Doubt about physics. A rotating triangle against walls

[bernardo]

Hello guys,

I have a rotating triangle colliding against the 4 walls of the device. It always "reflect" on the walls with the same angle, even though it collides with different angles at the wall. Is this the expected the behavior, physics wise? I'd expect it to bounce in different directions according to it's angle. Maybe I have something configured wrong?

For the walls and triangle:

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1. final FixtureDef objectFixtureDef = PhysicsFactory.createFixtureDef(0f, 1f, 0f);

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Thanks a lot

Edit: I've experimented making the triangle with 1f for density. The body then starts to rotate with a different velocity, but it's still bouncing in the same direction, which I find weird. It also seems to conservate energy, because when it's spinning too fast it gets very slow. Is there a way to make the linear velocity constant and be able to change the spinning speed indenpently on collisions?

Here's the whole source code

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1. package org.anddev.andengine.examples;
2.  
3. import static org.anddev.andengine.extension.physics.box2d.util.constants.PhysicsConstants.PIXEL_TO_METER_RATIO_DEFAULT;
4.  
5. import org.anddev.andengine.engine.Engine;
6. import org.anddev.andengine.engine.camera.Camera;
7. import org.anddev.andengine.engine.options.EngineOptions;
8. import org.anddev.andengine.engine.options.EngineOptions.ScreenOrientation;
9. import org.anddev.andengine.engine.options.resolutionpolicy.RatioResolutionPolicy;
10. import org.anddev.andengine.entity.primitive.Rectangle;
11. import org.anddev.andengine.entity.scene.Scene;
12. import org.anddev.andengine.entity.scene.Scene.IOnSceneTouchListener;
13. import org.anddev.andengine.entity.scene.background.ColorBackground;
14. import org.anddev.andengine.entity.shape.Shape;
15. import org.anddev.andengine.entity.sprite.AnimatedSprite;
16. import org.anddev.andengine.entity.util.FPSLogger;
17. import org.anddev.andengine.extension.physics.box2d.PhysicsConnector;
18. import org.anddev.andengine.extension.physics.box2d.PhysicsFactory;
19. import org.anddev.andengine.extension.physics.box2d.PhysicsWorld;
20. import org.anddev.andengine.input.touch.TouchEvent;
21. import org.anddev.andengine.opengl.texture.Texture;
22. import org.anddev.andengine.opengl.texture.TextureOptions;
23. import org.anddev.andengine.opengl.texture.region.TextureRegionFactory;
24. import org.anddev.andengine.opengl.texture.region.TiledTextureRegion;
25. import org.anddev.andengine.sensor.accelerometer.AccelerometerData;
26. import org.anddev.andengine.sensor.accelerometer.IAccelerometerListener;
27. import org.anddev.andengine.util.Debug;
28.  
29. import android.hardware.SensorManager;
30. import android.view.MotionEvent;
31. import android.widget.Toast;
32.  
33. import com.badlogic.gdx.math.Vector2;
34. import com.badlogic.gdx.physics.box2d.Body;
35. import com.badlogic.gdx.physics.box2d.FixtureDef;
36. import com.badlogic.gdx.physics.box2d.PolygonShape;
37. import com.badlogic.gdx.physics.box2d.BodyDef.BodyType;
38.  
39. /**
40.  * @author Nicolas Gramlich
41.  * @since 18:47:08 - 19.03.2010
42.  */
43. public class PhysicsExample extends BaseExample implements IAccelerometerListener, IOnSceneTouchListener {
44.         // ===========================================================
45.         // Constants
46.         // ===========================================================
47.  
48.         private static final int CAMERA_WIDTH = 720;
49.         private static final int CAMERA_HEIGHT = 480;
50.  
51.         // ===========================================================
52.         // Fields
53.         // ===========================================================
54.  
55.         private Texture mTexture;
56.  
57.         private TiledTextureRegion mBoxFaceTextureRegion;
58.         private TiledTextureRegion mCircleFaceTextureRegion;
59.         private TiledTextureRegion mTriangleFaceTextureRegion;
60.         private TiledTextureRegion mHexagonFaceTextureRegion;
61.  
62.         private PhysicsWorld mPhysicsWorld;
63.  
64.         private int mFaceCount = 0;
65.  
66.         // ===========================================================
67.         // Constructors
68.         // ===========================================================
69.  
70.         // ===========================================================
71.         // Getter & Setter
72.         // ===========================================================
73.  
74.         // ===========================================================
75.         // Methods for/from SuperClass/Interfaces
76.         // ===========================================================
77.  
78.         @Override
79.         public Engine onLoadEngine() {
80.                 Toast.makeText(this, "Touch the screen to add objects.", Toast.LENGTH_LONG).show();
81.                 final Camera camera = new Camera(0, 0, CAMERA_WIDTH, CAMERA_HEIGHT);
