A prismatic constraint is like the line constraint except it does not allow rotation about the anchor point. the Prismatic Joint is just like the Line Joint only it does not allow rotation about the anchor point. Because of this, we can formulate the Prismatic Joint by combining two joints: Line Joint and Angle Joint. This allows us to skip directly to the Jacobian:
// Line joint + Angle joint
b2_prismatic_joint.cpp of the full version Box2D gives the following useful guidance.
// Linear constraint (point-to-line)
// d = p2 - p1 = x2 + r2 - x1 - r1
// C = dot(perp, d)
// Cdot = dot(d, cross(w1, perp)) + dot(perp, v2 + cross(w2, r2) - v1 - cross(w1, r1))
// = -dot(perp, v1) - dot(cross(d + r1, perp), w1) + dot(perp, v2) + dot(cross(r2, perp), v2)
// J = [-perp, -cross(d + r1, perp), perp, cross(r2,perp)]
//
// Angular constraint
// C = a2 - a1 + a_initial
// Cdot = w2 - w1
// J = [0 0 -1 0 0 1]
//
// K = J * invM * JT
//
// J = [-a -s1 a s2]
// [0 -1 0 1]
// a = perp
// s1 = cross(d + r1, a) = cross(p2 - x1, a)
// s2 = cross(r2, a) = cross(p2 - x2, a)
// Motor/Limit linear constraint
// C = dot(ax1, d)
// Cdot = -dot(ax1, v1) - dot(cross(d + r1, ax1), w1) + dot(ax1, v2) + dot(cross(r2, ax1), v2)
// J = [-ax1 -cross(d+r1,ax1) ax1 cross(r2,ax1)]
// Predictive limit is applied even when the limit is not active.
// Prevents a constraint speed that can lead to a constraint error in one time step.
// Want C2 = C1 + h * Cdot >= 0
// Or:
// Cdot + C1/h >= 0
// I do not apply a negative constraint error because that is handled in position correction.
// So:
// Cdot + max(C1, 0)/h >= 0
// Block Solver
// We develop a block solver that includes the angular and linear constraints. This makes the limit stiffer.
//
// The Jacobian has 2 rows:
// J = [-uT -s1 uT s2] // linear
// [0 -1 0 1] // angular
//
// u = perp
// s1 = cross(d + r1, u), s2 = cross(r2, u)
// a1 = cross(d + r1, v), a2 = cross(r2, v)
Javascript implementation
class PrismaticJoint extends Joint {
constructor(b1, b2, anchor1 = b1.position, anchor2 = b2.position, dir, frequency = 30, dampingRatio = 1.0, jointMass = -1) {
super(b1, b2, frequency, dampingRatio, jointMass);
this.impulseSum = new Vector2();
if (body1.type == Type.Static && body2.type == Type.Static)
throw "Can't make prismatic constraint between static bodies";
if (body1.type == Type.Dynamic && body2.type == Type.Dynamic)
throw "Can't make prismatic constraint between dynamic bodies";
if (body2.type == Type.Static)
throw "Please make prismatic constraint by using the body1 as a static body";
this.localAnchor1 = Vector2.InverseRotateAndTranslate(this.body1.Rot, this.body1.position, anchor1);
this.localAnchor2 = Vector2.InverseRotateAndTranslate(this.body2.Rot, this.body2.position, anchor2);
this.initialAngle = body2.rotation - body1.rotation;
if (dir == undefined) {
let u = anchor2.sub(anchor1);
this.t = new Vector2(-u.y, u.x).normalized();
}
else {
this.t = new Vector2(-dir.y, dir.x).normalized();
}
Util.assert(this.t.squaredLength > 0);
}
preStep()
preStep(inv_dt) {
// Calculate Jacobian J and effective mass M
// J = [-t^t, -(ra + u)×t, t^t, rb×t] // Line
// [ 0, -1, 0, 1] // Angle
// M = (J · M^-1 · J^t)^-1
// Pre-compute anchors, mass matrix, and bias.
