i have a fixture which i set his density to 0 , but he is falling down as he HAS a weight .
b2PolygonShape spriteShape; //b2polygon-for box shape
spriteShape.SetAsBox(fruit2.contentSize.width/PTM_RATIO/10,fruit2.contentSize.height/PTM_RATIO/10); //for b2polygon
b2FixtureDef spriteShapeDef;
spriteShapeDef.shape = &spriteShape;
spriteShapeDef.density = 0.0;
the world g is -9.8 as usual.
How i set this fixture to fall down slowly ? why density of 0/1/2 doesnt do that ?
thanks
How do you want it to react? Do you want it to kind of float down like a feather? A feather floats down slowly because it has the force of the air pushing it back up. So in other words, apply a small enough force upwards on the object that it falls slowly, but doesn't go up. A very low density object will still fall fast without an opposing force to slow it down.
Related
I am editing the rollingball example map from Unreal (in Unreal Engine). I changed it so my ball you grow and melt (it's a snowball), gaining and losing size and mass.
My problem is the control of it. The example map moves the ball by adding torque. My problem is that my mass changes a lot, so I need a torque the changes depending on the size of my ball (not 1:1 as a bigger snowball should still moves slower).
My problem is that my ball seems to accumulate torque and spins a lot (I added a lot of friction to the ball, it did not helped, it just moved the problem a bit). As an example, if I press left for a while, it's gonna go left. Then if I press right, it goes right. But when I stop pressing right, it spins and goes left again.
This is my current code:
void ASnowballBall::MoveRight(float Val)
{
const FVector Torque = FVector(-1.f * getNewTorque(Val), 0.f, 0.f);
Ball->AddTorque(SpringArm->GetComponentRotation().RotateVector(Torque));
}
void ASnowballBall::MoveForward(float Val)
{
const FVector Torque = FVector(0.f, getNewTorque(Val), 0.f);
Ball->AddTorque(SpringArm->GetComponentRotation().RotateVector(Torque));
}
float ASnowballBall::getNewTorque(float Val)
{
return (Val * RollTorque * (log2(Ball->GetMass()))) / 10000;
}
Note: The log was a test to increase the torque slowly with the mass. It's not working well at all. Right now I am using return Val * RollTorque instead, but it's almost as bad.
The control is horrible and I would like to get an idea on how to fix it. If removing the torque would be better, I will change it. I simply want a responsive control that is fun, and where the player still have a sense of the size of the ball (slower when big compared to small).
PS: My original mass is about 500 kg and can go up to 20 000 kg (it's a puzzle game). I did not set the original mass, it's set by Unreal relative to it's size. I could change the mass scale though.
You could try using AddForce instead of AddTorque.
So the speed of the ball is modified every tick like this:
void APlayerBallBearing::Tick(float deltaSeconds)
{
Super::Tick(deltaSeconds);
BallMesh->AddForce(FVector(InputLongitude, InputLatitude, 0.0f) * ControllerForce * BallMesh->GetMass());
}
InputLongitude and InputLatitude are the values that are put in by the user in the current frame for movement in cardinal directions, they correspond to your MoveForward and MoveRight values I reckon.
The controller force is something you need to tweak in order to find the right value for your ball movement. With a value of at least 600.0f I started getting "decent" behaviour.
I cannot help you with the mass though you would need to try that out yourself, the default mass of the ball mesh I used was ~110.
I'm currently building a simplified Reaction Control System for a Satellite game, and need a way to use the system to align the satellite to a given unit direction in world-space coordinates. Because this is a game simulation, I am faking the system and just applying a torque force around the objects epicenter.
This is difficult because in my case, the Torque cannot be varied in strength, it is either on or off. It's either full force or no force. Calculating the direction that the torque needs to be applied in is relatively easy, but I'm having trouble getting it to align perfectly without spinning out of control and getting stuck in a logical loop. it needs to apply the opposing force at precisely the right 'time' to land on the target orientation with zero angular velocity.
