simpleFluidEmitter/simpleFluidEmitter.cpp

//-
// ==========================================================================
// Copyright 2015 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.
// ==========================================================================
//+

// DESCRIPTION:
// 
// Produces the dependency graph node "simpleFluidEmitter".
//
// This example demonstrates how to use the new MPxFluidEmitterNode class
// to implement most of the functionality in Maya's standard fluid emitters. 
//
// MEL usage:
//  
//  createNode simpleFluidEmitter -name simpleFluidEmitter;
//  // create particle object and connect to the plugin emitter node.
//  particle -name particles ;
//  connectDynamic -em simpleFluidEmitter particles;
//  setAttr "simpleFluidEmitter.rate" 200;
//  setAttr "simpleFluidEmitter.speed" 25;
//  playbackOptions -e -min 0.00 -max 60.0;
//  currentTime -e 0;
//  play -wait -forward true;
//  // make some keyframes on emitter
//  currentTime 0 ;
//  select -r simpleFluidEmitter ;
//  setKeyframe "simpleFluidEmitter.tx";
//  setKeyframe "simpleFluidEmitter.ty";
//  setKeyframe "simpleFluidEmitter.tz";
//  setKeyframe "simpleFluidEmitter.rx";
//  setKeyframe "simpleFluidEmitter.ry";
//  setKeyframe "simpleFluidEmitter.rz";
//  currentTime 30 ;
//  move -r -2.011944 6.283524 -2.668834 ;
//  move -r -ls -wd 0 0 12.97635 ;
//  rotate -r -os 0 -75.139762 0 ;
//  setKeyframe "simpleFluidEmitter.tx";
//  setKeyframe "simpleFluidEmitter.ty";
//  setKeyframe "simpleFluidEmitter.tz";
//  setKeyframe "simpleFluidEmitter.rx";
//  setKeyframe "simpleFluidEmitter.ry";
//  setKeyframe "simpleFluidEmitter.rz";
//  currentTime 60 ;
//  move -r 0 0 -14.526107 ;
//  move -r 0 -8.130523 0 ;
//  rotate -r -os 0 0 78.039751 ;
//  rotate -r -os 0 0 53.86918 ;
//  setKeyframe "simpleFluidEmitter.tx";
//  setKeyframe "simpleFluidEmitter.ty";
//  setKeyframe "simpleFluidEmitter.tz";
//  setKeyframe "simpleFluidEmitter.rx";
//  setKeyframe "simpleFluidEmitter.ry";
//  setKeyframe "simpleFluidEmitter.rz";
//

#include <maya/MIOStream.h>
#include <math.h>
#include <stdlib.h>

#include "simpleFluidEmitter.h"

#include <maya/MVectorArray.h>
#include <maya/MDoubleArray.h>
#include <maya/MIntArray.h>
#include <maya/MMatrix.h>

#include <maya/MFnDependencyNode.h>
#include <maya/MFnNumericAttribute.h>
#include <maya/MFnUnitAttribute.h>
#include <maya/MFnVectorArrayData.h>
#include <maya/MFnDoubleArrayData.h>
#include <maya/MFnArrayAttrsData.h>
#include <maya/MFnMatrixData.h>

#include <maya/MDagPath.h>
#include <maya/MMatrix.h>
#include <maya/MTransformationMatrix.h>
#include <maya/MFnDynSweptGeometryData.h>
#include <maya/MDynSweptTriangle.h>
#include <maya/MPlugArray.h>


MTypeId simpleFluidEmitter::id( 0x81020 );

simpleFluidEmitter::simpleFluidEmitter()
{
}


simpleFluidEmitter::~simpleFluidEmitter()
{
}


void *simpleFluidEmitter::creator()
{
    return new simpleFluidEmitter;
}


MStatus simpleFluidEmitter::initialize()
//
//  Descriptions:
//      Initialize the node, create user defined attributes.
//
{
    return( MS::kSuccess );
}


MStatus simpleFluidEmitter::compute(const MPlug& plug, MDataBlock& block)
//
//  Description:
//
//      Fluid emitters do not perform emission in their compute() method.
//      Instead, each fluid to which the emitter is connected will call the
//      fluidEmitter() method once per frame, to allow the emitter to emit
//      directly into the fluid.
//
//      It is ESSENTIAL that the compute routine return MS::kUnknownParameter
//      when the "emissionFunction" attribute is being evaluated.  Doing so
//      will trigger the base class default compute() method, which will
//      register this node's "fluidEmitter" function with the fluid.  The
//      mechanisms for doing this are not exposed through the API, so it is
//      important to let the default code handle this case.
//
//      For all other attributes, users can override the compute() method.
//
{
    if( plug.attribute() == mEmissionFunction )
    {
        //  ESSENTIAL!  Let the base class default compute method handle this
        //
        return MS::kUnknownParameter;
    }
    else
    {
        //  can add custom handling for other attributes here
        //
        return MS::kUnknownParameter;
    }
}


