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/*
* Simple Mechanics Simulator (SiMS)
* copyright (c) 2009 Jakob Odersky
* made available under the MIT License
*/
package sims.dynamics.joints
import sims.dynamics._
import sims.geometry._
/** DistanceJoints keep their connection points at a constant distance.
* @param node1 first associated body
* @param anchor1 first connection point
* @param node2 second associated body
* @param anchor2 second connection point*/
case class DistanceJoint(node1: Body, anchor1: Vector2D, node2: Body, anchor2: Vector2D) extends Joint{
def this(node1: Body, node2: Body) = this(node1, node1.pos, node2, node2.pos)
/**Distance between the two connection points at initialisation (the desired distance).*/
val distance = (anchor2 - anchor1).length
private val a1 = anchor1 - node1.pos
private val a2 = anchor2 - node2.pos
private val initRotation1 = node1.rotation
private val initRotation2 = node2.rotation
/**Returns the connection point on body one (in world coordinates).*/
def connection1 = (a1 rotate (node1.rotation - initRotation1)) + node1.pos
/**Returns the connection point on body two (in world coordinates).*/
def connection2 = (a2 rotate (node2.rotation - initRotation2)) + node2.pos
/**Relative position of the connection points.*/
def x = connection2 - connection1
/**Relative velocity of the connection points.*/
def v = node2.velocityOfPoint(connection2) - node1.velocityOfPoint(connection1)
/* x = connection2 - connection1
* C = ||x|| - L
* u = x / ||x||
* v = v2 + w2 cross r2 - v1 - w1 cross r1
* Cdot = u dot v
* J = [-u -(r1 cross u) u (r2 cross u)]
* 1/m = J * M^-1 * JT
* = 1/m1 * u * u + 1/m2 * u * u + 1/I1 * (r1 cross u)^2 + 1/I2 * (r2 cross u)^2*/
override def correctVelocity(h: Double) = {
val x = this.x //relative position
val v = this.v //relative velocity
val r1 = (connection1 - node1.pos)
val r2 = (connection2 - node2.pos)
val cr1 = r1 cross x.unit
val cr2 = r2 cross x.unit
val Cdot = x.unit dot v //velocity constraint
val invMass = 1/node1.mass + 1/node1.I * cr1 * cr1 + 1/node2.mass + 1/node2.I * cr2 * cr2 //=J M^-1 JT
val m = if (invMass == 0.0) 0.0 else 1/invMass //avoid division by zero
val lambda = -m * Cdot //=-JV/JM^-1JT
val impulse = x.unit * lambda //P=J lambda
node1.applyImpulse(-impulse, connection1)
node2.applyImpulse(impulse, connection2)
}
override def correctPosition(h: Double) = {
val C = x.length - distance
val cr1 = (connection1 - node1.pos) cross x.unit
val cr2 = (connection2 - node2.pos) cross x.unit
val invMass = 1/node1.mass + 1/node1.I * cr1 * cr1 + 1/node2.mass + 1/node2.I * cr2 * cr2
val m = if (invMass == 0.0) 0.0 else 1/invMass
val impulse = -x.unit * m * C
node1.pos -= impulse / node1.mass
node2.pos += impulse / node2.mass
node1.rotation -= ((connection1 - node1.pos) cross impulse) / node1.I
node2.rotation += ((connection2 - node2.pos) cross impulse) / node2.I
}
}
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