Rotate a Crosshair around a sprite - 2d worms style game?? (LibGDX)

Question

I'm currently learning to code 2d games in Libgdx. Right now I'm making a kind of worms clone. I am using Box2d and have just my main sprite and a flat ground object so far.

What im stuck on is making the crosshair for the main character which I would like to rotate around him at a distance of say 90px when the player presses UP or DOWN key.

Here is the code I have so far but I havent even been able to attempt the rotation code at all:

package com.supermegabrobro.apoca.Sprites;
import com.badlogic.gdx.Gdx;
import com.badlogic.gdx.Input;
import com.badlogic.gdx.graphics.Texture;
import com.badlogic.gdx.graphics.g2d.Batch;
import com.badlogic.gdx.graphics.g2d.Sprite;
import com.badlogic.gdx.math.Vector2;
import com.badlogic.gdx.physics.box2d.Body;
import com.badlogic.gdx.physics.box2d.BodyDef;
import com.badlogic.gdx.physics.box2d.CircleShape;
import com.badlogic.gdx.physics.box2d.FixtureDef;
import com.supermegabrobro.apoca.Screens.PlayScreen;

    public class ArmyDude extends Sprite {
PlayScreen playScreen;
BodyDef bodyDef;
Body body;
FixtureDef fixtureDef;

boolean crossHairShowing = false;

Texture crossHairTexture;
float crossHairDistance = 30f;
float crossHairAngle = 45f;
Vector2 crossHairPosition;


public ArmyDude(PlayScreen playScreen){
    super(new Texture("armydude.png"));
    this.playScreen = playScreen;
    bodyDef = new BodyDef();
    bodyDef.type = BodyDef.BodyType.DynamicBody;
    bodyDef.position.set(0,0);   // TODO: Make this random

    body = playScreen.world.createBody(bodyDef);
    CircleShape circleShape = new CircleShape();
    circleShape.setRadius(getWidth() / 2);
    fixtureDef = new FixtureDef();
    fixtureDef.shape = circleShape;
    body.createFixture(fixtureDef);

    crossHairTexture = new Texture("crosshair.png");
    crossHairPosition = new Vector2(0,0);





}

public void update(float dt){
    setPosition(body.getPosition().x - getWidth() / 2, body.getPosition().y - getHeight() / 2);

    if(Gdx.input.isKeyPressed(Input.Keys.UP)){
        crossHairShowing = true;
    }
}


@Override
public void draw(Batch batch) {
    super.draw(batch);
    if (crossHairShowing){

    }
}
}

Any help is massively appreciated thanks. (nb. I have read up on, and implemented in previous projects, normal rotation of sprites. But I beleive this is very different since I am trying to not rotate the actual sprite but instead draw a texture (of a crosshair) that rotates around this sprite.

I'm really stuck though my poor brain can't figure it out alone :(

Answer 1

Usually problems like this can be solved by using vector geometry. With the basic ideas of vectors, problems relating to the coordinate system can be much easier solved.

For your game the steps would be:

1. Create angle and crosshairVector at the constructor

   float angle = 45f;
   Vector2 crosshairVector = new Vector2;
   

2. In your method to get the crosshair position get the vector position of the player.

   Vector2 playerVector = new Vector2(getX(), getY());
   

3. Create a vector with a length of the desired distance between player and crosshair.

   Vector2 distanceVector = new Vector2(90, 0);
   

4. Rotate the vector by the value of the angle variable.

   distanceVector.setAngle(angle);
   

5. Add both vectors together to get the position vector of the crosshair.

   crosshairVector = playerVector.add(distanceVector);
   

Now you have the position of the crosshair in the Vector crosshairVector. With crosshairVector.x and crosshairVector.y you get the x and y values. To rotate the crosshair simply change the angle variable.

If you want to know more about the basics of vectors you can have a look on: https://www.mathsisfun.com/algebra/vectors.html