Vector2.Rotate(this.body1.Rot, this.localAnchor1, this.r1);
Vector2.Rotate(this.body2.Rot, this.localAnchor2, this.r2);
let p1 = this.body1.position.add(this.r1);
let p2 = this.body2.position.add(this.r2);
this.u = p2.sub(p1);
let sa = this.r1.add(this.u).cross(this.t);
let sb = this.r2.cross(this.t);
let k = new Matrix2();
k.m00 = this.body1.invMass + sa * sa * this.body1.invI + this.body2.invMass + sb * sb * this.body2.invI;
k.m01 = sa * this.body1.invI + sb * this.body2.invI;
k.m10 = sa * this.body1.invI + sb * this.body2.invI;
k.m11 = this.body1.invI + this.body2.invI;
k.m00 += this.gamma;
k.m11 += this.gamma;
this.m = k.inverted();
let error0 = this.u.dot(this.t);
let error1 = this.body2.rotation - this.body1.rotation - this.initialAngle;
if (Settings.positionCorrection)
this.bias = new Vector2(error0, error1).mul(this.beta * inv_dt);
else
this.bias = new Vector2(0.0, 0.0);
if (Settings.warmStarting)
this.applyImpulse(this.impulseSum);
}solve()
solve() {
// Calculate corrective impulse: Pc
// Pc = J^t · λ (λ: lagrangian multiplier)
// λ = (J · M^-1 · J^t)^-1 ⋅ -(J·v+b)
let jv0 = this.t.dot(this.body2.linearVelocity) + this.r2.cross(this.t) * this.body2.angularVelocity
- (this.t.dot(this.body1.linearVelocity) + this.r2.add(this.u).cross(this.t) * this.body1.angularVelocity)
+ this.gamma;
let jv1 = this.body2.angularVelocity - this.body1.angularVelocity;
let jv = new Vector2(jv0, jv1);
let lambda = this.m.mulVector(jv.add(this.bias).add(this.impulseSum.mul(this.gamma)).inverted());
this.applyImpulse(lambda);
if (Settings.warmStarting)
this.impulseSum.add(lambda);
}
applyImpulse(lambda) {
// V2 = V2' + M^-1 ⋅ Pc
// Pc = J^t ⋅ λ
let lambda0 = lambda.x;
let lambda1 = lambda.y;
this.body1.linearVelocity = this.body1.linearVelocity.sub(this.t.mul(lambda0 * this.body1.invMass));
this.body1.angularVelocity = this.body1.angularVelocity - this.r1.add(this.u).cross(this.t) * this.body1.invI;
this.body2.linearVelocity = this.body2.linearVelocity.add(this.t.mul(lambda0 * this.body2.invMass));
this.body2.angularVelocity = this.body2.angularVelocity + this.r2.cross(this.t) * this.body2.invI;
this.body1.angularVelocity = this.body1.angularVelocity - lambda1 * this.body1.invI;
this.body2.angularVelocity = this.body2.angularVelocity + lambda1 * this.body2.invI;
}
box2d version
// MIT License
// Copyright (c) 2019 Erin Catto
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
// The above copyright notice and this permission notice shall be included in all
// copies or substantial portions of the Software.
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
// SOFTWARE.
#include "box2d/b2_body.h"
#include "box2d/b2_draw.h"
#include "box2d/b2_prismatic_joint.h"
#include "box2d/b2_time_step.h"
// Linear constraint (point-to-line)
// d = p2 - p1 = x2 + r2 - x1 - r1
// C = dot(perp, d)
// Cdot = dot(d, cross(w1, perp)) + dot(perp, v2 + cross(w2, r2) - v1 - cross(w1, r1))
// = -dot(perp, v1) - dot(cross(d + r1, perp), w1) + dot(perp, v2) + dot(cross(r2, perp), v2)
// J = [-perp, -cross(d + r1, perp), perp, cross(r2,perp)]
//
// Angular constraint
// C = a2 - a1 + a_initial
// Cdot = w2 - w1
// J = [0 0 -1 0 0 1]
//
// K = J * invM * JT
//
// J = [-a -s1 a s2]
// [0 -1 0 1]
// a = perp
// s1 = cross(d + r1, a) = cross(p2 - x1, a)
// s2 = cross(r2, a) = cross(p2 - x2, a)
// Motor/Limit linear constraint
// C = dot(ax1, d)
// Cdot = -dot(ax1, v1) - dot(cross(d + r1, ax1), w1) + dot(ax1, v2) + dot(cross(r2, ax1), v2)
// J = [-ax1 -cross(d+r1,ax1) ax1 cross(r2,ax1)]
// Predictive limit is applied even when the limit is not active.