What I've determined so far is that I need to calculate the 'time' it will take to reach zero velocity based on my current angular velocity and the angle between the two vectors. If that exceeds the time until I reach angle zero, then it needs to apply the opposing torque. In theory this will also prevent it from 'bouncing' around the axis too much. I almost have it working, but in some cases it seems to get stuck applying force in one direction, so I'm hoping somebody can check the logic. My simulation does NOT take mass into account at the moment, so you can ignore the Inertia Tensor (unless it makes the calculation easier!)
For one axis, I'm currently doing it this way, but I figure someone will have a far more elegant solution that can actually compute both Yaw and Pitch axes at once (Roll is invalid).
Omega = Angular Velocity in Local-Space (Degrees Per Second)
Force = Strength of the Thrusters
// Calculate Time Variables
float Angle = AcosD(DotProduct(ForwardVector, DirectionVector));
float Time1 = Abs(Angle / Omega.Z); // Time taken to reach angle 0 at current velocity
float Time2 = Abs(DeltaTime * (Omega.Z / Force); // Time it will take to reach Zero velocity based on force strength.
// Calculate Direction we need to apply the force to rotate toward the target direction. Note that if we are at perfect opposites, this will be zero!
float AngleSign = Sign(DotProduct(RightVector, DirectionVector));
float Torque.Z = 0;
if (Time1 < Time2)
{
Torque.Z = AngleSign * Force;
}
else
{
Torque.Z = AngleSign * Force * -1.0f
}
// Torque is applied to object as a change in acceleration (no mass) and modified by DeltaSeconds for frame-rate independent force.
This is far from elegant and there are definitely some sign issues. Do you folks know a better way to achieve this?
EDIT:
If anybody understands Unreal Engine's Blueprint system, this is how I'm currently prototyping it before I move it to C++
Beginning from the "Calculate Direction" line, you could instead directly compute the correction torque vector in 3D, then modify its sign if you know that the previous correction is about to overshoot:
// Calculate Direction we need to apply the force to rotate toward the target direction
Torque = CrossProduct(DirectionVector, ForwardVector)
Torque = Normalize(Torque) * Force
if (Time2 < Time1)
{
Torque = -Torque
}
But you should handle the problematic cases:
// Calculate Direction we need to apply the force to rotate toward the target direction
Torque = CrossProduct(DirectionVector, ForwardVector)
if (Angle < 0.1 degrees)
{
// Avoid divide by zero in Normalize
Torque = {0, 0, 0}
}
else
{
// Handle case of exactly opposite direction (where CrossProduct is zero)
if (Angle > 179.9 degrees)
{
Torque = {0, 0, 1}
}
Torque = Normalize(Torque) * Force
if (Time2 < Time1)
{
Torque = -Torque
}
}
Okay well what i take from the pseudocode above is that you want to start braking when the time needed to break exceeds the time left till angle 0 is reached. Have you tried to slowly start breaking (in short steps because of the constant torque) BEFORE the time to break exceeds the time till angle 0?
When you do so and your satellite is near angle 0 and the velocity very low, you can just set velocity and angle to 0 so it doesn't wobble around anymore.
Did you ever figure this out? I'm working on a similar problem in UE4. I also have a constant force. I'm rotating to a new forward vector. I've realized time can't be predicted. Take for example you're rotating on Z axis at 100 degrees/second and a reverse force in exactly .015 seconds will nail your desired rotation and velocity but the next frame takes .016 seconds to render and you've just overshot it since you aren't changing your force. I think the solution is something like cheating by manually setting the forward vector once velocity is zeroed out.
I have a problem that I'm strugling to solve for a few days.
I'm trying to make a bowling game using bullet physics, but the pin shakes, jiggles and moves to the side after I position it and it falls to the floor.