82.                 return new Engine(new EngineOptions(true, ScreenOrientation.LANDSCAPE, new RatioResolutionPolicy(CAMERA_WIDTH, CAMERA_HEIGHT), camera));
83.         }
84.  
85.         @Override
86.         public void onLoadResources() {
87.                 this.mTexture = new Texture(64, 128, TextureOptions.BILINEAR);
88.                 TextureRegionFactory.setAssetBasePath("gfx/");
89.                 this.mBoxFaceTextureRegion = TextureRegionFactory.createTiledFromAsset(this.mTexture, this, "face_box_tiled.png", 0, 0, 2, 1); // 64x32
90.                 this.mCircleFaceTextureRegion = TextureRegionFactory.createTiledFromAsset(this.mTexture, this, "face_circle_tiled.png", 0, 32, 2, 1); // 64x32
91.                 this.mTriangleFaceTextureRegion = TextureRegionFactory.createTiledFromAsset(this.mTexture, this, "face_triangle_tiled.png", 0, 64, 2, 1); // 64x32
92.                 this.mHexagonFaceTextureRegion = TextureRegionFactory.createTiledFromAsset(this.mTexture, this, "face_hexagon_tiled.png", 0, 96, 2, 1); // 64x32
93.                 this.mEngine.getTextureManager().loadTexture(this.mTexture);
94.  
95.                 this.enableAccelerometerSensor(this);
96.         }
97.  
98.         @Override
99.         public Scene onLoadScene() {
100.                 this.mEngine.registerUpdateHandler(new FPSLogger());
101.  
102.                 final Scene scene = new Scene(2);
103.                 scene.setBackground(new ColorBackground(0, 0, 0));
104.                 scene.setOnSceneTouchListener(this);
105.  
106.                 this.mPhysicsWorld = new PhysicsWorld(new Vector2(0, 0), false);
107.  
108.                 final Shape ground = new Rectangle(0, CAMERA_HEIGHT - 2, CAMERA_WIDTH, 2);
109.                 final Shape roof = new Rectangle(0, 0, CAMERA_WIDTH, 2);
110.                 final Shape left = new Rectangle(0, 0, 2, CAMERA_HEIGHT);
111.                 final Shape right = new Rectangle(CAMERA_WIDTH - 2, 0, 2, CAMERA_HEIGHT);
112.  
113.                 final FixtureDef wallFixtureDef = PhysicsFactory.createFixtureDef(1f, 1f, 0f);
114.                 PhysicsFactory.createBoxBody(this.mPhysicsWorld, ground, BodyType.StaticBody, wallFixtureDef); //left
115.                 PhysicsFactory.createBoxBody(this.mPhysicsWorld, roof, BodyType.StaticBody, wallFixtureDef);
116.                 PhysicsFactory.createBoxBody(this.mPhysicsWorld, left, BodyType.StaticBody, wallFixtureDef);
117.                 PhysicsFactory.createBoxBody(this.mPhysicsWorld, right, BodyType.StaticBody, wallFixtureDef);
118.  
119.                 scene.getBottomLayer().addEntity(ground);
120.                 scene.getBottomLayer().addEntity(roof);
121.                 scene.getBottomLayer().addEntity(left);
122.                 scene.getBottomLayer().addEntity(right);
123.  
124.                 scene.registerUpdateHandler(this.mPhysicsWorld);
125.  
126.                 return scene;
127.         }
128.  
129.         public void onLoadComplete() {
130.  
131.         }
132.  
133.         @Override
134.         public boolean onSceneTouchEvent(final Scene pScene, final TouchEvent pSceneTouchEvent) {
135.                 if(this.mPhysicsWorld != null) {
136.                         if(pSceneTouchEvent.getAction() == MotionEvent.ACTION_DOWN) {
137.                                 this.runOnUpdateThread(new Runnable() {
138.                                         @Override
139.                                         public void run() {
140.                                                 PhysicsExample.this.addFace(pSceneTouchEvent.getX(), pSceneTouchEvent.getY());
141.                                         }
142.                                 });
143.                                 return true;
144.                         }
145.                 }
146.                 return false;
147.         }
148.  
149.         @Override
150.         public void onAccelerometerChanged(final AccelerometerData pAccelerometerData) {
151.                 //this.mPhysicsWorld.setGravity(new Vector2(pAccelerometerData.getY(), pAccelerometerData.getX()));
152.         }
153.  
154.         // ===========================================================
155.         // Methods
156.         // ===========================================================
157.  
158.         private void addFace(final float pX, final float pY) {
159.                 final Scene scene = this.mEngine.getScene();
160.  
161.                 this.mFaceCount++;
162.                 Debug.d("Faces: " + this.mFaceCount);
163.  
164.                 final AnimatedSprite face;
165.                 final Body body;
166.  
167.                 final FixtureDef objectFixtureDef = PhysicsFactory.createFixtureDef(1f, 1f, 0f);
168.  
169.                 if(this.mFaceCount % 4 == 0) {
170.                         face = new AnimatedSprite(pX, pY, this.mBoxFaceTextureRegion);
171.                         body = PhysicsFactory.createBoxBody(this.mPhysicsWorld, face, BodyType.DynamicBody, objectFixtureDef);
172.                 } else if (this.mFaceCount % 4 == 1) {
