Mass?

[LifeDrainingCoding]

Is there in FXGL 21 any way to configure Entity mass except adding fixture definitions with density? Btw, MassData is located in package that's not visible. I think it's essential to expect mass property of physical bodies in game engine but i didn't find any except density.

[AlmasB]

Hi, I think you are right. I don't remember any mass related use cases, so it's probably still not exposed publicly. Can you provide a minimal example of your use case / how you would use it in your context. Would exposing MassData help you implement this, or do you need any other APIs that are not public at the moment? Thanks.

[LifeDrainingCoding]

Here's a simple example

import com.almasb.fxgl.entity.Entity;
import com.almasb.fxgl.entity.SpawnData;
import com.almasb.fxgl.entity.components.BoundingBoxComponent;
import com.almasb.fxgl.entity.components.CollidableComponent;
import com.almasb.fxgl.entity.components.ViewComponent;
import com.almasb.fxgl.physics.BoundingShape;
import com.almasb.fxgl.physics.HitBox;
import com.almasb.fxgl.physics.PhysicsComponent;
import com.almasb.fxgl.physics.box2d.dynamics.BodyDef;
import com.almasb.fxgl.physics.box2d.dynamics.BodyType;
import com.almasb.fxgl.physics.box2d.dynamics.FixtureDef;
import com.lifedrained.fxglphysicstest.types.TypeOf;
import javafx.scene.paint.Color;
import javafx.scene.shape.Circle;

//just keys for spawn data
import static com.lifedrained.fxglphysicstest.SpawnDataKeys.*;

public class Marble extends Entity {

    protected PhysicsComponent physics;
    protected BodyDef bodyDef;
    protected FixtureDef fixtureDef;
    protected ViewComponent view;
    protected BoundingBoxComponent bbox;
    protected double radius, mass,area , density = 0;

    public Marble(SpawnData data) {
        super();
        setX(data.getX());
        setY(data.getY());
        setType(TypeOf.DYNAMIC_ENTITY);
        radius = data.get(RADIUS);
        mass = data.get(MASS);

        bodyDef = new BodyDef();
        fixtureDef = new FixtureDef();
        fixtureDef.setFriction(0.1f);
        fixtureDef.setRestitution(0.1f);
        updateMassRelated(0);

        view = getComponent(ViewComponent.class);
        view.clearChildren();
        view.addChild(new Circle(radius, Color.RED));

        physics = new PhysicsComponent();
        physics.setBodyDef(bodyDef);
        physics.setFixtureDef(fixtureDef);
        physics.setBodyType(BodyType.DYNAMIC);

        bbox = getBoundingBoxComponent();
        bbox.addHitBox(new HitBox(BoundingShape.circle(radius)));

        addComponent(new CollidableComponent(true));
        addComponent(physics);
    }



    public void setRadius(double radius) {
        this.radius = radius;

        updateMassRelated(density);
        updateBody();
    }
    public void setMass(double mass) {
        this.mass = mass;

        updateMassRelated(density);
    }

    public void setDensity(double density) {
        updateMassRelated(density);
    }


    /*This should be calculated automatically when changing mass\area\density in FXGL,
     but we have to do it manually */
    public void updateMassRelated(double density){
        area = Math.PI * radius * radius;
        
        //checking if density was changed, if not recalc density, if yes then calc mass 
        if (this.density == 0 || Double.compare(this.density, density) == 0) {
            this.density = mass/area;
        }else {
            mass = density*area;
            this.density = density;
        }

        fixtureDef.setDensity((float) this.density);
    }

    public void updateBody(){
        view.clearChildren();
        view.addChild(new Circle(radius, Color.RED));
        bbox.clearHitBoxes();
        bbox.addHitBox(new HitBox(BoundingShape.circle(radius)));
    }
}

Use cases? Reducing boilerplate code. Mass should be linked to density and area of bounding box and if we change one of this then those properties should be changed in chain like in updateMassRelated() as well . Every developer using FXGL physics forced to write this kind boilerplate code like method updateMassRelated()

Upd: i forgot to mention that i'm using FXGL 21.1