hairCollisionSolver.cpp

//-
// ==========================================================================
// Copyright 1995,2006,2008 Autodesk, Inc. All rights reserved.
//
// Use of this software is subject to the terms of the Autodesk
// license agreement provided at the time of installation or download,
// or which otherwise accompanies this software in either electronic
// or hard copy form.
// ==========================================================================
//+

#include <maya/MIOStream.h>
#include <maya/MStatus.h>
#include <maya/MDagPath.h>
#include <maya/MFloatPointArray.h>
#include <maya/MObject.h>
#include <maya/MPlugArray.h>
#include <maya/MFnDependencyNode.h>
#include <maya/MFnDagNode.h>
#include <maya/MFnMesh.h>
#include <maya/MHairSystem.h>
#include <maya/MFnPlugin.h>

#include <math.h> 

#define kPluginName             "hairCollisionSolver"

//
//
// OVERVIEW:
// This plug-in implements a custom collision solver for Maya's dynamic
// hair system. This allows users to override the following aspects of
// Maya's dynamic hair systems:
//
//              o Override the Maya dynamic hair system's object-to-hair
//                collision algorithm with a user-defined algorithm.
//              o Optionally perform global filtering on hair, such as freeze,
//                smoothing, etc.
//
// It should be noted that Maya's dynamic hair system involves four
// arears of collision detection, and this plug-in is specific to the
// Hair-to-Object aspect only. The four areas collision which the Maya
// dynamic hair system involves are:
//
//              1) Hair to object collision.
//              2) Hair to implicit object collision.
//              3) Hair to ground plane collision.
//              4) Self collision between hairs.
//
// This plug-in illustrates the first case, overriding Maya's internal
// Hair-to-Object collision solver. There is currently no API for over-
// riding the other three collision solvers.
//
//
// RATIONALE:
// There are several reasons why are user may wish to override Maya's
// internal hair to object collision algorithm. These are:
//
//              1) The internal algorithm may not be accurate enough. After all,
//                 it is a simulation of real-life physics and there are tradeoffs
//                 taken to provide reasonable performance. Note that there are
//                 means of increasing the accuracy without writing a custom
//                 implementation, such as decreasing the dynamics time step, or
//                 increasing the hair width.
//              2) The built-in algorithm might be too accurate. If you only want
//                 simplified collisions, such as against a bounding box repre-
//                 sentation instead of the internal algorithm's exhasutive test-
//                 ing against each surface of the object, you could write a
//                 lighter-weight implementation.
//              3) You might have a desire to process the hairs, such as smooth
//                 them out.
//
//
// STRATEGY:
// The basic idea is to implement a custom callback which is registered
// via MHairSystem::registerCollisionSolverCollide(). Your callback will
// then be invoked in place of Maya's internal collision solver. By simply
// registering a collision solver, you can completely implement a custom
// hair-to-object solution.
//
// However, since the collision solver is called once per hair times the
// number of solver iterations, it is wise to pre-process the data if
// possible to speed up the collision tests. For this reason, you can
// assign a pre-frame callback. One approach is to create a pre-processed
// representation of your object pre-frame (e.g. an octree representation)
// and then access this representation during the collision testing.
//
// For the purposes of our simple demo plug-in, our private data will
// consist of a COLLISION_INFO which contains an array of COLLISION_OBJ
// structures, each one holding the bounding box of the object in world
// space.
//
// One issue with pre-processing the data involves managing the private
// data. A collision object could be deleted or turned off during a
// simulation. One way to cleanly manage such data is to store your
// private data on a typed attribute which is added to the node. You
// would build your private data once at the start of simulation in your
// pre-frame callback (keep track of the current time, and if the curTime
// passed in is less than what you store locally, assume the playback
// has restarted from the beginning -- or the user is being silly and
// trying to play the simulation backwards :-) Since it is relatively
// expensive to look up a dynamic attribute value, and the collide()
// callback can get triggered 1000's of times per frame, for efficiency
// you can pass back your private data as a pointer from your pre-frame
// routine, and this pointer is then passed directly into your collide()
// callback.
// 
//
typedef struct {
        int                             numVerts;       // Number of vertices in object.
        double                  minx;           // Bounding box minimal extrema.
        double                  miny;           // Bounding box minimal extrema.
        double                  minz;           // Bounding box minimal extrema.
        double                  maxx;           // Bounding box maximal extrema.
        double                  maxy;           // Bounding box maximal extrema.
        double                  maxz;           // Bounding box maximal extrema.
} COLLISION_OBJ ;

typedef struct {
        int                             numObjs;        // Number of collision objects.
        COLLISION_OBJ   *objs;          // Array of per-object info.
} COLLISION_INFO ;