// Prevents a constraint speed that can lead to a constraint error in one time step.
// Want C2 = C1 + h * Cdot >= 0
// Or:
// Cdot + C1/h >= 0
// I do not apply a negative constraint error because that is handled in position correction.
// So:
// Cdot + max(C1, 0)/h >= 0
// Block Solver
// We develop a block solver that includes the angular and linear constraints. This makes the limit stiffer.
//
// The Jacobian has 2 rows:
// J = [-uT -s1 uT s2] // linear
// [0 -1 0 1] // angular
//
// u = perp
// s1 = cross(d + r1, u), s2 = cross(r2, u)
// a1 = cross(d + r1, v), a2 = cross(r2, v)
void b2PrismaticJointDef::Initialize(b2Body* bA, b2Body* bB, const b2Vec2& anchor, const b2Vec2& axis)
{
bodyA = bA;
bodyB = bB;
localAnchorA = bodyA->GetLocalPoint(anchor);
localAnchorB = bodyB->GetLocalPoint(anchor);
localAxisA = bodyA->GetLocalVector(axis);
referenceAngle = bodyB->GetAngle() - bodyA->GetAngle();
}
b2PrismaticJoint::b2PrismaticJoint(const b2PrismaticJointDef* def)
: b2Joint(def)
{
m_localAnchorA = def->localAnchorA;
m_localAnchorB = def->localAnchorB;
m_localXAxisA = def->localAxisA;
m_localXAxisA.Normalize();
m_localYAxisA = b2Cross(1.0f, m_localXAxisA);
m_referenceAngle = def->referenceAngle;
m_impulse.SetZero();
m_axialMass = 0.0f;
m_motorImpulse = 0.0f;
m_lowerImpulse = 0.0f;
m_upperImpulse = 0.0f;
m_lowerTranslation = def->lowerTranslation;
m_upperTranslation = def->upperTranslation;
b2Assert(m_lowerTranslation <= m_upperTranslation);
m_maxMotorForce = def->maxMotorForce;
m_motorSpeed = def->motorSpeed;
m_enableLimit = def->enableLimit;
m_enableMotor = def->enableMotor;
m_translation = 0.0f;
m_axis.SetZero();
m_perp.SetZero();
}
void b2PrismaticJoint::InitVelocityConstraints(const b2SolverData& data)
{
m_indexA = m_bodyA->m_islandIndex;
m_indexB = m_bodyB->m_islandIndex;
m_localCenterA = m_bodyA->m_sweep.localCenter;
m_localCenterB = m_bodyB->m_sweep.localCenter;
m_invMassA = m_bodyA->m_invMass;
m_invMassB = m_bodyB->m_invMass;
m_invIA = m_bodyA->m_invI;
m_invIB = m_bodyB->m_invI;
b2Vec2 cA = data.positions[m_indexA].c;
float aA = data.positions[m_indexA].a;
b2Vec2 vA = data.velocities[m_indexA].v;
float wA = data.velocities[m_indexA].w;
b2Vec2 cB = data.positions[m_indexB].c;
float aB = data.positions[m_indexB].a;
b2Vec2 vB = data.velocities[m_indexB].v;
float wB = data.velocities[m_indexB].w;
b2Rot qA(aA), qB(aB);
// Compute the effective masses.
b2Vec2 rA = b2Mul(qA, m_localAnchorA - m_localCenterA);
b2Vec2 rB = b2Mul(qB, m_localAnchorB - m_localCenterB);
b2Vec2 d = (cB - cA) + rB - rA;
float mA = m_invMassA, mB = m_invMassB;
float iA = m_invIA, iB = m_invIB;
// Compute motor Jacobian and effective mass.
{
m_axis = b2Mul(qA, m_localXAxisA);
m_a1 = b2Cross(d + rA, m_axis);
m_a2 = b2Cross(rB, m_axis);
m_axialMass = mA + mB + iA * m_a1 * m_a1 + iB * m_a2 * m_a2;
if (m_axialMass > 0.0f)
{
m_axialMass = 1.0f / m_axialMass;
}
}
// Prismatic constraint.