MStatus 
simpleFluidEmitter::fluidEmitter( 
    const MObject& fluidObject, 
    const MMatrix& worldMatrix, 
    int plugIndex 
)
//==============================================================================
//
//  Description:
//
//      Callback function that gets called once per frame by each fluid
//      into which this emitter is emitting.  Emits values directly into
//      the fluid object.  The MFnFluid object passed to this routine is
//      not pointing to a DAG object, it is pointing to an internal fluid
//      data structure that the fluid node is constructing, eventually to
//      be set into the fluid's output attribute.
//
//  Parameters:
//
//      fluid:          fluid into which we are emitting
//      worldMatrix:    object->world matrix for the fluid
//      plugIndex:      identifies which fluid connected to the emitter
//                      we are emitting into
//
//  Returns:
//
//      MS::kSuccess            if the method wishes to override the default
//                              emitter behaviour
//      MS::kUnknownParameter   if the method wishes to have the default
//                              emitter behaviour execute after this routine
//                              exits.
//
//  Notes:
//
//      The method first does some work common to all emitter types, then
//      calls one of 4 different methods to actually do the emission.
//      The methods are:
//
//          omniEmitter:    omni-directional emitter from a point,
//                          or from the vertices of an owner object.
//
//          volumeEmitter:  emits from the surface of an exact cube, sphere,
//                          cone, cylinder, or torus.
//
//          surfaceEmitter: emits from the surface of an owner object.
//
//==============================================================================
{
    //  make sure the fluid is valid.  If it isn't, return MS::kSuccess, indicating
    //  that no work needs to be done.  If we return a failure code, then the default
    //  fluid emitter code will try to run, which is pointless if the fluid is not
    //  valid.
    //
    MFnFluid fluid( fluidObject );
    if( fluid.object() != MObject::kNullObj )
    {
        return MS::kSuccess;
    }

    //  get a data block for the emitter, so we can get attribute values
    //
    MDataBlock block = forceCache();

    //  figure out the time interval for emission for the given fluid
    //
    double dTime = getDeltaTime( plugIndex, block ).as(MTime::kSeconds);
    if( dTime == 0.0 )
    {
        // shouldn't happen, but if the time interval is 0, then no fluid should
        // be emitted
        return MS::kSuccess;
    }

    //  if currentTime <= startTime, return. The startTime is connected to 
    //  the target fluid object.
    //
    MTime cTime = getCurrentTime( block );
    MTime sTime = getStartTime( plugIndex, block );

    //  if we are at or before the start time, reset the random number
    //  state to the appropriate seed value for the given fluid
    //
    if( cTime < sTime )
    {
        resetRandomState( plugIndex, block );
        return MS::kSuccess;
    }

    //  check to see if we need to emit anything into the target fluid.
    //  if the emission rate is 0, or if the fluid doesn't have a grid
    //  for one of the quantities, then we needn't do any emission
    //
    
    //  emission rates
    double density = fluidDensityEmission( block ); 
    double heat = fluidHeatEmission( block );   
    double fuel = fluidFuelEmission( block );   
    bool doColor = fluidEmitColor( block ); 

    //  fluid grid settings
    MFnFluid::FluidMethod densityMode, tempMode, fuelMode;
    MFnFluid::ColorMethod colorMode;
    MFnFluid::FluidGradient grad;
    MFnFluid::FalloffMethod falloffMode;
    fluid.getDensityMode( densityMode, grad );
    fluid.getTemperatureMode( tempMode, grad );
    fluid.getFuelMode( fuelMode, grad );
    fluid.getColorMode( colorMode );
    fluid.getFalloffMode( falloffMode );

    //  see if we need to emit density, heat, fuel, or color
    bool densityToEmit = (density != 0.0) && ((densityMode == MFnFluid::kDynamicGrid)||(densityMode == MFnFluid::kStaticGrid));
    bool heatToEmit = (heat != 0.0) && ((tempMode == MFnFluid::kDynamicGrid)||(tempMode == MFnFluid::kStaticGrid));
    bool fuelToEmit = (fuel != 0.0) && ((fuelMode == MFnFluid::kDynamicGrid)||(fuelMode == MFnFluid::kStaticGrid));
    bool colorToEmit = doColor && ((colorMode == MFnFluid::kDynamicColorGrid)||(colorMode == MFnFluid::kStaticColorGrid));
    bool falloffEmit = (falloffMode == MFnFluid::kStaticFalloffGrid);

