Demystifying b2Vec2 for a physics newbie

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Demystifying b2Vec2 for a physics newbie

(5 posts) (3 voices)

Started 2 years ago by eshirt
Latest reply from eshirt

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eshirt
Member

Can anyone help a physics newbie make sense of the b2Vec2 arguments? Through trial & error (and some dumb luck) I've managed to make my objects move though the world in a uniform fashion. :) But I don't fully understand the values that I'm putting into the b2Vec2 when I use it in body->ApplyForce(). I have no gravity (it's a top-down game) and I am trying to figure out how to set a body in motion given its angle. Something like this:

|-----|
| ------------>
|-----|

where the arrow is equivalent to the 0 angle if I use body->GetAngle(). (Is that in radians?)

How do I figure out what b2Vect to use to make the body move in the direction of its angle? If anyone has any plain english explanations (or links to the same) of the values that make up a b2Vec2 (with relation to the ApplyForce() method), I think I can figure things out from there. :)

Ultimately, what I'd like to do is use that value to set my object in motion along that angle, then as the angle changes each step, the body's path will randomly "curve". In plain english, I want to do something like:

Get the body's angle
Put the body in motion in that direction.
Get a random float between -0.4 and 0.4.
Add that number to the current angle.
Set the body's new angle. (using body->SetTransform(currentPos, newAngle))
Repeat.

Posted 2 years ago #

patrickC
Member

I haven't done it in box2d, but if it's of any help this is what I would do in chipmunk:

//get normalized vector representing angle of body (rot)
cpVect angleVector = body->rot;

//create a vector with only forward force (since you want to apply a forward motion to the body you said), so we want a positive X value and 0 for Y
cpVect forceVector = cpv(10,0);

//rotate the force vector we just created by the rotation vector of the body
forceVector = cpvrotate(forceVector, angleVector);

//get random value (other, better ways of doing it)
float randValue = rand() % 0.8 - 0.4;

//apply force to body
cpBodyApplyForce(body, forceVector, cpvzero);

//rotate body manually (I would avoid this in a physical simulation)
//can't remember :)

So there, that is dirty unchecked written-on-the-fly code and I'm tired and going to bed now.
Hope it's somewhat of help.

Posted 2 years ago #

eshirt
Member

Thanks patrickC - I'll take a look at the box2d methods and find the equivalent of the rotation vector.

So, for the cpV (equivalent to the b2Vec2???) the x value is horizontal force, the y value is vertical force? Is it that straightforward?

Posted 2 years ago #
Lam
Member

Yeah x value is horizontal and y value is vertical.

I think box2d doesn't have a rotation vector so you might have to do it yourself.
Which is:
x' = x*cos(a) - y*sin(a)
y' = x*sin(a) + y*cos(a)

If you want an explanation on this then ask but it'll be with geometry or algebraic formula to really explain rotation in a 2D euclidean space.

Posted 2 years ago #

eshirt
Member

Lam, thanks for the help. It's officially been 12 years since I've had to write sin and cos formulas :) I guess I'd better re-aquaint myself with them.

In case it helps anyone else trying to do the same thing, here's what I came up with.

Given a body, "b", I loop this method:

float angle = b->GetAngle();
float force = randomFloatBetween(1.0f, 5.0f);   // randomFloatBetween is a custom method that does exactly that
b->SetLinearDamping(1.0f);    // this keeps it from getting too fast
b->SetAngularDamping(1.0f);   // and this keeps it from spinning wildly
b->ApplyImpulse(b2Vec2(cosf(angle) * force, sinf(angle) * force), b->GetWorldCenter());  // apply impulse in the angle's direction
float randValue = randomFloatBetween(-1.0f, 1.0f) * 100;
b->ApplyTorque(randValue);  // apply torque to rotate the body before looping again.

Demystifying b2Vec2 for a physics newbie

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