{
m_perp = b2Mul(qA, m_localYAxisA);
m_s1 = b2Cross(d + rA, m_perp);
m_s2 = b2Cross(rB, m_perp);
float k11 = mA + mB + iA * m_s1 * m_s1 + iB * m_s2 * m_s2;
float k12 = iA * m_s1 + iB * m_s2;
float k22 = iA + iB;
if (k22 == 0.0f)
{
// For bodies with fixed rotation.
k22 = 1.0f;
}
m_K.ex.Set(k11, k12);
m_K.ey.Set(k12, k22);
}
if (m_enableLimit)
{
m_translation = b2Dot(m_axis, d);
}
else
{
m_lowerImpulse = 0.0f;
m_upperImpulse = 0.0f;
}
if (m_enableMotor == false)
{
m_motorImpulse = 0.0f;
}
if (data.step.warmStarting)
{
// Account for variable time step.
m_impulse *= data.step.dtRatio;
m_motorImpulse *= data.step.dtRatio;
m_lowerImpulse *= data.step.dtRatio;
m_upperImpulse *= data.step.dtRatio;
float axialImpulse = m_motorImpulse + m_lowerImpulse - m_upperImpulse;
b2Vec2 P = m_impulse.x * m_perp + axialImpulse * m_axis;
float LA = m_impulse.x * m_s1 + m_impulse.y + axialImpulse * m_a1;
float LB = m_impulse.x * m_s2 + m_impulse.y + axialImpulse * m_a2;
vA -= mA * P;
wA -= iA * LA;
vB += mB * P;
wB += iB * LB;
}
else
{
m_impulse.SetZero();
m_motorImpulse = 0.0f;
m_lowerImpulse = 0.0f;
m_upperImpulse = 0.0f;
}
data.velocities[m_indexA].v = vA;
data.velocities[m_indexA].w = wA;
data.velocities[m_indexB].v = vB;
data.velocities[m_indexB].w = wB;
}
void b2PrismaticJoint::SolveVelocityConstraints(const b2SolverData& data)
{
b2Vec2 vA = data.velocities[m_indexA].v;
float wA = data.velocities[m_indexA].w;
b2Vec2 vB = data.velocities[m_indexB].v;
float wB = data.velocities[m_indexB].w;
float mA = m_invMassA, mB = m_invMassB;
float iA = m_invIA, iB = m_invIB;
// Solve linear motor constraint
if (m_enableMotor)
{
float Cdot = b2Dot(m_axis, vB - vA) + m_a2 * wB - m_a1 * wA;
float impulse = m_axialMass * (m_motorSpeed - Cdot);
float oldImpulse = m_motorImpulse;
float maxImpulse = data.step.dt * m_maxMotorForce;
m_motorImpulse = b2Clamp(m_motorImpulse + impulse, -maxImpulse, maxImpulse);
impulse = m_motorImpulse - oldImpulse;
b2Vec2 P = impulse * m_axis;
float LA = impulse * m_a1;
float LB = impulse * m_a2;
vA -= mA * P;
wA -= iA * LA;
vB += mB * P;
wB += iB * LB;
}
if (m_enableLimit)
{
// Lower limit
{
float C = m_translation - m_lowerTranslation;
float Cdot = b2Dot(m_axis, vB - vA) + m_a2 * wB - m_a1 * wA;
float impulse = -m_axialMass * (Cdot + b2Max(C, 0.0f) * data.step.inv_dt);
float oldImpulse = m_lowerImpulse;
m_lowerImpulse = b2Max(m_lowerImpulse + impulse, 0.0f);
impulse = m_lowerImpulse - oldImpulse;
b2Vec2 P = impulse * m_axis;
float LA = impulse * m_a1;
float LB = impulse * m_a2;
vA -= mA * P;
wA -= iA * LA;
vB += mB * P;
wB += iB * LB;
}
// Upper limit
// Note: signs are flipped to keep C positive when the constraint is satisfied.
// This also keeps the impulse positive when the limit is active.