173.                         face = new AnimatedSprite(pX, pY, this.mCircleFaceTextureRegion);
174.                         body = PhysicsFactory.createCircleBody(this.mPhysicsWorld, face, BodyType.DynamicBody, objectFixtureDef);
175.                 } else if (this.mFaceCount % 4 == 2) {
176.                         face = new AnimatedSprite(pX, pY, this.mTriangleFaceTextureRegion);
177.                         body = PhysicsExample.createTriangleBody(this.mPhysicsWorld, face, BodyType.DynamicBody, objectFixtureDef);
178.                 } else {
179.                         face = new AnimatedSprite(pX, pY, this.mHexagonFaceTextureRegion);
180.                         body = PhysicsExample.createHexagonBody(this.mPhysicsWorld, face, BodyType.DynamicBody, objectFixtureDef);
181.                 }
182.  
183.                 //face.animate(200);
184.                 //face.setVelocity(100, 0);
185.                 //face.setAcceleration(8, 8);
186.                 body.setLinearVelocity(new Vector2(50, 0));
187.                 body.setAngularVelocity(1f);
188.                 face.setUpdatePhysics(true);
189.  
190.                 scene.getTopLayer().addEntity(face);
191.                 this.mPhysicsWorld.registerPhysicsConnector(new PhysicsConnector(face, body, true, true, false, false));
192.         }
193.  
194.         /**
195.          * Creates a {@link Body} based on a {@link PolygonShape} in the form of a triangle:
196.          * <pre>
197.          *  /\
198.          * /__\
199.          * </pre>
200.          */
201.         public static Body createTriangleBody(final PhysicsWorld pPhysicsWorld, final Shape pShape, final BodyType pBodyType, final FixtureDef pFixtureDef) {
202.                 /* Remember that the vertices are relative to the center-coordinates of the Shape. */
203.                 final float halfWidth = pShape.getWidthScaled() * 0.5f / PIXEL_TO_METER_RATIO_DEFAULT;
204.                 final float halfHeight = pShape.getHeightScaled() * 0.5f / PIXEL_TO_METER_RATIO_DEFAULT;
205.  
206.                 final float top = -halfHeight;
207.                 final float bottom = halfHeight;
208.                 final float left = -halfHeight;
209.                 final float centerX = 0;
210.                 final float right = halfWidth;
211.  
212.                 final Vector2[] vertices = {
213.                                 new Vector2(centerX, top),
214.                                 new Vector2(right, bottom),
215.                                 new Vector2(left, bottom)
216.                 };
217.  
218.                 return PhysicsFactory.createPolygonBody(pPhysicsWorld, pShape, vertices, pBodyType, pFixtureDef);
219.         }
220.  
221.         /**
222.          * Creates a {@link Body} based on a {@link PolygonShape} in the form of a hexagon:
223.          * <pre>
224.          *  /\
225.          * /  \
226.          * |  |
227.          * |  |
228.          * \  /
229.          *  \/
230.          * </pre>
231.          */
232.         public static Body createHexagonBody(final PhysicsWorld pPhysicsWorld, final Shape pShape, final BodyType pBodyType, final FixtureDef pFixtureDef) {
233.                 /* Remember that the vertices are relative to the center-coordinates of the Shape. */
234.                 final float halfWidth = pShape.getWidthScaled() * 0.5f / PIXEL_TO_METER_RATIO_DEFAULT;
235.                 final float halfHeight = pShape.getHeightScaled() * 0.5f / PIXEL_TO_METER_RATIO_DEFAULT;
236.  
237.                 /* The top and bottom vertex of the hexagon are on the bottom and top of hexagon-sprite. */
238.                 final float top = -halfHeight;
239.                 final float bottom = halfHeight;
240.  
241.                 final float centerX = 0;
242.  
243.                 /* The left and right vertices of the heaxgon are not on the edge of the hexagon-sprite, so we need to inset them a little. */
244.                 final float left = -halfWidth + 2.5f / PIXEL_TO_METER_RATIO_DEFAULT;
245.                 final float right = halfWidth - 2.5f / PIXEL_TO_METER_RATIO_DEFAULT;
246.                 final float higher = top + 8.25f / PIXEL_TO_METER_RATIO_DEFAULT;
247.                 final float lower = bottom - 8.25f / PIXEL_TO_METER_RATIO_DEFAULT;
248.  
249.                 final Vector2[] vertices = {
250.                                 new Vector2(centerX, top),
251.                                 new Vector2(right, higher),
252.                                 new Vector2(right, lower),
253.                                 new Vector2(centerX, bottom),
254.                                 new Vector2(left, lower),
255.                                 new Vector2(left, higher)
256.                 };
257.  
258.                 return PhysicsFactory.createPolygonBody(pPhysicsWorld, pShape, vertices, pBodyType, pFixtureDef);
259.         }
260.  
261.         // ===========================================================
262.         // Inner and Anonymous Classes
263.         // ===========================================================
264. }
265.  

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