    //  nothing to emit, do nothing
    //
    if( !densityToEmit && !heatToEmit && !fuelToEmit && !colorToEmit && !falloffEmit )
    {
        return MS::kSuccess;
    }

    //  get the dropoff rate for the fluid 
    //  
    double dropoff = fluidDropoff( block );

    //  modify the dropoff rate to account for fluids that have
    //  been scaled in worldspace - larger scales mean slower
    //  falloffs and vice versa
    //
    MTransformationMatrix xform( worldMatrix );
    double xformScale[3];
    xform.getScale( xformScale, MSpace::kWorld );
    double dropoffScale = sqrt( xformScale[0]*xformScale[0] + 
                                xformScale[1]*xformScale[1] + 
                                xformScale[2]*xformScale[2] );
    if( dropoffScale > 0.1 )
    {
        dropoff /= dropoffScale;
    }
    
    //  retrieve the current random state from the "randState" attribute, and
    //  store it in the member variable "randState".  We will use this member 
    //  value numerous times via the randgen() method.  Once we are done emitting,
    //  we will set the random state back into the attribute via setRandomState().
    //
    getRandomState( plugIndex, block );

    //  conversion value used to map user input emission rates into internal
    //  values.
    //
    double conversion = 0.01;

    MEmitterType emitterType = getEmitterType( block );
    switch( emitterType )
    {
        case kOmni:
            omniFluidEmitter( fluid, worldMatrix, plugIndex, block, dTime,
                              conversion, dropoff );
            break;

        case kVolume:
            volumeFluidEmitter( fluid, worldMatrix, plugIndex, block, dTime,
                                conversion, dropoff );
            break;
            
        case kSurface:
            surfaceFluidEmitter( fluid, worldMatrix, plugIndex, block, dTime,
                                 conversion, dropoff );
            break;

        default:
            break;
    }

    //  store the random state back into the datablock
    //  
    setRandomState( plugIndex, block );
    return MS::kSuccess;
}

#ifdef MIN
#undef MIN
#endif

#ifdef MAX
#undef MAX
#endif

#define MIN(x,y) ((x)<(y)?(x):(y))
#define MAX(x,y) ((x)>(y)?(x):(y))

void 
simpleFluidEmitter::omniFluidEmitter(
    MFnFluid&       fluid,
    const MMatrix&  fluidWorldMatrix,
    int             plugIndex,
    MDataBlock&     block,
    double          dt,
    double          conversion,
    double          dropoff
)
//==============================================================================
//
//  Method: 
//
//      simpleFluidEmitter::omniFluidEmitter
//
//  Description:
//
//      Emits fluid from a point, or from a set of object control points.
//
//  Parameters:
//
//      fluid:              fluid into which we are emitting
//      fluidWorldMatrix:   object->world matrix for the fluid
//      plugIndex:          identifies which fluid connected to the emitter
//                          we are emitting into
//      block:              datablock for the emitter, to retrieve attribute
//                          values
//      dt:                 time delta for this frame
//      conversion:         mapping from UI emission rates to internal units
//      dropoff:            specifies how much emission rate drops off as
//                          we move away from each emission point.
//
//  Notes:
//
//      If no owner object is present for the emitter, we simply emit from
//      the emitter position.  If an owner object is present, then we emit
//      from each control point of that object in an identical fashion.
//      
//      To associate an owner object with an emitter, use the
//      addDynamic MEL command, e.g. "addDynamic simpleFluidEmitter1 pPlane1".
//
//==============================================================================
{
    //  find the positions that we need to emit from
    //
    MVectorArray emitterPositions;
    
    //  first, try to get them from an owner object, which will have its
    //  "ownerPositionData" attribute feeding into the emitter.  These
    //  values are in worldspace
    //
    bool gotOwnerPositions = false;
    MObject ownerShape = getOwnerShape();
    if( ownerShape != MObject::kNullObj )
    {
        MStatus status;
        MDataHandle hOwnerPos = block.inputValue( mOwnerPosData, &status );
        if( status == MS::kSuccess )
        {
            MObject dOwnerPos = hOwnerPos.data();
            MFnVectorArrayData fnOwnerPos( dOwnerPos );
            MVectorArray posArray = fnOwnerPos.array( &status );
            if( status == MS::kSuccess )
            {
                // assign vectors from block to ownerPosArray.
                //
                for( unsigned int i = 0; i < posArray.length(); i ++ )
                {
                    emitterPositions.append( posArray[i] );
                }
                
                gotOwnerPositions = true;
            }
        }
    }
    
    //  there was no owner object, so we just use the emitter position for
    //  emission.
    //
    if( !gotOwnerPositions )
    {
        MPoint emitterPos = getWorldPosition();
        emitterPositions.append( emitterPos );
    }