//
// Synopsis:
//              bool    preFrame( hairSystem, curTime, privateData )
//
// Description:
//              This callback is invoked once at the start of each frame, allowing
//      the user to perform any pre-processing as they see fit, such as build-
//      ing or updating private collision-detection structures.
//
//              Note: it is possible for collision objects to change between
//      frames during a simulation (for example, the user could delete a col-
//      lision object), so if you choose to store your pre-processed data, it
//      is critical to track any edits or deletions to the collision object
//      to keep your pre-processed data valid.
//
//              There are lots of hints for writing an effective pre-frame call-
//      back in the STRATEGY section listed earlier in this file.
//
// Parameters:
//              MObject hairSystem      : (in)  The hair system shape node.
//              double  curTime         : (in)  Current time in seconds.
//              void    **privateData:(out)     Allows the user to return a private
//                                                                      data pointer to be passed into their
//                                                                      collision solver. If you store your
//                                                                      pre-processed data in data structure
//                                                                      which is difficult to access, such as
//                                                                      on a typed attribute, this provides
//                                                                      an easy way to provide the pointer.
//
// Returns:
//              bool    true            : Successfully performed any needed initial-
//                                                        isation for the hair simulation this frame.
//              bool    false           : An error was detected.
//


bool            preFrame(
                                const MObject           hairSystem,
                                const double            curTime,
                                void                            **privateData )
{
        MStatus status;

        // If you need want to perform any preprocessing on your collision
        // objects pre-frame, do it here. One option for storing the pre-
        // processed data is on a typed attribute on the hairSystem node.
        // That data could be fetched and updated here.
        //
        // In our example, we'll just compute a bounding box here and NOT use
        // attribute storage. That is an exercise for the reader.
        //
        MFnDependencyNode fnHairSystem( hairSystem, &status );
        CHECK_MSTATUS_AND_RETURN( status, false );
        fprintf( stderr,
                        "preFrame: calling hairSystem node=`%s', time=%g\n",
                        fnHairSystem.name().asChar(), curTime );
        MObjectArray    cols;
        MIntArray               logIdxs;
        CHECK_MSTATUS_AND_RETURN( MHairSystem::getCollisionObject( hairSystem,
                        cols, logIdxs ), false );
        int nobj = cols.length();

        // Allocate private data.
        // This allows us to pre-process data on a pre-frame basis to avoid
        // calculating it per hair inside the collide() call. As noted earlier
        // we could allocate this in preFrame() and hang it off the hairSystem
        // node via a dynamic attribute.
        // Instead we'll allocate it here.
        //
        COLLISION_INFO *collisionInfo = (COLLISION_INFO *) malloc(
                        sizeof( COLLISION_INFO ) );
        collisionInfo->objs = (COLLISION_OBJ *) malloc(
                        nobj * sizeof( COLLISION_OBJ ) );
        collisionInfo->numObjs = nobj;

        // Create the private data that we'll make available to the collide
        // method. The data should actually be stored in a way that it can
        // be cleaned up (such as storing the pointer on the hairSystem node
        // using a dynamic attribute). As it stands right now, there is a
        // memory leak with this plug-in because the memory we're allocating
        // for the private data is never cleaned up.
        //
        // Note that when using the dynamic attribute approach, it is still
        // wise to set *privateData because this avoids the need to look up
        // the plug inside the collide() routine which is a high-traffic
        // method.
        //
        *privateData = (void *) collisionInfo;

        // Loop through the collision objects and pre-process, storing the
        // results in the collisionInfo structure.
        //
        int        obj;
        for ( obj = 0; obj < nobj; ++obj ) {
                // Get the ith collision geometry we are connected to.
                //
                MObject colObj = cols[obj];

                // Get the DAG path for the collision object so we can transform
                // the vertices to world space.
                //
                MFnDagNode fnDagNode( colObj, &status );
                CHECK_MSTATUS_AND_RETURN( status, false );
                MDagPath path;
                status = fnDagNode.getPath( path );
                CHECK_MSTATUS_AND_RETURN( status, false );
                MFnMesh fnMesh( path, &status );
                if ( MS::kSuccess != status ) {
                        fprintf( stderr,
                                        "%s:%d: collide was not passed a valid mesh shape\n",
                                        __FILE__, __LINE__ );
                        return( false );
                }

