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#include "RigidBody.hpp"
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_pos[i] = _cmpr[i] = 0;
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void Gear::setPosition(float* position)
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for(i=0; i<3; i++) _pos[i] = position[i];
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void Gear::setCompression(float* compression)
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for(i=0; i<3; i++) _cmpr[i] = compression[i];
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void Gear::setSpring(float spring)
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void Gear::setDamping(float damping)
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void Gear::setStaticFriction(float sfric)
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void Gear::setDynamicFriction(float dfric)
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void Gear::setBrake(float brake)
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_brake = Math::clamp(brake, 0, 1);
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void Gear::setRotation(float rotation)
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void Gear::setExtension(float extension)
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_extension = Math::clamp(extension, 0, 1);
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void Gear::setCastering(bool c)
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void Gear::getPosition(float* out)
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for(i=0; i<3; i++) out[i] = _pos[i];
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void Gear::getCompression(float* out)
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for(i=0; i<3; i++) out[i] = _cmpr[i];
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float Gear::getSpring()
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float Gear::getDamping()
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float Gear::getStaticFriction()
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float Gear::getDynamicFriction()
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float Gear::getBrake()
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float Gear::getRotation()
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float Gear::getExtension()
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void Gear::getForce(float* force, float* contact)
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Math::set3(_force, force);
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Math::set3(_contact, contact);
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float Gear::getCompressFraction()
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bool Gear::getCastering()
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void Gear::calcForce(RigidBody* body, float* v, float* rot, float* ground)
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// Init the return values
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for(i=0; i<3; i++) _force[i] = _contact[i] = 0;
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// Don't bother if it's not down
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// First off, make sure that the gear "tip" is below the ground.
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// If it's not, there's no force.
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float a = ground[3] - Math::dot3(_pos, ground);
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// Now a is the distance from the tip to ground, so make b the
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// distance from the base to ground. We can get the fraction
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// (0-1) of compression from a/(a-b). Note the minus sign -- stuff
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// above ground is negative.
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Math::add3(_cmpr, _pos, tmp);
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float b = ground[3] - Math::dot3(tmp, ground);
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// Calculate the point of ground _contact.
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_contact[i] = _pos[i] + _frac*_cmpr[i];
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// Turn _cmpr into a unit vector and a magnitude
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float clen = Math::mag3(_cmpr);
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Math::mul3(1/clen, _cmpr, cmpr);
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// Now get the velocity of the point of contact
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body->pointVelocity(_contact, rot, cv);
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Math::add3(cv, v, cv);
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// Finally, we can start adding up the forces. First the spring
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// compression. (note the clamping of _frac to 1):
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_frac = (_frac > 1) ? 1 : _frac;
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float fmag = _frac*clen*_spring;
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// Then the damping. Use the only the velocity into the ground
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// (projection along "ground") projected along the compression
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// axis. So Vdamp = ground*(ground dot cv) dot cmpr
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Math::mul3(Math::dot3(ground, cv), ground, tmp);
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float dv = Math::dot3(cmpr, tmp);
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float damp = _damp * dv;
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if(damp > fmag) damp = fmag; // can't pull the plane down!
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if(damp < -fmag) damp = -fmag; // sanity
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// The actual force applied is only the component perpendicular to
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// the ground. Side forces come from velocity only.
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_wow = (fmag - damp) * -Math::dot3(cmpr, ground);
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Math::mul3(-_wow, ground, _force);
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// Castering gear feel no force in the ground plane
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// Wheels are funky. Split the velocity along the ground plane
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// into rolling and skidding components. Assuming small angles,
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// we generate "forward" and "left" unit vectors (the compression
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// goes "up") for the gear, make a "steer" direction from these,
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// and then project it onto the ground plane. Project the
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// velocity onto the ground plane too, and extract the "steer"
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// component. The remainder is the skid velocity.
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float gup[3]; // "up" unit vector from the ground
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Math::set3(ground, gup);
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Math::mul3(-1, gup, gup);
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float xhat[] = {1,0,0};
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float steer[3], skid[3];
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Math::cross3(gup, xhat, skid); // up cross xhat =~ skid
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Math::unit3(skid, skid); // == skid
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Math::cross3(skid, gup, steer); // skid cross up == steer
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// Correct for a (small) rotation
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Math::mul3(_rot, steer, tmp);
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Math::add3(tmp, skid, skid);
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Math::unit3(skid, skid);
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Math::cross3(skid, gup, steer);
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float vsteer = Math::dot3(cv, steer);
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float vskid = Math::dot3(cv, skid);
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float wgt = Math::dot3(_force, gup); // force into the ground
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float fsteer = _brake * calcFriction(wgt, vsteer);
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float fskid = calcFriction(wgt, vskid);
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if(vsteer > 0) fsteer = -fsteer;
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if(vskid > 0) fskid = -fskid;
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// Phoo! All done. Add it up and get out of here.
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Math::mul3(fsteer, steer, tmp);
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Math::add3(tmp, _force, _force);
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Math::mul3(fskid, skid, tmp);
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Math::add3(tmp, _force, _force);
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float Gear::calcFriction(float wgt, float v)
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// How slow is stopped? 10 cm/second?
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const float STOP = 0.1;
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const float iSTOP = 1/STOP;
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if(v < STOP) return v*iSTOP * wgt * _sfric;
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else return wgt * _dfric;
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}; // namespace yasim