libGDX apply linear velocity on a body

Newbie & Debugging Questions
#1

Hey guys, i’ve started playing a little bit around with libGDX and box2d. It’s pretty straight forward and very nice but somehow I cant manage to move a body with WASD. I googled a lot but could not finde a solution for my problem.

This is my method for handlingInput:

float velX = 0, velY = 0;
        if(Gdx.input.isKeyPressed(Input.Keys.W)) {
            velY = 200.0f ;
        } else if(Gdx.input.isKeyPressed(Input.Keys.D)) {
            velX = 200.0f;
        } else if(Gdx.input.isKeyPressed(Input.Keys.S)) {
            velY = -200.0f;
        } else if(Gdx.input.isKeyPressed(Input.Keys.A)) {
            velX = -200.0f;
        }
        else {
            velX = 0;
            velY = 0;
        }

        mPlayer.setLinearVelocity(velX, velY);

The position gets updated correctly if I check it with a sysout. My problem is that the box2ddebugrenderer which I use does not update the body accordingly. This might also be a camera problem I assume.

These are my draw and update methods which I call inside of render.

    /**
     * Used only for graphic to draw stuff
     */
    private void draw() {
        mBatch.setProjectionMatrix(mCamera.combined);
        mB2dr.render(mWorld, mCamera.combined);
    }

    /**
     * Main update method used for logic
     * @param delta delta time received from {@link PlayScreen#render(float)} method
     */
    private void update(final float delta) {
        mCamera.position.set(mPlayer.getPosition(), 0);
        mCamera.update();
        mWorld.step(delta, 6, 2);
    }

And last but not least the constructor for my screen:

public PlayScreen() {
        mBatch = new SpriteBatch();
        mWorld = new World(new Vector2(0,0), true);
        mB2dr = new Box2DDebugRenderer();
        mCamera = new OrthographicCamera(Gdx.graphics.getWidth(), Gdx.graphics.getHeight());
        mCamera.zoom = 10f;
        mViewport = new FitViewport(Gdx.graphics.getWidth() / 50, Gdx.graphics.getHeight() / 50, mCamera);
        mPlayer = ShapeFactory.createRectangle(new Vector2(0,0), new Vector2(32,32), BodyType.DynamicBody, mWorld, 0.0f);
    }

As I said the position gets updated correctly on pressing a button. But the box2renderer does not display the new position of the square. I couldnt find anything on the web regarding this.
If I press “W” for example the rectangle moves up (teleporting - only once) and when I release the button the rectangle gets teleported back to its starting position. The position is increased correctly though. Similiar behavior for the other buttons. I assume that somehow my camera setup is flawed ???

Thanks in advance!

PS. I also call

Gdx.gl.glClearColor(0, 0, 0, 1);
Gdx.gl.glClear(GL20.GL_COLOR_BUFFER_BIT);

inside of my render method.

#2

Hi, ral0r2, I assume you using your handlingInput in update method, so shouldn’t velX and velY to be variables of class where this method is.

I think isKeyPressed is not for the discrete input so, you don’t need this


else {
            velX = 0;
            velY = 0;
        }

MB you better do it like this



if(Gdx.input.isKeyPressed(Input.Keys.A)
  mPlayer.setLinearVelocity(-200, 0);
if(Gdx.input.isKeyPressed(Input.Keys.D)
  mPlayer.setLinearVelocity(200, 0);
if(Gdx.input.isKeyPressed(Input.Keys.W)
  mPlayer.setLinearVelocity(0, 200);
if(Gdx.input.isKeyPressed(Input.Keys.S)
  mPlayer.setLinearVelocity(0, -200);

I have to say i’m not familiar with Box2D ;D :stuck_out_tongue: but it looks like you better try it, am I right?

#3

Thanks! I agree with you :slight_smile: However this does not solve my problem and is rather a clean code advice :point:

#4

oh, I thought its an input problem :expressionless:

But can I ask one thing



private void update(final float delta) {
        mCamera.position.set(mPlayer.getPosition(), 0);
        mCamera.update();
        mWorld.step(delta, 6, 2);
    }

Why delta here is final?

#5

The camera is following your player, so of course the player will always remain at the center of the screen. If you add another shape to your game that remains still, you will see that your player moves relative to that shape.

#6

I’m dumb lol. I assumed that it would be visible somehow tho that the shape moves. That fixed it thanks!

@SugarBlood

I used a tutorial but I think it’s final because the parameter value of delta is not supposed to be altered inside of the method.

#7

ahah, sometimes things like that happens.

I see, also first time seeing an argument finalized like that :D.

#8

Stepping will apply the velocity and changes the position of your player.
So you should first step and then set the camera position to the player position.

 
private void update(final float delta) {
        mWorld.step(delta, 6, 2);
        mCamera.position.set(mPlayer.getPosition(), 0);
        mCamera.update();
}