                // Get the vertices of the object transformed to world space.
                //
                MFloatPointArray        verts;
                status = fnMesh.getPoints( verts, MSpace::kWorld );
                CHECK_MSTATUS_AND_RETURN( status, false );

                // Compute the bounding box for the collision object.
                // As this is a quick and dirty demo, we'll just support collisions
                // between hair and the bbox.
                //
                double minx, miny, minz, maxx, maxy, maxz, x, y, z;
                minx = maxx = verts[0].x;
                miny = maxy = verts[0].y;
                minz = maxz = verts[0].z;
                int nv = verts.length();
                int i;
                for ( i = 1; i < nv; ++i ) {
                        x = verts[i].x;
                        y = verts[i].y;
                        z = verts[i].z;

                        if ( x < minx ) {
                                minx = x;
                        }
                        if ( y < miny ) {
                                miny = y;
                        }
                        if ( z < minz ) {
                                minz = z;
                        }

                        if ( x > maxx ) {
                                maxx = x;
                        }
                        if ( y > maxy ) {
                                maxy = y;
                        }
                        if ( z > maxz ) {
                                maxz = z;
                        }
                }

                // Store this precomputed informantion into our private data
                // structure.
                //
                collisionInfo->objs[obj].numVerts = nv;
                collisionInfo->objs[obj].minx = minx;
                collisionInfo->objs[obj].miny = miny;
                collisionInfo->objs[obj].minz = minz;
                collisionInfo->objs[obj].maxx = maxx;
                collisionInfo->objs[obj].maxy = maxy;
                collisionInfo->objs[obj].maxz = maxz;
                fprintf( stderr, "Inside preFrameInit, bbox=%g %g %g %g %g %g\n",
                                minx,miny,minz,maxx,maxy,maxz);
        }

        return( true );
}

//
// Synopsis:
//              bool    belowCollisionPlane( co, pnt )
//
// Description:
//              Test if `pnt' is directly below the collision plane of `co'.
//
// Parameters:
//              COLLISION_OBJ   *co     : (in)  The collision object to test against.
//              MVector                 &pnt: (in)      Point to test.
//
// Returns:
//              bool    true            : The `pnt' is directly below the collision
//                                                        plane specified by `co'.
//              bool    false           : The `pnt' is not directly below the collis-
//                                                        ion plane specified by `co'.
//


bool    belowCollisionPlane( const COLLISION_OBJ *co, const MVector &pnt )
{
        return(    pnt.x > co->minx && pnt.x < co->maxx
                                            && pnt.y < co->maxy
                        && pnt.z > co->minz && pnt.z < co->maxz );
}

//
// Synopsis:
//              MStatus collide( hairSystem, follicleIndex, hairPositions,
//                                              hairPositionsLast, hairWidths, startIndex,
//                                              endIndex, curTime, friction, privateData )
//
// Description:
//              This callback is invoked by Maya to test if a collision exists be-
//      tween the follicle (defined by `hairPositions', `hairPositionsLast'
//      and `hairWidths') and the collision objects associated with the
//      hairSystem. If a collision is detected, this routine should adjust
//      `hairPositions' to compensate. The `hairPositionsLast' can also be
//      modified to adjust the velocity, but this should only be a dampening
//      effect as the hair solver expects collisions to be dampened,
//
//              This method is invoked often (actually its once per hair times the
//      hairSystem shape's iterations factor). Thus with 10,000 follicles it
//      would be called 80,000 times per frame (Note: as of Maya 7.0, the
//      iterations factor is multipled by 2, so at its default value of 4, you
//      get 8x calls. However, if you set iterations=0, it clamps to 1x calls).
//
// Parameters:
//              MObject hairSystem      : (in)  The hair system shape node.
//              int             follicleIndex:(in)      Which follicle we are processing.
//                                                                      You can get the follicle node if you
//                                                                      wish via MHairSystem::getFollicle().
//              MVectorArray &hairPositions:
//                                                        (mod) Array of locations in world space
//                                                                      where the hair system is trying to
//                                                                      move the follicle. The first entry
//                                                                      corresponds to the root of the hair
//                                                                      and the last entry to the tip. If a
//                                                                      collision is detected, these values
//                                                                      should be updated appropriately.
//                                                                      Note that hairPositions can change
//                                                                      from iteration to iteration on the
//                                                                      same hair and same frame. You can set
//                                                                      a position, and then find the hair has
//                                                                      moved a bit the next iteration. There
//                                                                      are two reasons for this phenomenom:
//                                                                        1) Other collisions could occur,
//                                                                               including self collision.
//                                                                        2) Stiffness is actually applied PER
//                                                                               ITERATION.
//              MVectorArray &hairPositionsLast:
//                                                        (mod) Array of the position at the previous
//                                                                      time for each entry in `hairPositions'.
//              MDoubleArray &hairWidths:
//                                                        (in)  Array of widths, representing the
//                                                                      width of the follcle at each entry in
//                                                                      `hairPositions'.
//              int             startIndex      : (in)  First index in `hairPositions' we
//                                                                      can move. This will be 0 unless the
//                                                                      root is locked in which case it will
//                                                                      be 1.
//              int             endIndex        : (in)  Last index in `hairPositions' we can
//                                                                      move. Will be the full array (N-1)
//                                                                      unless the tip is locked.
//              double  curTime         : (in)  Start of current time interval.
//              double  friction        : (in)  Frictional coefficient.
//              void    *privateData: (in)      If a privateData record was returned
//                                                                      from preFrame() it will be passed in
//                                                                      here. This is an optimisation to save
//                                                                      expensive lookups of your private data
//                                                                      if stored on the node.
//
// Returns:
//              bool    true            : Successfully performed collision testing and
//                                                        adjustment of the hair array data.
//              bool    false           : An error occurred.
//