Here is a GIF of what happens:
http://imgur.com/7Mg41sf
Here is how I create a Pin:
btCollisionShape* shape = createShape(pinVertices);
btScalar bodyMass = 1.6f;
btVector3 bodyInertia(0,0,0);
shape->calculateLocalInertia(bodyMass, bodyInertia);
btRigidBody::btRigidBodyConstructionInfo bodyCI = btRigidBody::btRigidBodyConstructionInfo(bodyMass, nullptr, shape, bodyInertia);
bodyCI.m_restitution = 0.7;
bodyCI.m_friction = 0.9f;
_physicsBody = std::unique_ptr<btRigidBody>(new btRigidBody(bodyCI));
_physicsBody->setUserPointer(this);
And here is how I create a floor:
btCollisionShape* shape = createShape(laneVertices);
btScalar bodyMass = 0.0f;
btVector3 bodyInertia(0,0,0);
shape->calculateLocalInertia(bodyMass, bodyInertia);
btRigidBody::btRigidBodyConstructionInfo bodyCI = btRigidBody::btRigidBodyConstructionInfo(bodyMass, nullptr, shape, bodyInertia);
bodyCI.m_restitution = 0.6;
bodyCI.m_friction = 0.5;
_physicsBody = std::unique_ptr<btRigidBody>(new btRigidBody(bodyCI));
_physicsBody->setUserPointer(this);
Right now the floor is a btBoxShape and a pin is a btConvexHullShape, but I've tried using cylinder or cone and they also slide.
Been struggling for few days especially taking into account Bullet Physics website and forum are down.
Looks entirely reasonable to me. A rigid body isn't exactly going to bounce back up, nor is it going to shatter.
You have further issues with the imperfect approximation of reality. The bottom of your pin is probably flat, which means it theoretically hits the floor instantly over multiple points. Furthermore, due to the limited FP accuracy, the pin won't be exactly round, but then that part is realistic.
So the horizontal movements are probably because the small bit of freefall introduced a minor deviation from pure vertical fall. When hitting the ground this component wasn't cancelled, but the friction on moving did eventually bring the pin to a halt. Since the pin had only a small horizontal speed, the friction was not enough to topple the pin.
Perhaps you should set the restitution (bounce) of the pin and floor to something lower (try first with 0.0) This should solve it if the pin is bouncing.
Another thing you could try is to deactivate the pin after creating it. I don't know in Bullet, but in JBullet it's done like this:
body.setActivationState( CollisionObject.WANTS_DEACTIVATION );
This will stop your pin until some other object like the ball or other pin hits it.
I am sorry to ask such a basic question about bullet. However, I am having an issue. Here is the setup:
I have a world with no gravity. When I press a key, a 1x1x1 box is created in the center of the world. When I right click, a box of size 0.05*0.05*0.05 is create at the camera position, and is 'shot' in the direction you are looking. Here is where the trouble begins.
When a small cube hits a large cube, the interaction seems wrong. You would expcet a box 1/8000th the size of another to have very little effect. Yet the large cube goes flying, as if it been with a cube of its same size. I assumed it is because I created both objects with the same mass.
To confirm this, I apply an upward force of 1 newton (or whatever unit bullet uses). Both objects accelerate at the same rate.
My code for creating objects is as follows:
btMotionState *state = new btDefaultMotionState(btTransform(btQuaternion(0, 0, 0, 1), btVector3(0, 0, 0)));
btConvexHullShape* shape = new btConvexHullShape();
for(unsigned int i = 0; i < vertices.size(); ++i) {
shape->addPoint(toBt(vertices[i]));
}
shape->setMargin(0.01f);
btScalar mass = 1.f;
btVector3 inertia;
shape->calculateLocalInertia(mass, inertia);
shape->setMargin(0.01f);
btRigidBody::btRigidBodyConstructionInfo ci(mass, state, shape, inertia);
body = new btRigidBody(ci);
I expect that the btScalar mass = 1.f; line is the culprit.
Am I expected to calculate the mass of my objects? This is simple enough for a cube, but what about some weird convex shape? Is there any way I can get bullet to do this for me?
Yes, mass is the culprit. Think about your big box being an empty wooden crate and your small box being a solid cube of iron. Huge difference in size but equal mass. Now your physics seem correct, right?
Yes, you are expected to provide the mass. Whether you calculate it or just state "this box weighs 20kg" is up to you. Actually expecting someone else to calculate the mass for you just makes your problem worse, because you'd have to specify materials, material density, and material distribution of your objects besides its geometry. If you want to go that way there are plenty of other tools available to aid you with such calculations. But I'm sure you agree that just stating some mass through trial and error is easier by far.
