LibGDX Box2D Physics Direction Inverted - kotlin

Question

For some reason in my world physics direction is inverted.I mean -9.8 gravity is acting like 9.8 gravity so objects are going up.Horizontal velocity is also inverted.Something must be wrong with x,y coordinate system.
world = World(Vector2(0f,-9.8f),false) pushes objects up
is there a problem with PPM ?
package com.mygdx.game
import com.badlogic.gdx.ApplicationAdapter
import com.badlogic.gdx.Gdx
import com.badlogic.gdx.Input
import com.badlogic.gdx.graphics.GL20
import com.badlogic.gdx.math.Vector2
import com.badlogic.gdx.physics.box2d.*
import com.badlogic.gdx.graphics.OrthographicCamera
import com.badlogic.gdx.math.Vector3
import com.badlogic.gdx.physics.box2d.World
import com.badlogic.gdx.physics.box2d.Box2DDebugRenderer
class MyGdxGame : ApplicationAdapter() {
val PPM = 32f
private lateinit var camera : OrthographicCamera
private lateinit var world :World
private lateinit var player:Body
private lateinit var platform:Body
private lateinit var b2dr:Box2DDebugRenderer
override fun create() {
var w = Gdx.graphics.width
var h = Gdx.graphics.height
camera = OrthographicCamera()
camera.setToOrtho(false, w / 2f, h / 2f)
world = World(Vector2(0f,-9.8f),false)
b2dr = Box2DDebugRenderer()
player = createBox(2f,10f,32,32,false)
platform = createBox(0f,0f,64,32,true)
}
override fun render() {
update(Gdx.graphics.deltaTime)
Gdx.gl.glClearColor(0f, 0f, 0f, 1f)
Gdx.gl.glClear(GL20.GL_COLOR_BUFFER_BIT)
b2dr.render(world,camera.combined.scl(PPM))
}
override fun resize(width: Int, height: Int) {
camera.setToOrtho(false, width / 2f, height / 2f)
}
override fun dispose() {
world.dispose()
b2dr.dispose()
}
fun update(delta:Float){
world.step(1/60f,6,2)
inputUpdate(delta)
cameraUpdate(delta)
}
fun inputUpdate(delta:Float){
var horizontalForce = 0f;
if(Gdx.input.isKeyPressed(Input.Keys.LEFT)){
horizontalForce -=1
}
if(Gdx.input.isKeyPressed(Input.Keys.RIGHT)){
horizontalForce +=1
}
if(Gdx.input.isKeyJustPressed(Input.Keys.UP)){
player.applyForceToCenter(0f,300f,false)
}
player.setLinearVelocity(horizontalForce*5,player.linearVelocity.y)
}
fun cameraUpdate(delta:Float){
var position:Vector3 = camera.position
position.x = player.position.x * PPM // if you get box2d units multiply with ppm
position.y = player.position.y * PPM
camera.position.set(position)
camera.update()
}
fun createBox(x:Float,y:Float,width:Int,height:Int,isStatic:Boolean):Body{
var pBody:Body
var def:BodyDef
def = BodyDef()
if(isStatic){
def.type = BodyDef.BodyType.StaticBody
}
else{
def.type = BodyDef.BodyType.DynamicBody
}
def.position.set(x/PPM,y/PPM)
def.fixedRotation = true
pBody = world.createBody(def)
var shape:PolygonShape = PolygonShape()
shape.setAsBox(width/2/PPM,height/2/PPM) // takes from center this is 32x32 box
pBody.createFixture(shape,1.0f)
shape.dispose()//clean it
return pBody
}
}

Answer 1

It's all correct with box2d and your code.
-9.81 pushes objects up and 9.81 pull objects down.
In the World constructor, you set the gravity of the world.
And Gravity is a natural phenomenon by which all things with mass or
energy—including planets, stars, galaxies, and even light—are brought
toward (or gravitate toward) one another.
Basic Gravity is the force with which object attract each other.
So on Earth gravity is the force with which objects are attracted to the center of the earth.
On Earth, we have a gravity of ~ 9.81 means all Objects will pull to the center of the earth with a force of ~ 9.81 m/s².
Finally is it logical that if gravity is a force which pulls objects down that -gravity will push objects up.
On Earth, we have a gravity of ~ 9.81 not a gravity of ~ -9.81.
If it were, we would all end up outside in the universe.