Noob question - box2d revolute joint

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Noob question - box2d revolute joint

(9 posts) (3 voices)

Started 1 year ago by jonathan0382
Latest reply from PAj

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jonathan0382
Member

Sup community,

I'm trying to learn box2d but am stuck on what should be easy. I'm defining a revolute joint with motor etc, and am later, upon ccTouchesBegan, trying to increase the motor speed.

In the .h I have:

b2RevoluteJointDef jd2;
	b2Joint *rev2_joint;

In the init I have:

b2Vec2 anchor2;
	anchor2.Set(bd2.position.x, bd2.position.y);//bd2 is the position of a body
	jd2.Initialize(swingarm, rwheel, anchor2);
	jd2.collideConnected = false;
	jd2.motorSpeed = -10.0f;
	jd2.maxMotorTorque = 100000.0f;
	jd2.enableMotor = true;
	rev2_joint = _world -> CreateJoint(&jd2);

Upon run I get a nice turning revolute joint, at speed -10. Now I want to change this speed, like the backwheel on a car upon acceleration. I'm finding myself unable to update it. The following

rev2_joint->SetMotorSpeed = -100.0f;//didn't work - error says request for member motorSpeed is not in b2Joint
	rev2_joint.motorSpeed = -10000.0f;//didn't work - error says request for member motorSpeed is not in b2Joint

Any suggestions? Am I defining the joints all wrong?

Thanks

Posted 1 year ago #

indy2005
Member

I am using it like this:

leftJoint->SetMotorSpeed(mspeed);

try dropping the = syntax...

-10000 radians per second though seems a ridiculously high speed?

Posted 1 year ago #
jonathan0382
Member
```
rev2_joint->SetMotorSpeed(-50);
```
Just worked for me. I was going to post it up, but you beat me to it indy2005. Thanks bro.

And yeah, the speed was ridiculous. I just wanted to really notice it :D
Posted 1 year ago #
indy2005
Member

good luck!

Posted 1 year ago #
PAj
Member
Weird but I get errors for all the ways to adjust motor speed as simulation is running as posted in this thread ..

in .h I have b2Joint & b2RevoluteJoint:
```
b2RevoluteJointDef jointdef1;
	b2Joint *rev1_joint;
```
in .mm
```
b2Vec2 anchor1;
anchor1.Set(winSize.width/2/PTM_RATIO, 425/PTM_RATIO);//bd2 is the position of a body
		jointdef1.Initialize(_paddle1Body, _paddle2Body, anchor1);
		jointdef1.collideConnected = false;
		//jointdef1.userData = jointdef1data;
		jointdef1.maxMotorTorque = 1000.0f;
		jointdef1.motorSpeed = 100.0f;
		jointdef1.enableMotor = false;
		rev1_joint = _world -> CreateJoint(&jointdef1);
```
Works fine to move the motor but ...

In my simulation step section I do:

rev1_joint->SetMotorSpeed(10);
377: error: 'class b2Joint' has no member named 'SetMotorSpeed'

which is true after checking so try:

jointdef1->SetMotorSpeed = -100.0f
377: error: base operand of '->' has non-pointer type 'b2RevoluteJointDef'
but b2RevoluteJoint.h does have SetMotorSpeed!?!?!

rev2_joint->SetMotorSpeed = -100.0f
error: 'class b2Joint' has no member named 'SetMotorSpeed'

This is kinda blowing my mind here ... any other ideas?
Posted 1 year ago #
PAj
Member
Sorry
```
rev2_joint->SetMotorSpeed = -100.0f
was suppose to be
rev1_joint->SetMotorSpeed = -100.0f
```
but it still same errors ... :-(

So how does one move a joint a specific angle if not done with a motor?

Trying to animate a character made up of joints and bodies ...
Posted 1 year ago #
PAj
Member
Can it be done if I get the user data for the joint?

For a body you assign the body user data to be a Sprite.
```
CCSprite *paddle1 = [CCSprite spriteWithFile:@"paddle-img.png"];
paddle1BodyDef.userData = paddle1;'

and that seems to work fine. But for a joint with 2 or more sprites attached it wouldn't appear I could do the same.

b2joint.h relevant section claims:
```
/// Get the user data pointer.
void* GetUserData() const;

/// Set the user data pointer.
void SetUserData(void* data);`

It's like a body so I should be able to get joint user data, yes?

:-/
Posted 1 year ago #

PAj
Member

Sorry to post again but this is getting pretty frustrating ...

Any help will come with my first born child if you like!!!

Back to original attempt to get info on joint.

from the box2d manual it claims one can do something like this:

b2Vec2 anchor1;
anchor1.Set((startPosX, startPosY);
jointdef1.Initialize(mainBody, tail1Body, anchor1);
jointdef1.collideConnected = false;
jointdef1.lowerAngle = 1.0f * b2_pi; // -90 degrees
jointdef1.upperAngle = 1.25f * b2_pi; // 45 degrees
jointdef1.enableLimit = true;
jointdef1.maxMotorTorque = 10000.0f;
jointdef1.motorSpeed = 0.0f;
jointdef1.enableMotor = true;
rev1_joint = _world -> CreateJoint(&jointdef1);'

Joint works fine. Great!

Now when I try to grab ANY info on the joint in tick such as:

<code>float32 angle = jointdef1->GetJointAngle();</code>

I continue to get error:
"Base operand of -> has non-pointer type b2RevoluteJointDef"
despite the fact in .h I have:

b2RevoluteJointDef jointdef1;
b2Joint *rev1_joint;`

Ok so it's not a pointer so I try:

float32 angle = jointdef1.GetJointAngle();

"struct b2RevoluteJointDef' has no member named 'GetJointAngle"

...

Seriously, what the begeezus am I doing wrong here?????

I get the feeling I'm going be really embarrassed by this one :-/

Posted 1 year ago #

PAj
Member

Thanks to Erin over at another forum!

In my.h define not just a b2joint but a b2RevoluteJoint

b2RevoluteJoint *revJ1_joint;

During creation do this

revJ1_joint = (b2RevoluteJoint*)_world->CreateJoint(&jointdef1);

and now this call works fine:
float32 angle = revJ1_joint->GetJointAngle();

Hope it helps someone else!

Now to figure out how to get the userData ...

Posted 1 year ago #

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