{
float C = m_upperTranslation - m_translation;
float Cdot = b2Dot(m_axis, vA - vB) + m_a1 * wA - m_a2 * wB;
float impulse = -m_axialMass * (Cdot + b2Max(C, 0.0f) * data.step.inv_dt);
float oldImpulse = m_upperImpulse;
m_upperImpulse = b2Max(m_upperImpulse + impulse, 0.0f);
impulse = m_upperImpulse - oldImpulse;
b2Vec2 P = impulse * m_axis;
float LA = impulse * m_a1;
float LB = impulse * m_a2;
vA += mA * P;
wA += iA * LA;
vB -= mB * P;
wB -= iB * LB;
}
}
// Solve the prismatic constraint in block form.
{
b2Vec2 Cdot;
Cdot.x = b2Dot(m_perp, vB - vA) + m_s2 * wB - m_s1 * wA;
Cdot.y = wB - wA;
b2Vec2 df = m_K.Solve(-Cdot);
m_impulse += df;
b2Vec2 P = df.x * m_perp;
float LA = df.x * m_s1 + df.y;
float LB = df.x * m_s2 + df.y;
vA -= mA * P;
wA -= iA * LA;
vB += mB * P;
wB += iB * LB;
}
data.velocities[m_indexA].v = vA;
data.velocities[m_indexA].w = wA;
data.velocities[m_indexB].v = vB;
data.velocities[m_indexB].w = wB;
}
// A velocity based solver computes reaction forces(impulses) using the velocity constraint solver.Under this context,
// the position solver is not there to resolve forces.It is only there to cope with integration error.
//
// Therefore, the pseudo impulses in the position solver do not have any physical meaning.Thus it is okay if they suck.
//
// We could take the active state from the velocity solver.However, the joint might push past the limit when the velocity
// solver indicates the limit is inactive.
bool b2PrismaticJoint::SolvePositionConstraints(const b2SolverData& data)
{
b2Vec2 cA = data.positions[m_indexA].c;
float aA = data.positions[m_indexA].a;
b2Vec2 cB = data.positions[m_indexB].c;
float aB = data.positions[m_indexB].a;
b2Rot qA(aA), qB(aB);
float mA = m_invMassA, mB = m_invMassB;
float iA = m_invIA, iB = m_invIB;
// Compute fresh Jacobians
b2Vec2 rA = b2Mul(qA, m_localAnchorA - m_localCenterA);
b2Vec2 rB = b2Mul(qB, m_localAnchorB - m_localCenterB);
b2Vec2 d = cB + rB - cA - rA;
b2Vec2 axis = b2Mul(qA, m_localXAxisA);
float a1 = b2Cross(d + rA, axis);
float a2 = b2Cross(rB, axis);
b2Vec2 perp = b2Mul(qA, m_localYAxisA);
float s1 = b2Cross(d + rA, perp);
float s2 = b2Cross(rB, perp);
b2Vec3 impulse;
b2Vec2 C1;
C1.x = b2Dot(perp, d);
C1.y = aB - aA - m_referenceAngle;
float linearError = b2Abs(C1.x);
float angularError = b2Abs(C1.y);
bool active = false;
float C2 = 0.0f;
if (m_enableLimit)
{
float translation = b2Dot(axis, d);
if (b2Abs(m_upperTranslation - m_lowerTranslation) < 2.0f * b2_linearSlop)
{
C2 = translation;
linearError = b2Max(linearError, b2Abs(translation));
active = true;
}
else if (translation <= m_lowerTranslation)
{
C2 = b2Min(translation - m_lowerTranslation, 0.0f);
linearError = b2Max(linearError, m_lowerTranslation - translation);
active = true;
}
else if (translation >= m_upperTranslation)
{
C2 = b2Max(translation - m_upperTranslation, 0.0f);
linearError = b2Max(linearError, translation - m_upperTranslation);
active = true;
}
}
if (active)
{
