

NB: 1024 is the width of screen. And my player is 50 pixels wide, thus the 974. I am only working with X at the momemnt, to try getting it to work.

Answer 1

What that code is doing is limiting motion to only if you're inside the screen. What you should instead do is something like this:

//control code here

if(player.getx() > 1024){
    player.setx(1024);
}else if(player.getx() < 0){
    player.setx(0);
}

that way if the player exceeds the bounds of the screen the position will be set back to the bounds.

Answer 2

Try this:

 if(player1.getX()+5 < 1024 && player1.getX()-5 > 0) {

        //Player 1
        if(Keyboard.isKeyDown(Input.KEY_D)) {
            player1.addForce(5, "d");
        }
        if(Keyboard.isKeyDown(Input.KEY_A)) {
            player1.addForce(5, "a");
        }
        if(Keyboard.isKeyDown(Input.KEY_W)) {
            player1.addForce(5, "w");
        }
        if(Keyboard.isKeyDown(Input.KEY_S)) {
            player1.addForce(5, "s");
        }
    } else if(player1.xlocation>=1024) {
        player1.addForce(5,"a");
    } else if(player1.xlocation<=0) {
        player1.addForce(5,"d");
    }

Answer 3

Compute for the future position first before updating the position of your player:

if( RIGHT is pressed )
{
    var futurePositionX = currentPositionX + increment;

    if( futurePositionX < rightScreenBound )
    {
        currentPositionX = futurePostionX;
    }
    else
    {
        // if futureposition will exceed bound, set position to bound
        // (rightScreenBound is the rightmost possible position)
        currentPositionX = rightScreenBound;
    }
}

Answer 4

Flickering behavior is correct. It is like throwing ball at the wall and when it bounces away, you punch it back to the wall. It is because you are setting forces instead of positions.

First, you must decide, wheter you want to control the position, or force (speed = derivative of position). If you want to set forces, then:

• you can not simply add force when button is down. It may grow to infinity.

• when object reaches the border, set force to zero! don't add force in another direction!

Answer 5

The reason it's flickering is probably because you update the possition, then update the image then check for collision then update the possition again and then update the image again.

Instead try to put the collision detection before you update the image.