#define EPSILON 0.0001

bool    collide(
                                const MObject           hairSystem,
                                const int                       follicleIndex,
                                MVectorArray            &hairPositions,
                                MVectorArray            &hairPositionsLast,
                                const MDoubleArray      &hairWidths,
                                const int                       startIndex,
                                const int                       endIndex,
                                const double            curTime,
                                const double            friction,
                                const void                      *privateData )
{
        // Get the private data for the collision objects which was returned
        // from preFrame().
        //
        COLLISION_INFO *ci = (COLLISION_INFO *) privateData;
        if ( !ci ) {
                fprintf( stderr,"%s:%d: collide() privateData pointer is NULL\n",
                                __FILE__, __LINE__ );
                return( false );
        }

        // If object has no vertices, or hair has no segments, then there is
        // nothing to collide. In our example, we'll return here, but if you
        // want to implement your own hair processing such as smoothing or
        // freeze on the hair, you could proceed and let the processing happen
        // after the object loop so that the data gets processed even if no
        // collisions occur.
        //
        if ( ci->numObjs <= 0 || hairPositions.length() <= 0 ) {
                return( true );
        }

        int             obj;
        for ( obj = 0; obj < ci->numObjs; ++obj ) {
                COLLISION_OBJ *co = &ci->objs[obj];

                // For our plug-in example, we just collide the segments of the
                // hair against the top of the bounding box for the geometry.
                // In an implementation where you only care about hair falling
                // downwards onto flat objects, this might be OK. However, in the
                // most deluxe of implementation, you should do the following:
                //
                //              o  Determine the motion of each face of your collision
                //                 object during the time range.
                //              o  Step through the follicle, and consider each segment
                //                 to be a moving cylinder where the start and end radii
                //                 can differ. The radii come from `hairWidths' and the
                //                 motion comes from the difference between `hairPositions'
                //                 and `hairPositionsLast'.
                //              o  Intersect each moving element (e.g. moving triangle
                //                 if you have a mesh obect) with each hair segment
                //                 e.g. moving cylinder). This intersection may occur
                //                 at a point within the frame. (Remember that the
                //                 hairPositions[] array holds the DESIRED location where
                //                 the hair system wants the hair to go.
                //              o  There can be multiple collisions along a hair segment.
                //                 Use the first location found and the maximal velocity.
                //
                // If a collision is detected, the `hairPositions' array should be
                // updated. `hairPositionsLast' may also be updated to provide a
                // dampening effect only.
                //

                // Loop through the follicle, starting at the root and advancing
                // toward the tip.
                //
                int             numSegments = hairPositions.length() - 1;
                int             seg;
                for ( seg = 0; seg < numSegments; ++seg ) {
                        // Get the desired position of the segment (i.e. where the
                        // solver is trying to put the hair for the current frame)
                        // and the velocity required to get to that desired position.
                        //
                        // Thus,
                        //              P = hairPositions               // Desired pos'n at cur frame.
                        //              L = hairPositionsLast   // Position at prev frame.
                        //              V = P - L                               // Desired velocity of hair.
                        //
                        MVector pStart = hairPositions[seg];
                        MVector pEnd = hairPositions[seg + 1];