float k11 = mA + mB + iA * s1 * s1 + iB * s2 * s2;
float k12 = iA * s1 + iB * s2;
float k13 = iA * s1 * a1 + iB * s2 * a2;
float k22 = iA + iB;
if (k22 == 0.0f)
{
// For fixed rotation
k22 = 1.0f;
}
float k23 = iA * a1 + iB * a2;
float k33 = mA + mB + iA * a1 * a1 + iB * a2 * a2;
b2Mat33 K;
K.ex.Set(k11, k12, k13);
K.ey.Set(k12, k22, k23);
K.ez.Set(k13, k23, k33);
b2Vec3 C;
C.x = C1.x;
C.y = C1.y;
C.z = C2;
impulse = K.Solve33(-C);
}
else
{
float k11 = mA + mB + iA * s1 * s1 + iB * s2 * s2;
float k12 = iA * s1 + iB * s2;
float k22 = iA + iB;
if (k22 == 0.0f)
{
k22 = 1.0f;
}
b2Mat22 K;
K.ex.Set(k11, k12);
K.ey.Set(k12, k22);
b2Vec2 impulse1 = K.Solve(-C1);
impulse.x = impulse1.x;
impulse.y = impulse1.y;
impulse.z = 0.0f;
}
b2Vec2 P = impulse.x * perp + impulse.z * axis;
float LA = impulse.x * s1 + impulse.y + impulse.z * a1;
float LB = impulse.x * s2 + impulse.y + impulse.z * a2;
cA -= mA * P;
aA -= iA * LA;
cB += mB * P;
aB += iB * LB;
data.positions[m_indexA].c = cA;
data.positions[m_indexA].a = aA;
data.positions[m_indexB].c = cB;
data.positions[m_indexB].a = aB;
return linearError <= b2_linearSlop && angularError <= b2_angularSlop;
}
b2Vec2 b2PrismaticJoint::GetAnchorA() const
{
return m_bodyA->GetWorldPoint(m_localAnchorA);
}
b2Vec2 b2PrismaticJoint::GetAnchorB() const
{
return m_bodyB->GetWorldPoint(m_localAnchorB);
}
b2Vec2 b2PrismaticJoint::GetReactionForce(float inv_dt) const
{
return inv_dt * (m_impulse.x * m_perp + (m_motorImpulse + m_lowerImpulse - m_upperImpulse) * m_axis);
}
float b2PrismaticJoint::GetReactionTorque(float inv_dt) const
{
return inv_dt * m_impulse.y;
}
float b2PrismaticJoint::GetJointTranslation() const
{
b2Vec2 pA = m_bodyA->GetWorldPoint(m_localAnchorA);
b2Vec2 pB = m_bodyB->GetWorldPoint(m_localAnchorB);
b2Vec2 d = pB - pA;
b2Vec2 axis = m_bodyA->GetWorldVector(m_localXAxisA);
float translation = b2Dot(d, axis);
return translation;
}
float b2PrismaticJoint::GetJointSpeed() const
{
b2Body* bA = m_bodyA;
b2Body* bB = m_bodyB;
b2Vec2 rA = b2Mul(bA->m_xf.q, m_localAnchorA - bA->m_sweep.localCenter);
b2Vec2 rB = b2Mul(bB->m_xf.q, m_localAnchorB - bB->m_sweep.localCenter);
b2Vec2 p1 = bA->m_sweep.c + rA;
b2Vec2 p2 = bB->m_sweep.c + rB;
b2Vec2 d = p2 - p1;
b2Vec2 axis = b2Mul(bA->m_xf.q, m_localXAxisA);
b2Vec2 vA = bA->m_linearVelocity;
b2Vec2 vB = bB->m_linearVelocity;
float wA = bA->m_angularVelocity;
float wB = bB->m_angularVelocity;
float speed = b2Dot(d, b2Cross(wA, axis)) + b2Dot(axis, vB + b2Cross(wB, rB) - vA - b2Cross(wA, rA));
return speed;
}
bool b2PrismaticJoint::IsLimitEnabled() const
{
return m_enableLimit;
}
void b2PrismaticJoint::EnableLimit(bool flag)
{
if (flag != m_enableLimit)
{
m_bodyA->SetAwake(true);
m_bodyB->SetAwake(true);
m_enableLimit = flag;
m_lowerImpulse = 0.0f;
m_upperImpulse = 0.0f;
}
}
float b2PrismaticJoint::GetLowerLimit() const
{
return m_lowerTranslation;
}
float b2PrismaticJoint::GetUpperLimit() const
{
return m_upperTranslation;
}