    //  get emission rates for density, fuel, heat, and emission color
    //  
    double densityEmit = fluidDensityEmission( block );
    double fuelEmit = fluidFuelEmission( block );
    double heatEmit = fluidHeatEmission( block );
    bool doEmitColor = fluidEmitColor( block );
    MColor emitColor = fluidColor( block );

    //  rate modulation based on frame time, user value conversion factor, and
    //  standard emitter "rate" value (not actually exposed in most fluid
    //  emitters, but there anyway).
    //
    double theRate = getRate(block) * dt * conversion;

    //  get voxel dimensions and sizes (object space)
    //
    double size[3];
    unsigned int res[3];
    fluid.getDimensions( size[0], size[1], size[2] );
    fluid.getResolution( res[0], res[1], res[2] );
    
    //  voxel sizes
    double dx = size[0] / res[0];
    double dy = size[1] / res[1];
    double dz = size[2] / res[2];
    
    //  voxel centers
    double Ox = -size[0]/2;
    double Oy = -size[1]/2;
    double Oz = -size[2]/2; 

    //  emission will only happen for voxels whose centers lie within
    //  "minDist" and "maxDist" of an emitter position
    //
    double minDist = getMinDistance( block );
    double maxDist = getMaxDistance( block );

    //  bump up the min/max distance values so that they
    //  are both > 0, and there is at least about a half
    //  voxel between the min and max values, to prevent aliasing
    //  artifacts caused by emitters missing most voxel centers
    //
    MTransformationMatrix fluidXform( fluidWorldMatrix );
    double fluidScale[3];
    fluidXform.getScale( fluidScale, MSpace::kWorld );
    
    //  compute smallest voxel diagonal length
    double wsX =  fabs(fluidScale[0]*dx);
    double wsY = fabs(fluidScale[1]*dy);
    double wsZ = fabs(fluidScale[2]*dz);
    double wsMin = MIN( MIN( wsX, wsY), wsZ );
    double wsMax = MAX( MAX( wsX, wsY), wsZ );
    double wsDiag  = wsMin * sqrt(3.0);

    //  make sure emission range is bigger than 0.5 voxels
    if ( maxDist <= minDist || maxDist <= (wsDiag/2.0) ) {
        if ( minDist < 0 ) minDist = 0;

        maxDist = minDist + wsDiag/2.0;
        dropoff = 0;
    }

    //  Now, it's time to actually emit into the fluid:
    //  
    //  foreach emitter point
    //      foreach voxel
    //          - select some points in the voxel
    //          - compute a dropoff function from the emitter point
    //          - emit an appropriate amount of fluid into the voxel
    //
    //  Since we've already expanded the min/max distances to cover
    //  the smallest voxel dimension, we should only need 1 sample per
    //  voxel, unless the voxels are highly non-square.  We increase the
    //  number of samples in these cases.
    //
    //  If the "jitter" flag is enabled, we jitter each sample position,
    //  using the rangen() function, which keeps track of independent 
    //  random states for each fluid, to make sure that results are
    //  repeatable for multiple simulation runs.
    //  

    // basic sample count
    int numSamples = 1;

    // increase samples if necessary for non-square voxels
    if(wsMin >.00001) 
    {
        numSamples = (int)(wsMax/wsMin + .5);
        if(numSamples > 8) 
            numSamples = 8;
        if(numSamples < 1)
            numSamples = 1;
    }
    
    bool jitter =  fluidJitter(block);
    if( !jitter )
    {
        //  I don't have a good uniform sample generator for an 
        //  arbitrary number of samples.  It would be a good idea to use
        //  one here.  For now, just use 1 sample for the non-jittered case.
        //
        numSamples = 1;
    }

    for( unsigned int p = 0; p < emitterPositions.length(); p++ )
    {
        MPoint emitterWorldPos = emitterPositions[p];

        //  loop through all voxels, looking for ones that lie at least
        //  partially within the dropoff field around this emitter point
        //
        for( unsigned int i = 0; i < res[0]; i++ )
        {
            double x = Ox + i*dx;
            
            for( unsigned int j = 0; j < res[1]; j++ )
            {
                double y = Oy + j*dy;
                
                for( unsigned int k = 0; k < res[2]; k++ )
                {
                    double z = Oz + k*dz;
    
                    int si;
                    for( si = 0; si < numSamples; si++ )
                    {
                        //  compute sample point (fluid object space)
                        //
                        double rx, ry, rz;
                        if( jitter )
                        {
                            rx = x + randgen()*dx;
                            ry = y + randgen()*dy;
                            rz = z + randgen()*dz;
                        }
                        else
                        {
                            rx = x + 0.5*dx;
                            ry = y + 0.5*dy;
                            rz = z + 0.5*dz;
                        }

