cocos2d for iPhone

A fast, easy to use, free, and community supported 2D game engine

1. 

   indy2005
   Member

   Hi,

   I need to check the velocity of a b2Body, in the direction it is facing. So if the body is moving forwards it would be positive, if it is moving perfectly laterally, it would be zero, and if any velocity is away from the direction the b2Body is facing, it is negative.

   I can calculate the length of the vector, but the length is obviously always positive...and I need to know the component of the velocity in terms of the direction.

   // Get the linear velocity vector of the body
   	b2Vec2 linearVelocity = body_->GetLinearVelocity();
   	_currentLinearVelocity = linearVelocity.Length();
   
   	//now find out the velocity component in the direction of the bodies rotation
   	b2Vec2 facingVelocity = body_->GetTransform().R.col2;
   	facingVelocity *= b2Dot(linearVelocity, facingVelocity);
   	_currentForwardVelocity = facingVelocity.Length(); // this is always positive...after all, a length is always positive
   
   	CCLOG(@"--->>>--->>>  VELOCITY IN FACING DIRECTION OF SHIP IS %f", _currentForwardVelocity);
   	CCLOG(@"--->>>--->>>  Linear Velocity OF SHIP IS %f", _currentLinearVelocity);
   
   	b2Vec2 thrustVec = body_->GetTransform().R.col2;
   	thrustVec *= _thrustForce;
   
   	body_->ApplyForce(thrustVec, body_->GetPosition());

   as always, any advice appreciated.

   Regards

   i

   Posted 1 year ago #
2. 

   bryce
   Member

   All you'll have to do is linearVelocity.x and if it's going left, it will be negative, going right will be positive. Same for going up and down, linearVelocity.y.

   Posted 1 year ago #
3. 

   toadkick
   Member

   when you take the dot product of 2 normalized vectors, you are projecting one vector onto the other (in other words, the result indicates to what degree the two are facing the same way). More specifically, the result is the cosine of the angle between them.

   so:

   b2Vec2 linearVelocity = body_->GetLinearVelocity().Normalize();
   b2Vec2 facingVector = body_->GetTransform().R.col2.Normalize();
   
   float dot = b2Dot(linearVelocity, facingVector);
   
   // if dot == 1, they are facing exactly the same direction
   // if dot == -1, they are facing exactly the opposite direction
   // if dot > 0, they are facing roughly the same direction
   // if dot < 0, they are facing roughly the opposite direction

   hope that helps.

   Posted 1 year ago #
4. 

   indy2005
   Member

   Hi,

   Thanks, so I use the dot product to tell if the velocity is in the direction of the orientation. How do I work out the magnitude of that velocity. I only want to apply a force impulse if the body hasn't already reached maximum velocity in the exact direction of orientation. I can use the normalised dot product trick to determine if I need to do the velocity calculation at all (between 0 and 1) and then just use the length of the non normalised dot product to get what I need (I think!?).

   Many thanks

   i

   Posted 1 year ago #
5. 

   alechko03
   Member

   Hey Indy 2005!

   Your code work for me thank you! The only thing i needed to change was the R.col2 to R.col1. Now I have a button that when pressed gives my player object a thrust in the direction it is facing. Again thank you, hopefully you will find a solution to your issues.

   all the best!

   Posted 7 months ago #

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