void b2PrismaticJoint::SetLimits(float lower, float upper)
{
b2Assert(lower <= upper);
if (lower != m_lowerTranslation || upper != m_upperTranslation)
{
m_bodyA->SetAwake(true);
m_bodyB->SetAwake(true);
m_lowerTranslation = lower;
m_upperTranslation = upper;
m_lowerImpulse = 0.0f;
m_upperImpulse = 0.0f;
}
}
bool b2PrismaticJoint::IsMotorEnabled() const
{
return m_enableMotor;
}
void b2PrismaticJoint::EnableMotor(bool flag)
{
if (flag != m_enableMotor)
{
m_bodyA->SetAwake(true);
m_bodyB->SetAwake(true);
m_enableMotor = flag;
}
}
void b2PrismaticJoint::SetMotorSpeed(float speed)
{
if (speed != m_motorSpeed)
{
m_bodyA->SetAwake(true);
m_bodyB->SetAwake(true);
m_motorSpeed = speed;
}
}
void b2PrismaticJoint::SetMaxMotorForce(float force)
{
if (force != m_maxMotorForce)
{
m_bodyA->SetAwake(true);
m_bodyB->SetAwake(true);
m_maxMotorForce = force;
}
}
float b2PrismaticJoint::GetMotorForce(float inv_dt) const
{
return inv_dt * m_motorImpulse;
}
void b2PrismaticJoint::Dump()
{
// FLT_DECIMAL_DIG == 9
int32 indexA = m_bodyA->m_islandIndex;
int32 indexB = m_bodyB->m_islandIndex;
b2Dump(" b2PrismaticJointDef jd;\n");
b2Dump(" jd.bodyA = bodies[%d];\n", indexA);
b2Dump(" jd.bodyB = bodies[%d];\n", indexB);
b2Dump(" jd.collideConnected = bool(%d);\n", m_collideConnected);
b2Dump(" jd.localAnchorA.Set(%.9g, %.9g);\n", m_localAnchorA.x, m_localAnchorA.y);
b2Dump(" jd.localAnchorB.Set(%.9g, %.9g);\n", m_localAnchorB.x, m_localAnchorB.y);
b2Dump(" jd.localAxisA.Set(%.9g, %.9g);\n", m_localXAxisA.x, m_localXAxisA.y);
b2Dump(" jd.referenceAngle = %.9g;\n", m_referenceAngle);
b2Dump(" jd.enableLimit = bool(%d);\n", m_enableLimit);
b2Dump(" jd.lowerTranslation = %.9g;\n", m_lowerTranslation);
b2Dump(" jd.upperTranslation = %.9g;\n", m_upperTranslation);
b2Dump(" jd.enableMotor = bool(%d);\n", m_enableMotor);
b2Dump(" jd.motorSpeed = %.9g;\n", m_motorSpeed);
b2Dump(" jd.maxMotorForce = %.9g;\n", m_maxMotorForce);
b2Dump(" joints[%d] = m_world->CreateJoint(&jd);\n", m_index);
}
void b2PrismaticJoint::Draw(b2Draw* draw) const
{
const b2Transform& xfA = m_bodyA->GetTransform();
const b2Transform& xfB = m_bodyB->GetTransform();
b2Vec2 pA = b2Mul(xfA, m_localAnchorA);
b2Vec2 pB = b2Mul(xfB, m_localAnchorB);
b2Vec2 axis = b2Mul(xfA.q, m_localXAxisA);
b2Color c1(0.7f, 0.7f, 0.7f);
b2Color c2(0.3f, 0.9f, 0.3f);
b2Color c3(0.9f, 0.3f, 0.3f);
b2Color c4(0.3f, 0.3f, 0.9f);
b2Color c5(0.4f, 0.4f, 0.4f);
draw->DrawSegment(pA, pB, c5);
if (m_enableLimit)
{
b2Vec2 lower = pA + m_lowerTranslation * axis;
b2Vec2 upper = pA + m_upperTranslation * axis;
b2Vec2 perp = b2Mul(xfA.q, m_localYAxisA);
draw->DrawSegment(lower, upper, c1);
draw->DrawSegment(lower - 0.5f * perp, lower + 0.5f * perp, c2);
draw->DrawSegment(upper - 0.5f * perp, upper + 0.5f * perp, c3);
}
else
{
draw->DrawSegment(pA - 1.0f * axis, pA + 1.0f * axis, c1);
}
draw->DrawPoint(pA, 5.0f, c1);
draw->DrawPoint(pB, 5.0f, c4);
}
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