                        //  compute distance from sample to emitter point
                        //  
                        MPoint point( rx, ry, rz );
                        point *= fluidWorldMatrix;
                        MVector diff = point - emitterWorldPos;
                        double distSquared = diff * diff;
                        double dist = diff.length();
                    
                        //  discard if outside min/max range
                        //
                        if( (dist < minDist) || (dist > maxDist) )
                        {
                            continue;
                        }
                        
                        //  drop off the emission rate according to the falloff
                        //  parameter, and divide to accound for multiple samples
                        //  in the voxel
                        //
                        double distDrop = dropoff * distSquared;
                        double newVal = theRate * exp( -distDrop ) / (double)numSamples;

                        //  emit density/heat/fuel/color into the current voxel
                        //
                        if( newVal != 0 )
                        {
                            fluid.emitIntoArrays( (float) newVal, i, j, k, (float)densityEmit, (float)heatEmit, (float)fuelEmit, doEmitColor, emitColor );
                        }

                        float *fArray = fluid.falloff();
                        if( fArray != NULL )
                        {
                            MPoint midPoint( x+0.5*dx, y+0.5*dy, z+0.5*dz );
                            midPoint.x *= 0.2;
                            midPoint.y *= 0.2;
                            midPoint.z *= 0.2;

                            float fdist = (float) sqrt( midPoint.x*midPoint.x + midPoint.y*midPoint.y + midPoint.z*midPoint.z );
                            fdist /= sqrtf(3.0f);
                            fArray[fluid.index(i,j,k)] = 1.0f-fdist;
                        }
                    }
                }
            }
        }
    }
}

void 
simpleFluidEmitter::volumeFluidEmitter(
    MFnFluid&       fluid,
    const MMatrix&  fluidWorldMatrix,
    int             plugIndex,
    MDataBlock&     block,
    double          dt,
    double          conversion,
    double          dropoff
)
//==============================================================================
//
//  Method: 
//
//      simpleFluidEmitter::volumeFluidEmitter
//
//  Description:
//
//      Emits fluid from points distributed over the surface of the 
//      emitter's owner object.
//
//  Parameters:
//
//      fluid:              fluid into which we are emitting
//      fluidWorldMatrix:   object->world matrix for the fluid
//      plugIndex:          identifies which fluid connected to the emitter
//                          we are emitting into
//      block:              datablock for the emitter, to retrieve attribute
//                          values
//      dt:                 time delta for this frame
//      conversion:         mapping from UI emission rates to internal units
//      dropoff:            specifies how much emission rate drops off as
//                          we move away from the local y-axis of the 
//                          volume emitter shape.
//
//==============================================================================
{
    //  get emitter position and relevant matrices 
    //  
    MPoint emitterPos = getWorldPosition();
    MMatrix emitterWorldMatrix = getWorldMatrix();
    MMatrix fluidInverseWorldMatrix = fluidWorldMatrix.inverse();
    
    //  get emission rates for density, fuel, heat, and emission color
    //  
    double densityEmit = fluidDensityEmission( block );
    double fuelEmit = fluidFuelEmission( block );
    double heatEmit = fluidHeatEmission( block );
    bool doEmitColor = fluidEmitColor( block );
    MColor emitColor = fluidColor( block );
    
    //  rate modulation based on frame time, user value conversion factor, and
    //  standard emitter "rate" value (not actually exposed in most fluid
    //  emitters, but there anyway).
    //
    double theRate = getRate(block) * dt * conversion;
    
    //  find the voxels that intersect the bounding box of the volume
    //  primitive associated with the emitter
    //
    MBoundingBox bbox;
    if( !volumePrimitiveBoundingBox( bbox ) )
    {
        //  shouldn't happen
        //
        return;
    }
    
    //  transform volume primitive into fluid space
    //
    bbox.transformUsing( emitterWorldMatrix );
    bbox.transformUsing( fluidInverseWorldMatrix );
    MPoint lowCorner = bbox.min();
    MPoint highCorner = bbox.max();

    // see if autoresize to emitter is on, so we can resize before emitting
    // 
    // unfortunately, we need the fluidShape itself in order to check
    // we'll just look at the first connected fluid,
    // walking the multi is left as an excercise for the reader
    // similarly, determining whether there is start frame emission
    // and resizing at the start frame, or not, is left as an excercise
    // and finally, emission with autoResize will need to include the dynamicOffset
    // 
    MObject         thisObj = thisMObject();
    MFnDependencyNode   nodeFn(thisObj);
    
    bool autoResize = false;
    bool resizeToEmitter = false;
    MPlug fnPlug = nodeFn.findPlug("emissionFunction", true);
    if(fnPlug.isConnected()) {
        MPlugArray connections;
        fnPlug.connectedTo(connections, false, true);
        if (connections.length() > 0) {
            MObject sourceNode = connections[0].node();
            if (sourceNode.hasFn(MFn::kFluid)) {
                MFnFluid fluidFn(sourceNode);
                autoResize = fluidFn.isAutoResize();
                resizeToEmitter =  fluidFn.isResizeToEmitter();
                if(autoResize && resizeToEmitter) {
                    fluidFn.expandToInclude(lowCorner, highCorner);
                }
            }
        }
    }
    if(autoResize && resizeToEmitter) {
        fluid.updateGrid ();
    }