                        MVector vStart = pStart - hairPositionsLast[seg];
                        MVector vEnd = pEnd - hairPositionsLast[seg + 1];

                        // The proper way to time sample is to intersect the moving
                        // segment of width `hairWidths[seg] to hairWidths[seg + 1]'
                        // with the moving collision object. For the purposes of our
                        // demo plug-in, we will simply assume the collision object is
                        // static, the hair segment has zero width, and instead of
                        // intersecting continuously in time, we will sample discrete
                        // steps along the segment.
                        //
                        #define STEPS 4
                        int             step;
                        for ( step = 0; step < STEPS; ++step ) {
                                // Compute the point for this step and its corresponding
                                // velocity. This is a "time swept" point:
                                //      p1 = desired position at current time
                                //      p0 = position at previous time to achieve desired pos
                                //
                                MVector pCur, pPrev, v;
                                double fracAlongSeg = step / ( (double) STEPS );
                                v = vStart * ( 1.0 - fracAlongSeg ) + vEnd * fracAlongSeg;
                                MVector p1 = pStart * ( 1.0 - fracAlongSeg )
                                                + pEnd * fracAlongSeg;
                                MVector p0 = p1 - v;

                                // See if BOTH endpoints are outside of the bounding box
                                // on the same side. If so, then the segment cannot
                                // intersect the bounding box. Note that we assume the
                                // bounding box is static.
                                //
                                if (       p0.x < co->minx && p1.x < co->minx
                                                || p0.y < co->miny && p1.y < co->miny
                                                || p0.z < co->minz && p1.z < co->minz
                                                || p0.x > co->maxx && p1.x > co->maxx
                                                || p0.y > co->maxy && p1.y > co->maxy
                                                || p0.z > co->maxz && p1.z > co->maxz ) {
                                        continue;
                                }

                                // For the purposes of this example plug-in, we'll assume
                                // the hair always moves downwards (due to gravity and
                                // thus in the negative Y direction). As such, we only
                                // need to test for collisions with the TOP of the
                                // bounding box. Expanding the example to all 6 sides is
                                // left as an exercise for the reader.
                                //
                                // Remember that p1 is the point at current time, and
                                // p0 is the point at the previous time. Since we assume
                                // the bounding box to be static, this simplifies things.
                                //
                                MVector where(-100000,-100000,-100000); // Loc'n of collision
                                double  fracTime;       // Time at which collision happens 0..1

                                if ( fabs( v.y ) < EPSILON              // velocity is zero
                                                && fabs( p1.y - co->maxy ) < EPSILON ) {        // right on the bbox
                                        // Velocity is zero and the desired location (p1) is
                                        // right on top of the bounding box.
                                        //
                                        where = p1;
                                        fracTime = 1.0;         // Collides right at end;
                                } else {
                                        fracTime = ( co->maxy -  p0.y ) / v.y;
                                        if ( fracTime >= 0.0 && fracTime <= 1.0 ) {
                                                // Compute the collision of the swept point with the
                                                // plane defined by the top of the bounding box.
                                                //
                                                where = p0 + v * fracTime;
                                        }
                                }

                                // If `seg' lies between startIndex and endIndex
                                // we can move it. If its <= startIndex, the root is
                                // locked and if >= endIndex the tip is locked.
                                //
                                if ( seg >= startIndex && seg <= endIndex ) {
                                        // Since we are just colliding with the top of the
                                        // bounding box, the normal where we hit is (0,1,0).
                                        // For the object velocity, we SHOULD measure the
                                        // relative motion of the object during the time
                                        // interval, but for our example, we'll assume its
                                        // not moving (0,0,0).
                                        //
                                        bool segCollides = false;
                                        MVector normal(0.0,1.0,0.0);    // normal is always up
                                        MVector objectVel(0.0,0.0,0.0); // assume bbox is static

