Box2d set sprite to body that rotates around specific point

Question

I want to rotate kinematic body around a specific point (I already done this part using

shape.setAsBox(1.8f / PPM, 0.2f / PPM, new Vector2(5.2f, 0), 0);

What I try is this:

PolygonShape shape = new PolygonShape(); shape.setAsBox(1.8f / PPM, 0.2f / PPM, new Vector2(5.2f, 0), 0); // !!!! FixtureDef fdef = new FixtureDef(); fdef.shape = shape; fdef.filter.categoryBits = categoryBits; fdef.filter.maskBits = Constants.PLAYER; bDef.position.set(((V_WIDTH / 32 - 3) / 2) / PPM, y / PPM); body = world.createBody(bDef).createFixture(fdef).getBody(); body.setAngularVelocity(1f); body.setUserData(new BodyUserData(categoryBits, colorArray[0])); sprite.setOrigin(sprite.getWidth() / 2, sprite.getHeight() / 2); sprite.setPosition( body.getPosition().x * 32 - sprite.getWidth() / 2, body.getPosition().y * 32 - sprite.getHeight() / 2 );

and here how I draw sprite

public void draw(SpriteBatch sb){ sprite.setRotation(body.getAngle() * MathUtils.radiansToDegrees); sprite.draw(sb); }

Unfortunately my sprite position is not the same as my body. It seems that sprite is rotating around it's center, not my point. Is it possible to fix that?

Community · Accepted Answer · 2017-04-13 12:18:49Z

Just setting your bodys angular velocity means that it rotates around its center. You never definded the point it should rotate around.

Sadly in Box2d you can't define a rotation origin, but you could try this:

Define a static body for your fixed point
Define your rotating body
Use a Joint to link these 2 bodies
Apply a force to your rotating body
Update your sprites position to the bodys position each time you draw it

Or you could calculate the circle of rotation by yourself and set the sprites position before each draw, like @Hamza Hasan described in his answer

If you want your sprite to rotate around a specific point, change its origin relative to that point:

// This sets the sprites origin to its center, letting it rotate around its center sprite.setOrigin(sprite.getWidth() / 2, sprite.getHeight() / 2); // If you want it to rotate around a point (e.g. 50 pixels to the left) you could try sprite.setOrigin(-50, sprite.getHeight() / 2);

You just have to calculate the distance of your sprite to the point:

Vector2 specificPoint = new Vector2(200, 400); Vector2 position = sprite.getPosition(); Vector2 distance = new Vector2(position.x + sprite.getWidth() / 2 - specificPoint.x, position.y + sprite.getHeight() / 2 - specificPoint.y); sprite.setOrigin(distance);

Hamza Hasan · Accepted Answer · 2016-03-14 13:55:37Z

I don't know libgdx, but I know something that might help you, in fact some trigonometry will help you at this point.

Normally I use parametric equation of circle for this purpose if I am unable to do it by all the other ways.

Here is the parametric equation of circle

x = r * Cos(theta) + displacementX y = r * Sin(theta) + displacementY

where,

r = radius
theta = angle that will change upon time. (in gameloop)
displacementX and displacementY would be the center point of the body in center. ( if displacementX and displacementY both are zero then this equation will rotate the body around origin )

Lets say, you have a box as centre point and a circle as the sprite or object that will rotate around the box.

pseudo-code:

r = 5; // radius or distance from the box theta = 0; // could be any, up to you rotationSpeed = 0.5; // Change in angle, also represents the rotation speed // public void draw(SpriteBatch sb) in your case Gameloop() { displacementX = box.x displacementY = box.y circle.x = r * Cos(theta) + displacementX circle.y = r * Sin(theta) + displacementY theta += rotationSpeed // from this you can control the rotation speed if (theta >= 360) theta = 0; }

\$\begingroup\$ if (theta >= 360) theta -= 360; \$\endgroup\$

Jon
– Jon

2016-03-08 14:48:03 +00:00
Commented Mar 8, 2016 at 14:48 — Jon
– Jon, Commented Mar 8, 2016 at 14:48

TripleSky · Accepted Answer · 2016-03-07 20:17:53Z

-3

You need to set sprite position to body's position each render frame, not only in initialization method.

answered Mar 7, 2016 at 20:17

TripleSky

214 bronze badges

\$\begingroup\$ I already tried this, unfortunately this doesn't worked for mr. \$\endgroup\$

user3334375
– user3334375

2016-03-08 07:52:36 +00:00
Commented Mar 8, 2016 at 7:52

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