    //  get voxel dimensions and sizes (object space)
    //
    double size[3];
    unsigned int res[3];
    fluid.getDimensions( size[0], size[1], size[2] );
    fluid.getResolution( res[0], res[1], res[2] );

    //  voxel sizes
    double dx = size[0] / res[0];
    double dy = size[1] / res[1];
    double dz = size[2] / res[2];
    
    //  voxel centers
    double Ox = -size[0]/2;
    double Oy = -size[1]/2;
    double Oz = -size[2]/2; 

    //  get fluid voxel coord range of bounding box
    //
    int3 lowCoords;
    int3 highCoords;
    fluid.toGridIndex( lowCorner, lowCoords );
    fluid.toGridIndex( highCorner, highCoords );
    
    int i;
    for ( i = 0; i < 3; i++ )
    {
        if ( lowCoords[i] < 0 ) {
            lowCoords[i] = 0;
        } else if ( lowCoords[i] > ((int)res[i])-1 ) {
            lowCoords[i] = ((int)res[i])-1;
        }

        if ( highCoords[i] < 0 ) {
            highCoords[i] = 0;
        } else if ( highCoords[i] > ((int)res[i])-1 ) {
            highCoords[i] = ((int)res[i])-1;
        }
        
    }

    //  figure out the emitter size relative to the voxel size, and compute
    //  a per-voxel sampling rate that uses 1 sample/voxel for emitters that
    //  are >= 2 voxels big in all dimensions.  For smaller emitters, use up
    //  to 8 samples per voxel.
    //
    double emitterVoxelSize[3];
    emitterVoxelSize[0] = (highCorner[0]-lowCorner[0])/dx;
    emitterVoxelSize[1] = (highCorner[1]-lowCorner[1])/dy;
    emitterVoxelSize[2] = (highCorner[2]-lowCorner[2])/dz;
        
    double minVoxelSize = MIN(emitterVoxelSize[0],MIN(emitterVoxelSize[1],emitterVoxelSize[2]));
    if( minVoxelSize < 1.0 )
    {
        minVoxelSize = 1.0;
    }
    int maxSamples = 8;
    int numSamples = (int)(8.0/(minVoxelSize*minVoxelSize*minVoxelSize) + 0.5);
    if( numSamples < 1 ) numSamples = 1;
    if( numSamples > maxSamples ) numSamples = maxSamples;
    
    //  non-jittered, just use one sample in the voxel center.  Should replace
    //  with uniform sampling pattern.
    //
    bool jitter = fluidJitter(block);
    if( !jitter )
    {
        numSamples = 1;
    }
    
    //  for each voxel that could potentially intersect the volume emitter
    //  primitive, take some samples in the voxel.  For those inside the
    //  volume, compute their dropoff relative to the primitive's local y-axis,
    //  and emit an appropriate amount into the voxel.
    //
    for( i = lowCoords[0]; i <= highCoords[0]; i++ )
    {
        double x = Ox + (i+0.5)*dx;
            
        for( int j = lowCoords[1]; j < highCoords[1]; j++ )
        {
            double y = Oy + (j+0.5)*dy;

            for( int k = lowCoords[2]; k < highCoords[2]; k++ )
            {
                double z = Oz + (k+0.5)*dz;
                
                for ( int si = 0; si < numSamples; si++) {
                    
                    //  compute voxel sample point (object space)
                    //
                    double rx, ry, rz;
                    if(jitter) {
                        rx = x + dx*(randgen() - 0.5);
                        ry = y + dy*(randgen() - 0.5);
                        rz = z + dz*(randgen() - 0.5);
                    } else {
                        rx = x;
                        ry = y;
                        rz = z;
                    }
                    
                    //  to world space
                    MPoint pt( rx, ry, rz );
                    pt *= fluidWorldMatrix;

                    //  test to see if point is inside volume primitive
                    //
                    if( volumePrimitivePointInside( pt, emitterWorldMatrix ) )
                    {
                        //  compute dropoff
                        //
                        double dist = pt.distanceTo( emitterPos );
                        double distDrop = dropoff * (dist*dist);
                        double newVal = (theRate * exp( -distDrop )) / (double)numSamples;
                        