                                        // If we get the this point, then the intersection
                                        // point `where' is on the plane of the bounding box
                                        // top. See if it lies within the actual bounds of the
                                        // boxtop, and if so compute the position and velocity
                                        // information and apply to the hair segment.
                                        //
                                        if ( where.x >= co->minx && where.x <= co->maxx
                                                        && where.z >= co->minz && where.z <= co->maxz
                                                        && fracTime >= 0.0 && fracTime <= 0.0 ) {
                                                // We have a collision at `where' with the plane.
                                                // Compute the new velocity for the hair at the
                                                // point of collision.
                                                //
                                                MVector objVelAlongNormal = (objectVel * normal)
                                                                * normal;
                                                MVector objVelAlongTangent = objectVel
                                                                - objVelAlongNormal;
                                                MVector pntVelAlongTangent = v - ( v * normal )
                                                                * normal;
                                                MVector reflectedPntVelAlongTangent
                                                                = pntVelAlongTangent * ( 1.0 - friction )
                                                                + objVelAlongTangent * friction;
                                                MVector newVel = objVelAlongNormal
                                                                + reflectedPntVelAlongTangent;

                                                // Update the hair position. It actually looks
                                                // more stable from a simulation standpoint to
                                                // just move the closest segment endpoint, but
                                                // you are free to experiment. What we'll do in
                                                // our example is move the closest segment end-
                                                // point by the amount the collided point (where)
                                                // has to move to have velocity `newVel' yet still
                                                // pass through `where' at time `fracTime'.
                                                //
                                                if ( fracAlongSeg > 0.5 ) {
                                                        MVector deltaPos = -newVel * fracTime;
                                                        hairPositionsLast[seg + 1] += deltaPos;
                                                        hairPositions[seg + 1] = hairPositionsLast[seg]
                                                                        + newVel;
                                                } else {
                                                        MVector deltaPos = newVel * fracTime;
                                                        hairPositionsLast[seg] += deltaPos;
                                                        hairPositions[seg] = hairPositionsLast[seg]
                                                                        + newVel;
                                                }
                                                segCollides = true;
                                        }

                    // Check for segment endpoints that may still be
                                        // inside. Note that segments which started out being
                                        // totally inside will never collide using the
                                        // algorithm we use above, so we'll simply clamp them
                                        // here. One might expect an inside-the-bounding-box
                                        // test instead. However, this does not work if the
                                        // collision object is a very thin object.
                                        //
                                        bool inside = false;
                    if ( belowCollisionPlane( co, hairPositions[seg] ) ) {
                                                hairPositions[seg].y = co->maxy + EPSILON;
                                                hairPositionsLast[seg] = hairPositions[seg] - objectVel;
                                                inside = true;
                    }
                    if ( belowCollisionPlane( co, hairPositions[seg+1] ) ) {
                                                hairPositions[seg+1].y = co->maxy + EPSILON;
                                                hairPositionsLast[seg+1] = hairPositions[seg+1] - objectVel;
                                                inside = true;
                    }

                                        // If we collided, go onto the next segment.
                                        //
                                        if ( segCollides || inside ) {
                                                goto nextSeg;
                                        }
                                }
                        }
nextSeg:;
        }
        }

        // You could perform any global filtering that you want on the hair
        // right here. For example: smoothing. This code is independent of
        // whether or not a collision occurred, and note that collide() is
        // called even with zero collision objects, so you will be guaranteed
        // of reaching here once per hair per iteration.
        //

        return( true );
}

//
// Synopsis:
//              MStatus initializePlugin( MObject obj )
//
// Description:
//              Invoked upon plug-in load to register the plug-in and initialise.
//
// Parameters:
//              MObject obj                     : (in)  Plug-in object being loaded.
//
// Returns:
//              MStatus::kSuccess       : Successfully performed any needed initial-
//                                                        isation for the plug-in.
//              MStatus statusCode      : Error was detected.
//


MStatus initializePlugin( MObject obj )
{
        MFnPlugin       plugin( obj, "Autodesk", "8.0", "Any" );

        CHECK_MSTATUS( MHairSystem::registerCollisionSolverCollide( collide ) );
        CHECK_MSTATUS( MHairSystem::registerCollisionSolverPreFrame( preFrame ) );

        return( MS::kSuccess );
}

//
// Synopsis:
//              MStatus uninitializePlugin( MObject obj )
//
// Description:
//              Invoked upon plug-in unload to deregister the plug-in and clean up.
//
// Parameters:
//              MObject obj                     : (in)  Plug-in object being unloaded.
//
// Returns:
//              MStatus::kSuccess       : Successfully performed any needed cleanup.
//              MStatus statusCode      : Error was detected.
//


MStatus uninitializePlugin( MObject obj )
{
        MFnPlugin       plugin( obj );

        CHECK_MSTATUS( MHairSystem::unregisterCollisionSolverCollide() );
        CHECK_MSTATUS( MHairSystem::unregisterCollisionSolverPreFrame() );

        return( MS::kSuccess );
}

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