Is it possible to set rotation of join with screen control?

[Sleepy_Weasel]

I've successfully set a joint, and I've got a screen control for rotation, but now Im wondering if its possible to set the rotation of the joint to the same degree of the rotation of the control. Has anyone done this?

[RealMayo]

Yes I've done something similar to this with RevoluteJoint and AnalogOnScreenControl. You just setAngularVelocity on the body attached to the joint based on logic surrounding the X Y coordinates of the movement of the control.

[Sleepy_Weasel]

Do you have an example of this by any chance?

[RealMayo]

Here you go (see below). Note that in Box2D I don't think you can have the body instantly exist at the new angle based on the control position. Therefore, you need to have the body move at a moderate pace from its starting angle to its ending angle. Its possible that collisions could occur while the body is moving if there are other bodies in its path.

In the following code, I have setup (as a sample) the left joystick to manipulate the body (that is attached to the RevoluteJoint) by using setAngularVelocity. Conversely, the right joystick manipulates the body by using setLinearVelocity. These are two different ways to make the body move about its joint.

setAngularVelocity however will cause the body to infinitely rotate until the joystick is let go, or until the body reaches a pre-established angle limit, or until you apply custom code to stop its movement at the desired time. A "limit" can be applied by setting values on the joint itself similar to this:
enableLimit = true
lowerAngle = MathUtils.degToRad(25)
upperAngle = MathUtils.degToRad(135)
Alternatively you can choose to constantly detect the current angle of the body, and then stop all movement (by no longer calling setAngularVelocity) when you have reached a desired angle, by using this:
myBody.getAngle()

setLinearVelocity on the other hand will naturally (gradually) bring the body to an angle that somewhat correlates with the position of the joystick. And it will stop there even if the joystick is still being held down. It will only move again if the joystick is moved in a different direction.

Note that I am using "multipliers" such as 40f and 10f. This is necessary due to the density of your body as well as the torque of your joint. If you give your body a high value for density or a high value for torque, then you need to use a high value for the multiplier when trying to move the body.

For your RevoluteJoint, you can start experimenting by setting the following values:
motorSpeed = 0
maxMotorTorque = 20
density = 2 (this one is applied to your body, not the joint)

Syntax: [ Download ] [ Hide ]

Syntax: [ Download ] [ Show ]

Using java Syntax Highlighting

1.                 private void myJoysticks() {
2. // SETUP THE LEFT JOYSTICK
3.                         final float x1 = 0;
4.                         final float y1 = mCameraHeight - this.mJoystickBaseTextureRegion.getHeight();
5.                         leftJoystick = new AnalogOnScreenControl(x1, y1, this.mCamera, this.mJoystickBaseTextureRegion, this.mJoystickKnobTextureRegion, 0.1f, this.getVertexBufferObjectManager(), new IAnalogOnScreenControlListener() {
6.                                 @Override
7.                                 public void onControlChange(final BaseOnScreenControl pBaseOnScreenControl, final float pValueX, final float pValueY) {
8.                                                 myBody.setAngularVelocity(pValueY * 40f);                                              
9.                                 }
10.  
11.                                 @Override
12.                                 public void onControlClick(final AnalogOnScreenControl pAnalogOnScreenControl) {
13.                                         // Do Nothing
14.                                 }
15.                         });
16.                         leftJoystick.getControlBase().setBlendFunction(GLES20.GL_SRC_ALPHA, GLES20.GL_ONE_MINUS_SRC_ALPHA);
17.                         leftJoystick.getControlBase().setAlpha(0.5f);
18.                         leftJoystick.refreshControlKnobPosition();
19.                         this.mScene.setChildScene(leftJoystick);
20.  
21. // SETUP THE RIGHT JOYSTICK                    
22.                         final float x2 = mCameraWidth - this.mJoystickBaseTextureRegion.getWidth();
23.                         final float y2 = mCameraHeight - this.mJoystickBaseTextureRegion.getHeight();
24.                         rightJoystick = new AnalogOnScreenControl(x2, y2, this.mCamera, this.mJoystickBaseTextureRegion, this.mJoystickKnobTextureRegion, 0.1f, this.getVertexBufferObjectManager(), new IAnalogOnScreenControlListener() {
25.                                 @Override
26.                                 public void onControlChange(final BaseOnScreenControl pBaseOnScreenControl, final float pValueX, final float pValueY) {                        
27.                                         final Vector2 velocity;
28.                                         velocity = Vector2Pool.obtain(pValueX * 10f, pValueY * 10f);                                   
29.                                         myBody.setLinearVelocity(velocity);
30.                                         Vector2Pool.recycle(velocity);                                                                                 
31.                                 }
32.  
33.                                 @Override
34.                                 public void onControlClick(final AnalogOnScreenControl pAnalogOnScreenControl) {
35.                                         // Do Nothing
36.                                 }
37.                         });
38.                         rightJoystick.getControlBase().setBlendFunction(GLES20.GL_SRC_ALPHA, GLES20.GL_ONE_MINUS_SRC_ALPHA);
39.                         rightJoystick.getControlBase().setAlpha(0.5f);
40.                         rightJoystick.refreshControlKnobPosition();
41.                         leftJoystick.setChildScene(rightJoystick);
42.                 }
43.  

Parsed in 0.038 seconds, using GeSHi 1.0.8.4

[Sleepy_Weasel]

Working fantastic. Thanks again!

[Sleepy_Weasel]

Im trying to eliminate movement of the joint when the player moves. In essence when the joystick is over 0,0 or not moving, enable limits on the joint, but when the joystick is moved to remove those limits. Is there a reason this wont work?

Syntax: [ Download ] [ Hide ]

Syntax: [ Download ] [ Show ]

1. if (pValueX == x1 && pValueY == x1) { 
2. AxeHandleBody.getAngle(); 
3. revjoint2.enableLimit = true; 
4. revjoint2.lowerAngle=AxeHandleBody.getAngle(); 
5. revjoint2.upperAngle=AxeHandleBody.getAngle(); 
6. } else { 
7. revjoint2.enableLimit=false; 
8. velocity2 = Vector2Pool.obtain(pValueX * 6f, pValueY * 6f); 
9. AxeHandleBody.setLinearVelocity(velocity2); 
10. Vector2Pool.recycle(velocity2); 
11. swinging = false; 
12.  
13. } 

The enable limit = false doesnt seem to allow the joint to move. Do I need to redo the upper/lower angles?

[Sleepy_Weasel]

Anyone have a solution to the fixed joint rotation?

[RealMayo]

I've had the same need, to limit the joint when the player moves. The only solution I have been able to come up with is to keep the joint active and limitless, but increase the Density and Torque A LOT so that player movement does not provide enough force to affect the joint.

[Sleepy_Weasel]

Ahh good idea. I'll do that. Thanks.