                        //  emit into arrays
                        //
                        if( newVal != 0.0 )
                        {
                            fluid.emitIntoArrays( (float) newVal, i, j, k, (float)densityEmit, (float)heatEmit, (float)fuelEmit, doEmitColor, emitColor );
                        }
                    }
                }
            }
        }
    }
}

void 
simpleFluidEmitter::surfaceFluidEmitter(
    MFnFluid&       fluid,
    const MMatrix&  fluidWorldMatrix,
    int             plugIndex,
    MDataBlock&     block,
    double          dt,
    double          conversion,
    double          dropoff
)
//==============================================================================
//
//  Method: 
//
//      simpleFluidEmitter::surfaceFluidEmitter
//
//  Description:
//
//      Emits fluid from one of a predefined set of volumes (cube, sphere,
//      cylinder, cone, torus).
//
//  Parameters:
//
//      fluid:              fluid into which we are emitting
//      fluidWorldMatrix:   object->world matrix for the fluid
//      plugIndex:          identifies which fluid connected to the emitter
//                          we are emitting into
//      block:              datablock for the emitter, to retrieve attribute
//                          values
//      dt:                 time delta for this frame
//      conversion:         mapping from UI emission rates to internal units
//      dropoff:            specifies how much emission rate drops off as
//                          the surface points move away from the centers
//                          of the voxels in which they lie.
//
//  Notes:
//      
//      To associate an owner object with an emitter, use the
//      addDynamic MEL command, e.g. "addDynamic simpleFluidEmitter1 pPlane1".
//
//==============================================================================
{
    //  get relevant world matrices
    //
    MMatrix fluidInverseWorldMatrix = fluidWorldMatrix.inverse();
    
    //  get emission rates for density, fuel, heat, and emission color
    //  
    double densityEmit = fluidDensityEmission( block );
    double fuelEmit = fluidFuelEmission( block );
    double heatEmit = fluidHeatEmission( block );
    bool doEmitColor = fluidEmitColor( block );
    MColor emitColor = fluidColor( block );
    
    //  rate modulation based on frame time, user value conversion factor, and
    //  standard emitter "rate" value (not actually exposed in most fluid
    //  emitters, but there anyway).
    //
    double theRate = getRate(block) * dt * conversion;

    //  get voxel dimensions and sizes (object space)
    //
    double size[3];
    unsigned int res[3];
    fluid.getDimensions( size[0], size[1], size[2] );
    fluid.getResolution( res[0], res[1], res[2] );
        
    //  voxel sizes
    double dx = size[0] / res[0];
    double dy = size[1] / res[1];
    double dz = size[2] / res[2];
    
    //  voxel centers
    double Ox = -size[0]/2;
    double Oy = -size[1]/2;
    double Oz = -size[2]/2; 

    //  get the "swept geometry" data for the emitter surface.  This structure
    //  tracks the motion of each emitter triangle over the time interval
    //  for this simulation step.  We just use positions on the emitter
    //  surface at the end of the time step to do the emission.
    //
    MDataHandle sweptHandle = block.inputValue( mSweptGeometry );
    MObject sweptData = sweptHandle.data();
    MFnDynSweptGeometryData fnSweptData( sweptData );

    //  for "non-jittered" sampling, just reset the random state for each 
    //  triangle, which gives us a fixed set of samples all the time.
    //  Sure, they're still jittered, but they're all jittered the same,
    //  which makes them kinda uniform.
    //
    bool jitter = fluidJitter(block);
    if( !jitter )
    {
        resetRandomState( plugIndex, block );
    }

    if( fnSweptData.triangleCount() > 0 )
    {
        //  average voxel face area - use this as the canonical unit that
        //  receives the emission rate specified by the users.  Scale the
        //  rate for other triangles accordingly.
        //
        double vfArea = pow(dx*dy*dz, 2.0/3.0);
        
        //  very rudimentary support for textured emission rate and
        //  textured emission color.  We simply sample each texture once
        //  at the center of each emitter surface triangle.  This will 
        //  cause aliasing artifacts when these triangles are large.
        //
        MFnDependencyNode fnNode( thisMObject() );
        MObject rateTextureAttr = fnNode.attribute( "textureRate" );
        MObject colorTextureAttr = fnNode.attribute( "particleColor" );

        bool texturedRate = hasValidEmission2dTexture( rateTextureAttr );
        bool texturedColor = hasValidEmission2dTexture( colorTextureAttr );
        
        //  construct texture coordinates for each triangle center
        //  
        MDoubleArray uCoords, vCoords;
        if( texturedRate || texturedColor )
        {
            uCoords.setLength( fnSweptData.triangleCount() );
            vCoords.setLength( fnSweptData.triangleCount() );
            
            int t;
            for( t = 0; t < fnSweptData.triangleCount(); t++ )
            {
                MDynSweptTriangle tri = fnSweptData.sweptTriangle( t );
                MVector uv0 = tri.uvPoint(0);
                MVector uv1 = tri.uvPoint(1);
                MVector uv2 = tri.uvPoint(2);
                
