libgdx让我的物理颠倒了吗？

Question

出于某种原因，当我在没有libgdx的先前物理测试中使用相同的代码时，它工作正常。但是对于libgdx，它认为地面在天空中。

package com.me.randomtests;

import com.badlogic.gdx.ApplicationListener;
import...

public class Randomtest implements ApplicationListener {
ShapeRenderer SR;
int x = 100, y = 100, radius = 10;
double deltax = 0, deltay = 0, gravity = 15, energyloss = .65, dt = .2;


@Override
public void create() {      
    SR=new ShapeRenderer();
}


@Override
public void dispose() {

}

@Override
public void render() {  
    Gdx.gl.glClearColor(1, 1, 1, 1);
    Gdx.gl.glClear(GL10.GL_COLOR_BUFFER_BIT);

  SR.begin(ShapeType.FilledCircle);
    SR.filledCircle(x, y, radius);
    SR.setColor(Color.BLUE);    
    SR.end();

    Update();

}

public void Update(){
    System.out.println(y);
    if(x + deltax > Gdx.graphics.getWidth() - radius -1 ){
        x = Gdx.graphics.getWidth() - radius - 1;
        deltax = - deltax;
    }else if(x + deltax < 0 + radius){
        x = 0+radius;
        deltax= - deltax;
    }else{
        x += deltax;
    }

    if(y>Gdx.graphics.getHeight() - radius - 1){
        y = Gdx.graphics.getHeight() - radius - 1;
        deltay *=.9;
        deltay = -deltay;
    }else{
        //vel formuka
        deltay += gravity *dt; 
        //pos formula
        y +=deltay*dt + .5*gravity*dt*dt;
    }
}

@Override
public void resize(int width, int height) {
}

@Override
public void pause() {
}

@Override
public void resume() {
}
}

我的问题在这里？也有人知道为什么0在libgdx底部？