Box2D线速度导致两个接触体之间出现间隙

Question

I'm working on an Android project using Box2D through LibGDX in which I plan on having multiple dynamic bodies that can be dragged and thrown around the screen. There will be a few static bodies on the screen as well and the dynamic bodies should never be allowed to pass through them. I coded up a simple application that contains a static body platform and a dynamic body square which falls onto the platform as a first attempt.

There seems to be a bit of a problem relating to the behavior of the box. When I start this app, gravity pulls the box from the top of the screen down to the platform where it rests. If I touch the box and try to drag it downward through the platform, the box actually levitates upward a bit and shoots upward when I release my finger (sometimes it even quickly rotates back and forth before shooting up). However, if I drag the box around and manually place it on the platform, then try to pull the box through the platform, it behaves as I would expect in that it does not move at all and does not shoot upward when I release my finger. The issue only shows when gravity is responsible for pulling it to the platform. My questions regarding this are below:

Why does the box levitate/rotate when I try to drag it through the platform after it has naturally rested on the platform?

Why does the box behave correctly (sits still) when I drag it around a bit, then try to drag it through the platform?

Currently, I'm setting the restitution on the box to zero whenever it is touched and resetting it back to 0.3 when it is released and this issue is showing. The strange thing is, if I set the restitution to zero when the box is created and leave it as such, this issue seems to disappear completely. I don't want to leave the restitution set to zero though.

I was dragging down on the box (default Bad Logic logo) in this screenshot. Notice the gap between it and the platform. Screenshot Image

Here's my code. Any help will be greatly appreciated! package com.mygdx.game;

import com.badlogic.gdx.ApplicationAdapter;
import com.badlogic.gdx.Gdx;
import com.badlogic.gdx.InputProcessor;
import com.badlogic.gdx.graphics.GL20;
import com.badlogic.gdx.graphics.OrthographicCamera;
import com.badlogic.gdx.graphics.Texture;
import com.badlogic.gdx.graphics.g2d.Sprite;
import com.badlogic.gdx.graphics.g2d.SpriteBatch;
import com.badlogic.gdx.math.Rectangle;
import com.badlogic.gdx.math.Vector2;
import com.badlogic.gdx.math.Vector3;
import com.badlogic.gdx.physics.box2d.Body;
import com.badlogic.gdx.physics.box2d.BodyDef;
import com.badlogic.gdx.physics.box2d.Box2DDebugRenderer;
import com.badlogic.gdx.physics.box2d.FixtureDef;
import com.badlogic.gdx.physics.box2d.PolygonShape;
import com.badlogic.gdx.physics.box2d.World;

public class MyGdxGame extends ApplicationAdapter implements InputProcessor {
    private static float VIEWPORT_WIDTH = 4.8f;
    private static float VIEWPORT_HEIGHT = 8f;
    private static float PPM = 300f;
    private static float BOX_RESTITUTION = 0.3f;

    World world;
    OrthographicCamera camera;
    Box2DDebugRenderer renderer;
    Body squareBody;
    SpriteBatch batch;
    Sprite box;
    boolean draggingBox = false;
    Vector2 touchPoint;

    @Override
    public void create () {
        Gdx.input.setInputProcessor(this);

        this.batch = new SpriteBatch();
        this.world = new World(new Vector2(0f, -9.81f), true);
        this.renderer = new Box2DDebugRenderer();
        this.camera = new OrthographicCamera();
        this.camera.setToOrtho(false, VIEWPORT_WIDTH, VIEWPORT_HEIGHT);

        // Platform
        BodyDef platformBodyDef = new BodyDef();
        platformBodyDef.type = BodyDef.BodyType.StaticBody;
        platformBodyDef.position.set(VIEWPORT_WIDTH / 2, 2f);
        Body platformBody = this.world.createBody(platformBodyDef);
        PolygonShape platformShape = new PolygonShape();
        platformShape.setAsBox(VIEWPORT_WIDTH / 2 - 0.2f, 0.5f / 2);
        FixtureDef platformFixtureDef = new FixtureDef();
        platformFixtureDef.shape = platformShape;
        platformFixtureDef.friction = 1f;
        platformFixtureDef.restitution = 0f;
        platformFixtureDef.density = 3f;
        platformBody.createFixture(platformFixtureDef);

        // Box sprite
        this.box = new Sprite(new Texture("badlogic.jpg"));