                MVector uvMid = (uv0+uv1+uv2)/3.0;
                uCoords[t] = uvMid[0];
                vCoords[t] = uvMid[1];
            }
        }

        //  evaluate textured rate and color values at the triangle centers
        //
        MDoubleArray texturedRateValues;
        if( texturedRate )
        {
            texturedRateValues.setLength( uCoords.length() );
            evalEmission2dTexture( rateTextureAttr, uCoords, vCoords, NULL, &texturedRateValues );
        }
        
        MVectorArray texturedColorValues;
        if( texturedColor )
        {
            texturedColorValues.setLength( uCoords.length() );
            evalEmission2dTexture( colorTextureAttr, uCoords, vCoords, &texturedColorValues, NULL );
        }
        
        for( int t = 0; t < fnSweptData.triangleCount(); t++ )
        {
            //  calculate emission rate and color values for this triangle
            //
            double curTexturedRate = texturedRate ? texturedRateValues[t] : 1.0;
            MColor curTexturedColor;
            if( texturedColor )
            {
                MVector& curVec = texturedColorValues[t];
                curTexturedColor.r = (float)curVec[0];
                curTexturedColor.g = (float)curVec[1];
                curTexturedColor.b = (float)curVec[2];
                curTexturedColor.a = 1.0;
            }
            else
            {
                curTexturedColor = emitColor;
            }

            MDynSweptTriangle tri = fnSweptData.sweptTriangle( t );
            MVector v0 = tri.vertex(0);
            MVector v1 = tri.vertex(1);
            MVector v2 = tri.vertex(2);

            //  compute number of samples for this triangle based on area,
            //  with large triangles receiving approximately 1 sample for 
            //  each voxel that they intersect
            //
            double triArea = tri.area();
            int numSamples = (int)(triArea / vfArea);
            if( numSamples < 1 ) numSamples = 1;
            
            //  compute emission rate for the points on the triangle.
            //  Scale the canonical rate by the area ratio of this triangle
            //  to the average voxel size, then split it amongst all the samples.
            //
            double triRate = (theRate*(triArea/vfArea))/numSamples;
            
            triRate *= curTexturedRate;
            
            for( int j = 0; j < numSamples; j++ )
            {
                //  generate a random point on the triangle,
                //  map it into fluid local space
                //
                double r1 = randgen();
                double r2 = randgen();
                
                if( r1 + r2 > 1 )
                {
                    r1 = 1-r1;
                    r2 = 1-r2;
                }
                double r3 = 1 - (r1+r2);
                MPoint randPoint = r1*v0 + r2*v1 + r3*v2;
                randPoint *= fluidInverseWorldMatrix;
                
                //  figure out where the current point lies
                //
                int3 coord;
                fluid.toGridIndex( randPoint, coord );
                
                if( (coord[0]<0) || (coord[1]<0) || (coord[2]<0) ||
                    (coord[0]>=(int)res[0]) || (coord[1]>=(int)res[1]) || (coord[2]>=(int)res[2]) )
                {
                    continue;
                }
                
                //  do some falloff based on how far from the voxel center 
                //  the current point lies
                //
                MPoint gridPoint;
                gridPoint.x = Ox + (coord[0]+0.5)*dx;
                gridPoint.y = Oy + (coord[1]+0.5)*dy;
                gridPoint.z = Oz + (coord[2]+0.5)*dz;
                
                MVector diff = gridPoint - randPoint;
                double distSquared = diff * diff;
                double distDrop = dropoff * distSquared;
                
                double newVal = triRate * exp( -distDrop );
        
                //  emit into the voxel
                //
                if( newVal != 0 )
                {
                    fluid.emitIntoArrays( (float) newVal, coord[0], coord[1], coord[2], (float)densityEmit, (float)heatEmit, (float)fuelEmit, doEmitColor, curTexturedColor );      
                }
            }
        }
    }
}

MStatus initializePlugin(MObject obj)
{
    MStatus status;
    MFnPlugin plugin(obj, PLUGIN_COMPANY, "3.0", "Any");

    status = plugin.registerNode( "simpleFluidEmitter", simpleFluidEmitter::id,
                            &simpleFluidEmitter::creator, &simpleFluidEmitter::initialize,
                            MPxNode::kFluidEmitterNode );
    if (!status) {
        status.perror("registerNode");
        return status;
    }

    return status;
}

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

    status = plugin.deregisterNode( simpleFluidEmitter::id );
    if (!status) {
        status.perror("deregisterNode");
        return status;
    }

    return status;
}