btGeneric6DofSpringConstraint.cpp

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/*
Bullet Continuous Collision Detection and Physics Library, http://bulletphysics.org
Copyright (C) 2006, 2007 Sony Computer Entertainment Inc. 

This software is provided 'as-is', without any express or implied warranty.
In no event will the authors be held liable for any damages arising from the use of this software.
Permission is granted to anyone to use this software for any purpose, 
including commercial applications, and to alter it and redistribute it freely, 
subject to the following restrictions:

1. The origin of this software must not be misrepresented; you must not claim that you wrote the original software. If you use this software in a product, an acknowledgment in the product documentation would be appreciated but is not required.
2. Altered source versions must be plainly marked as such, and must not be misrepresented as being the original software.
3. This notice may not be removed or altered from any source distribution.
*/

#include "btGeneric6DofSpringConstraint.h"
#include "BulletDynamics/Dynamics/btRigidBody.h"
#include "LinearMath/btTransformUtil.h"


btGeneric6DofSpringConstraint::btGeneric6DofSpringConstraint(btRigidBody& rbA, btRigidBody& rbB, const btTransform& frameInA, const btTransform& frameInB ,bool useLinearReferenceFrameA)
        : btGeneric6DofConstraint(rbA, rbB, frameInA, frameInB, useLinearReferenceFrameA)
{
        for(int i = 0; i < 6; i++)
        {
                m_springEnabled[i] = false;
                m_equilibriumPoint[i] = btScalar(0.f);
                m_springStiffness[i] = btScalar(0.f);
                m_springDamping[i] = btScalar(1.f);
        }
}


void btGeneric6DofSpringConstraint::enableSpring(int index, bool onOff)
{
        btAssert((index >= 0) && (index < 6));
        m_springEnabled[index] = onOff;
        if(index < 3)
        {
                m_linearLimits.m_enableMotor[index] = onOff;
        }
        else
        {
                m_angularLimits[index - 3].m_enableMotor = onOff;
        }
}



void btGeneric6DofSpringConstraint::setStiffness(int index, btScalar stiffness)
{
        btAssert((index >= 0) && (index < 6));
        m_springStiffness[index] = stiffness;
}


void btGeneric6DofSpringConstraint::setDamping(int index, btScalar damping)
{
        btAssert((index >= 0) && (index < 6));
        m_springDamping[index] = damping;
}


void btGeneric6DofSpringConstraint::setEquilibriumPoint()
{
        calculateTransforms();
        int i;

        for( i = 0; i < 3; i++)
        {
                m_equilibriumPoint[i] = m_calculatedLinearDiff[i];
        }
        for(i = 0; i < 3; i++)
        {
                m_equilibriumPoint[i + 3] = m_calculatedAxisAngleDiff[i];
        }
}



void btGeneric6DofSpringConstraint::setEquilibriumPoint(int index)
{
        btAssert((index >= 0) && (index < 6));
        calculateTransforms();
        if(index < 3)
        {
                m_equilibriumPoint[index] = m_calculatedLinearDiff[index];
        }
        else
        {
                m_equilibriumPoint[index] = m_calculatedAxisAngleDiff[index - 3];
        }
}



void btGeneric6DofSpringConstraint::internalUpdateSprings(btConstraintInfo2* info)
{
        // it is assumed that calculateTransforms() have been called before this call
        int i;
        btVector3 relVel = m_rbB.getLinearVelocity() - m_rbA.getLinearVelocity();
        for(i = 0; i < 3; i++)
        {
                if(m_springEnabled[i])
                {
                        // get current position of constraint
                        btScalar currPos = m_calculatedLinearDiff[i];
                        // calculate difference
                        btScalar delta = currPos - m_equilibriumPoint[i];
                        // spring force is (delta * m_stiffness) according to Hooke's Law
                        btScalar force = delta * m_springStiffness[i];
                        btScalar velFactor = info->fps * m_springDamping[i] / btScalar(info->m_numIterations);
                        m_linearLimits.m_targetVelocity[i] =  velFactor * force;
                        m_linearLimits.m_maxMotorForce[i] =  btFabs(force) / info->fps;
                }
        }
        for(i = 0; i < 3; i++)
        {
                if(m_springEnabled[i + 3])
                {
                        // get current position of constraint
                        btScalar currPos = m_calculatedAxisAngleDiff[i];
                        // calculate difference
                        btScalar delta = currPos - m_equilibriumPoint[i+3];
                        // spring force is (-delta * m_stiffness) according to Hooke's Law
                        btScalar force = -delta * m_springStiffness[i+3];
                        btScalar velFactor = info->fps * m_springDamping[i+3] / btScalar(info->m_numIterations);
                        m_angularLimits[i].m_targetVelocity = velFactor * force;
                        m_angularLimits[i].m_maxMotorForce = btFabs(force) / info->fps;
                }
        }
}


void btGeneric6DofSpringConstraint::getInfo2(btConstraintInfo2* info)
{
        // this will be called by constraint solver at the constraint setup stage
        // set current motor parameters
        internalUpdateSprings(info);
        // do the rest of job for constraint setup
        btGeneric6DofConstraint::getInfo2(info);
}