        // Square
        BodyDef squareBodyDef = new BodyDef();
        squareBodyDef.type = BodyDef.BodyType.DynamicBody;
        squareBodyDef.position.set(VIEWPORT_WIDTH / 2, 7.5f);
        this.squareBody = this.world.createBody(squareBodyDef);
        PolygonShape squareShape = new PolygonShape();
        squareShape.setAsBox(this.box.getWidth() / PPM / 2, this.box.getHeight() / PPM / 2);
        FixtureDef squareFixtureDef = new FixtureDef();
        squareFixtureDef.shape = squareShape;
        squareFixtureDef.friction = 1f;
        squareFixtureDef.restitution = BOX_RESTITUTION;
        squareFixtureDef.density = 1f;
        this.squareBody.createFixture(squareFixtureDef);
    }

    @Override
    public void render () {
        Gdx.gl.glClear(GL20.GL_COLOR_BUFFER_BIT);
        this.camera.update();
        this.world.step(1/60f, 6, 2);
        this.renderer.render(this.world, this.camera.combined);

        // Set the velocity so the box will follow the users finger when dragged.
        if (this.draggingBox == true) {
            Vector2 velocity = this.touchPoint.cpy();
            velocity.sub(new Vector2(this.box.getX() / PPM, this.box.getY() / PPM));
            velocity.scl(10f);
            this.squareBody.setLinearVelocity(velocity);
        }

        // Update the position/rotation of the box sprite to match the physics body.
        this.box.setPosition(
                this.squareBody.getPosition().x * PPM - (this.box.getWidth() / 2),
                this.squareBody.getPosition().y * PPM - (this.box.getHeight() / 2));
        this.box.setRotation((float) Math.toDegrees(this.squareBody.getAngle()));

        this.batch.begin();
        this.box.draw(this.batch);
        this.batch.end();
    }

    @Override
    public boolean touchDown(int screenX, int screenY, int pointer, int button) {
        Vector3 touchPoint3D = this.camera.unproject(new Vector3(screenX, screenY, 0));
        Vector2 touchPoint2D = new Vector2(touchPoint3D.x, touchPoint3D.y);

        // Get the box boundary and convert it to meters.
        Rectangle boxBoundary = this.box.getBoundingRectangle();
        boxBoundary.setX(boxBoundary.getX() / PPM);
        boxBoundary.setY(boxBoundary.getY() / PPM);
        boxBoundary.setWidth(boxBoundary.getWidth() / PPM);
        boxBoundary.setHeight(boxBoundary.getHeight() / PPM);

        // If the box boundary contains the touch point, start the dragging and remove restitution.
        if (boxBoundary.contains(touchPoint2D)) {
            this.draggingBox = true;
            this.touchPoint = touchPoint2D;
            this.squareBody.getFixtureList().get(0).setRestitution(0);
        }

        return true;
    }

    @Override
    public boolean touchUp(int screenX, int screenY, int pointer, int button) {
        this.draggingBox = false;
        this.touchPoint = null;
        this.squareBody.getFixtureList().get(0).setRestitution(BOX_RESTITUTION);
        return true;
    }

    @Override
    public boolean touchDragged(int screenX, int screenY, int pointer) {
        Vector3 touchPoint3D = this.camera.unproject(new Vector3(screenX, screenY, 0));
        this.touchPoint = new Vector2(touchPoint3D.x, touchPoint3D.y);

        return true;
    }

    @Override
    public void dispose () {
        this.box.getTexture().dispose();
        batch.dispose();
    }

    @Override
    public boolean keyDown(int keycode) {
        return false;
    }

    @Override
    public boolean keyUp(int keycode) {
        return false;
    }

    @Override
    public boolean keyTyped(char character) {
        return false;
    }

    @Override
    public boolean mouseMoved(int screenX, int screenY) {
        return false;
    }

    @Override
    public boolean scrolled(int amount) {
        return false;
    }
}

Answer 1

It seems Box2D does not work well with changes to restitution within a fixture after the fixture has been created. This line is the culprit:

this.squareBody.getFixtureList().get(0).setRestitution(0);

I found a work around for this issue though. If I destroy the fixture from the body, create a new fixture with the desired restitution, and add the new fixture to the body, this problem disappears. Something like below (although you'd probably want to extract the Fixture creation into a new function):

// Destroy the current Fixture that exists on the Body. Multiple Fixtures can be assigned, but in this case there is only one.
this.squareBody.destroyFixture(this.squareBody.getFixtureList().get(0));

// Create a new shape to define a new Fixture.
PolygonShape squareShape = new PolygonShape();
squareShape.setAsBox(this.box.getWidth() / PPM / 2, this.box.getHeight() / PPM / 2);

// Create a new Fixture with the desired restitution value.
FixtureDef squareFixtureDef = new FixtureDef();
squareFixtureDef.shape = squareShape;
squareFixtureDef.friction = 1f;
squareFixtureDef.restitution = 0;
squareFixtureDef.density = 1f;

// Add the new Fixture to the Body.
this.squareBody.createFixture(squareFixtureDef);