I looked at a bunch of similar questions, and I cannot seem to find one that particularly answers my question. I am coding a simple 3d game, and I am trying to allow the player to pick up and move entities around my map. I essentially want to get a velocity vector that will "push" the physics object a distance from the player's eyes, wherever they are looking. Here's an example of this being done in another game (the player is holding a chair entity in front of his eyes).
To do this, I find out the player's eye angles, then get the forward vector from the angles, then calculate the velocity of the object. Here is my working code:
void Player::PickupOtherEntity( Entity& HoldingEntity )
{
QAngle eyeAngles = this->GetPlayerEyeAngles();
Vector3 vecPos = this->GetEyePosition();
Vector3 vecDir = eyeAngles.Forward();
Vector3 holdingEntPos = HoldingEntity.GetLocation();
// update object by holding it a distance away
vecPos.x += vecDir.x * DISTANCE_TO_HOLD;
vecPos.y += vecDir.y * DISTANCE_TO_HOLD;
vecPos.z += vecDir.z * DISTANCE_TO_HOLD;
Vector3 vecVel = vecPos - holdingEntPos;
vecVel = vecVel.Scale(OBJECT_SPEED_TO_MOVE);
// set the entity's velocity as to "push" it to be in front of the player's eyes
// at a distance of DISTANCE_TO_HOLD away
HoldingEntity.SetVelocity(vecVel);
}
All that is great, but I want to convert my math so that I can apply an impulse. Instead of setting a completely new velocity to the object, I want to "add" some velocity to its existing velocity. So supposing I have its current velocity, what kind of math do I need to "add" velocity? This is essentially a game physics question. Thank you!
A very simple implementation could be like this:
velocity(t+delta) = velocity(t) + delta * acceleration(t)
acceleration(t) = force(t) / mass of the object
velocity, acceleration and force are vectors. t, delta and mass scalars.
This only works reasonably well for small and equally spaced deltas. What you are essentially trying to achieve with this is a simulation of bodies using classical mechanics.
An Impulse is technically F∆t for a constant F. Here we might want to assume a∆t instead because mass is irrelevant. If you want to animate an impulse you have to decide what the change in velocity should be and how long it needs to take. It gets complicated real fast.
To be honest an impulse isn't the correct thing to do. Instead it would be preferable to set a constant pick_up_velocity (people don't tend to pick things up using an impulse), and refresh the position each time the object rises up velocity.y, until it reaches the correct level:
while(entPos.y < holdingEntPos.y)
{
entPos.y += pickupVel.y;
//some sort of short delay
}
And as for floating in front of the player's eyes, set an EyeMovementEvent of some sort that also sends the correct change in position to any entity the player is holding.
And if I missed something and that's what you are already doing, remember that when humans apply an impulse, it is generally really high acceleration for a really short time, much less than a frame. You wouldn't see it in-game anyways.
basic Newtonian/D'Alembert physics dictate:
derivate(position)=velocity
derivate(velocity)=acceleration
and also backwards:
integrate(acceleration)=velocity
integrate(velocity)=position
so for your engine you can use:
rectangle summation instead of integration (numerical solution of integral). Define time constant dt [seconds] which is the interval between updates (timer or 1/fps). So the update code (must be periodically called every dt:
vx+=ax*dt;
vy+=ay*dt;
vz+=az*dt;
x+=vx*dt;
y+=vy*dt;
z+=vz*dt;
where:
a{x,y,z} [m/s^2] is actual acceleration (in your case direction vector scaled to a=Force/mass)
v{x,y,z} [m/s] is actual velocity
x,y,z [m] is actual position
These values have to be initialized a,v to zero and x,y,z to init position
all objects/players... have their own variables
full stop is done by v=0; a=0;
driving of objects is done only by change of a
in case of collision mirror v vector by collision normal
and maybe multiply by some k<1.0 (0.95 for example) to account energy loss on impact
You can add gravity or any other force field by adding g vector:
vx+=ax*dt+gx*dt;
vy+=ay*dt+gy*dt;
vz+=az*dt+gz*dt;
also You can add friction and anything else you need
PS. the same goes for angles just use angle/omega/epsilon/I instead of x/a/v/m
to be clear by angles I mean rotation (pitch,yaw,roll) around mass center