java - Libgdx Box2d 身体在坠落时失去速度

Question

我对 Box2d 很陌生，我正在尝试制作一个简单的程序，让身体掉落并在其上绘制一个精灵。当我运行我的程序时，一切正常，除了身体的速度不断下降，但仍在加速下降。例如，速度变为 0、-10、-20、-30、-40、-50、-60、-20、-30、-40、-50... 等等。我很可能做错了什么，因为我对此很陌生。感谢您的帮助！

package com.davejones.spritetest;

import com.badlogic.gdx.ApplicationAdapter;
import com.badlogic.gdx.Gdx;
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.Vector2;
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.Fixture;
import com.badlogic.gdx.physics.box2d.FixtureDef;
import com.badlogic.gdx.physics.box2d.PolygonShape;
import com.badlogic.gdx.physics.box2d.World;

public class SpriteTest extends ApplicationAdapter {

private SpriteBatch batch;
private World world;
private BodyDef bodyDef;
private Body body;
private Texture texture;
private Sprite sprite;
private OrthographicCamera camera;
private Box2DDebugRenderer debugRenderer;

@Override
public void create () {
    debugRenderer = new Box2DDebugRenderer();
    debugRenderer.setDrawBodies(false);
    camera = new OrthographicCamera();
    batch = new SpriteBatch();
    texture = new Texture("badlogic.jpg");
    sprite = new Sprite(texture);

    sprite.setPosition(Gdx.graphics.getWidth() / 2 - sprite.getWidth() / 2, Gdx.graphics.getHeight() / 2);

    world = new World(new Vector2(0, -10), true);

    bodyDef = new BodyDef();

    bodyDef.type = BodyDef.BodyType.DynamicBody;

    bodyDef.position.set(sprite.getX(), sprite.getY());
    body = world.createBody(bodyDef);

    PolygonShape shape = new PolygonShape();
    shape.setAsBox(sprite.getWidth()/2, sprite.getHeight()/2);

    FixtureDef fixtureDef = new FixtureDef();
    fixtureDef.shape = shape;
    fixtureDef.density = 1f;

    Fixture fixture = body.createFixture(fixtureDef);

    shape.dispose();
}

@Override
public void resize(int width, int height) {
    super.resize(width, height);
    float cameraWidth = Gdx.graphics.getWidth();
    float cameraHeight = Gdx.graphics.getHeight();
    camera.setToOrtho(false, cameraWidth, cameraHeight);
    camera.position.set(Gdx.graphics.getWidth() / 2 - sprite.getWidth() / 2, Gdx.graphics.getHeight() / 2 - sprite.getHeight() / 2, 0);
}


@Override
public void render () {
    world.step(Gdx.graphics.getDeltaTime(), 8, 3);

    sprite.setPosition(body.getPosition().x, body.getPosition().y);

    Gdx.gl.glClearColor(0, 0, 0, 1);
    Gdx.gl.glClear(GL20.GL_COLOR_BUFFER_BIT);
    batch.begin();
    batch.draw(sprite, sprite.getX(), sprite.getY());
    batch.end();

    System.out.println(body.getLinearVelocity().y);

    camera.update();
    debugRenderer.render(world, camera.combined);
}

@Override
public void dispose () {
    batch.dispose();
    texture.dispose();
    world.dispose();
    debugRenderer.dispose();
}

}

标签： javaandroidlibgdxbox2d

Answer 1

Box2d 最好与 0.1 到 10 的值一起使用，因为超过这些大小的值开始失去精度。每个 Box2D 单元应该代表 1 米，因此您的对象是 256 米乘 256 米，这是巨大的。再加上 Box2D 对物体的最大速度限制为每步 2 米，这意味着您的 256m 正方形物体以 60 FPS 的速度以超过 250 mph 的速度以每秒 120 米的速度下落。

这通常通过在屏幕单位和 box2d 单位之间使用某种形式的转换来解决。使用常量值将 256 转换为 0.1 到 10 之间的范围，或者使用相机矩阵在屏幕上渲染更大的对象。

我已更新您的示例以使用视口。这不会解决 Box2D 最大速度的问题，但它应该有助于通过使用与 Box2D 设计最适合使用的单位匹配的较小对象来减少影响。

package com.mygdx.gtest;

import com.badlogic.gdx.ApplicationAdapter;
import com.badlogic.gdx.Gdx;
import com.badlogic.gdx.graphics.GL20;
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.Vector2;
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.Fixture;
import com.badlogic.gdx.physics.box2d.FixtureDef;
import com.badlogic.gdx.physics.box2d.PolygonShape;
import com.badlogic.gdx.physics.box2d.World;
import com.badlogic.gdx.utils.viewport.ExtendViewport;

public class Test extends ApplicationAdapter {

    private SpriteBatch batch;
    private World world;
    private BodyDef bodyDef;
    private Body body;
    private Texture texture;
    private Sprite sprite;
    private Box2DDebugRenderer debugRenderer;
    private ExtendViewport viewport;

    @Override
    public void create() {
        debugRenderer = new Box2DDebugRenderer(true,true,true,true, true, true);
        viewport = new ExtendViewport(64,48); // adding viewport so we can use multiple resolutions easily
        viewport.getCamera().position.set(32, 24, 0); // set cam position to make 0,0 bottom left corner
        batch = new SpriteBatch();
        batch.setProjectionMatrix(viewport.getCamera().combined); // tell the batch how things should be rendered 
        texture = new Texture("badlogic.jpg");
        sprite = new Sprite(texture);
        sprite.setSize(10,10); // set the size of the sprite in box2d units (10 meters x 10 meters)

        // not needed as position is set in update method
        //sprite.setPosition(Gdx.graphics.getWidth() / 2 - sprite.getWidth() / 2,   Gdx.graphics.getHeight() / 2);

        world = new World(new Vector2(0, -10), true);

        bodyDef = new BodyDef();

        bodyDef.type = BodyDef.BodyType.DynamicBody;

        bodyDef.position.set(32 , 47);
        body = world.createBody(bodyDef);

        PolygonShape shape = new PolygonShape();
        shape.setAsBox(5,5);

        FixtureDef fixtureDef = new FixtureDef();
        fixtureDef.shape = shape;
        fixtureDef.density = 1f;

        Fixture fixture = body.createFixture(fixtureDef);

        shape.dispose();
    }

    @Override
    public void resize(int width, int height) {
        viewport.update(width, height); 
    }

    @Override
    public void render() {
        world.step(Gdx.graphics.getDeltaTime(), 8, 3);

        sprite.setPosition(body.getPosition().x - (sprite.getWidth()/2), body.getPosition().y -(sprite.getHeight()/2));

        Gdx.gl.glClearColor(0, 0, 0, 1);
        Gdx.gl.glClear(GL20.GL_COLOR_BUFFER_BIT);
        batch.setProjectionMatrix(viewport.getCamera().combined);  // tell the batch how things should be rendered incase of resized window
        batch.begin();
        batch.draw(sprite, sprite.getX(), sprite.getY(),sprite.getWidth(),sprite.getHeight());
        batch.end();

        System.out.println(body.getLinearVelocity().y);

        debugRenderer.render(world, viewport.getCamera().combined);
    }

    @Override
    public void dispose() {
        batch.dispose();
        texture.dispose();
        world.dispose();
        debugRenderer.dispose();
    }
}