Editable Initial State - Attempt 11

README.md

⬅️ Back

Putting it all together, I'll borrow from the Maya devkit examples testNsolverNode and testNobjectNode for an event loop

Table of contents

• DG
• Parallel
• DG and Multiple Rigids
• Parallel and Multiple Rigids
• Build & Run

Source Code

Roughly 800 lines in total, including comments.

• main.cpp Single translation unit, single output plug-in
• rigid.hpp The rigid, separated into its own file for readability, but is #included into main.cpp
• solver.hpp Also included, contains the solver part.

Architecture

Here's the intended evaluation from start to finish, except for Parallel which doesn't play by the rules.

1. pCube is about to be rendered
2. pCube1.translateX is evaluated
3. rigid.outputTranslateX is pulled
4. solver.nextState is pulled
5. rigid.startState is pulled
6. solver steps simulation, updates the position of all rigids
7. rigid.outputTranslateX is clean
8. pCube1.translateX is clean
9. Finish

On subsequent frames, rigid.currentState is pulled instead, so we can handle initialisation once and only once.

DG

Almost works with only a single rigid, the cruxes are:

1. Cycle on framestep (but not playback)
2. Caches previous frames for some reason
3. Reverts passive values whenever you edit them

DG, unlike Parallel, pulls on currentState and startState reliably and doesn't cycle when pulling pCube1.matrix during evaluation of pCube1.translateX. It does however cycle when you framestep, rather than playback. Unclear as to why.

Notes	Test
Fundamentals works well, no cycle
It for some reason pulls on startState whenever manipulation begins, which resets it to the previous position. I expect the passive values to take precedence.
Finally, a viewport bug. Doesn't happen in VP1. I've resorted to calling ogs -reset on the start frame in this example, which is bad. This only happens in DG, not parallel.

Parallel

Parallel just doesn't do as advertised. Why is startState pulled every frame? It doesn't cycle during playback, nor during frame step. But there is no way to initialise our data once, resulting in the velocity being zeroed out each time.

DG and Multiple Rigids

At this point, everything stops working.

One of them works, sometimes. Notice how one of the translates snaps back and simulates from where it was?

This tells me cycles are still happening even if not reported, and that the overall approach is flawed.

Parallel and Multiple Rigids

Parallel also cannot cope.

Build & Run

From Powershell.

cd c:\
git clone https://gist.github.com/681edd78c71a449c0b520f220a49c1e9.git initialStateNode12
mkdir initialStateNode12/build
cd initialStateNode10/build
$env:DEVKIT_LOCATION="c:\path\to\your\devkit"
cmake .. -G Ninja
ninja

Then from the Script Editor.

import os
from maya import cmds

fname = r"c:\initialState12\build\initialStateNode.mll"
cmds.loadPlugin(fname)

time = "time1"
solver = cmds.createNode("solverNode", parent=cmds.createNode("transform", name="solver"))
cmds.setAttr(solver + ".gravity", 0, -9.81, 0)
cmds.setAttr(solver + ".startTime", cmds.getAttr(time + ".outTime"))
cmds.connectAttr(time + ".outTime", solver + ".currentTime")

parent = None
for i in range(2):
    cube, _ = cmds.polyCube()

    if parent:
        cmds.parent(cube, parent, relative=True)
        cmds.move(3, 0, 0, relative=True)
    else:
        cmds.move(-3, 15, -7)

    rigid = cmds.createNode("rigidNode", parent=cube)

    cmds.connectAttr(time + ".outTime", rigid + ".currentTime")
    cmds.connectAttr(cube + ".parentInverseMatrix", rigid + ".inParentInverseMatrix")
    cmds.connectAttr(cube + ".matrix", rigid + ".inMatrix")
    cmds.connectAttr(rigid + ".outputTranslateX", cube + ".translateX")
    cmds.connectAttr(rigid + ".outputTranslateY", cube + ".translateY")
    cmds.connectAttr(rigid + ".outputTranslateZ", cube + ".translateZ")

    # Connect to scene
    cmds.connectAttr(rigid + ".startState", solver + ".startState[%d]" % i)
    cmds.connectAttr(rigid + ".currentState", solver + ".currentState[%d]" % i)
    cmds.connectAttr(solver + ".nextState[%d]" % i, rigid + ".nextState")

    #parent = cube  # This only makes matters worse


#cmds.file(new=True, force=True)
#cmds.unloadPlugin(os.path.basename(fname))

CMakeLists.txt

cmake_minimum_required(VERSION 2.8)

include($ENV{DEVKIT_LOCATION}/cmake/pluginEntry.cmake)

set(PROJECT_NAME initialStateNode)

set(SOURCE_FILES
   main.cpp
)

set(LIBRARIES
    OpenMaya
    OpenMayaRender
    OpenMayaUI
    Foundation
)

add_compile_definitions("/D_DEBUG /Zi")

build_plugin()

main.cpp

#include <string.h>
#include <unordered_map>
#include <cassert>

#include <maya/MPxNode.h>

#include <maya/MFnNumericAttribute.h>
#include <maya/MFnMatrixAttribute.h>
#include <maya/MFnUnitAttribute.h>
#include <maya/MFnPlugin.h>

#include <maya/MTypeId.h> 
#include <maya/MPlug.h>
#include <maya/MString.h>
#include <maya/MVector.h>
#include <maya/MTransformationMatrix.h>
#include <maya/MMatrix.h>
#include <maya/MDataBlock.h>
#include <maya/MDataHandle.h>
#include <maya/MStreamUtils.h>
#include <maya/MNodeMessage.h>
#include <maya/MFnDependencyNode.h>
#include <maya/MFnMatrixData.h>
#include <maya/MCallbackIdArray.h>
#include <maya/MGlobal.h>

#define Print                           MStreamUtils::stdErrorStream
#define Translate(mat)                  MTransformationMatrix(mat).translation(MSpace::kTransform)
#define AddAttribute(attr)              assert(addAttribute(attr) == MS::kSuccess);
#define AttributeAffects(attr, affects) assert(attributeAffects(attr, affects) == MS::kSuccess);

// Global counter, for a unique UID per rigid
static int gUniqueIdCounter { 1000 };

struct RigidBody {
    int uid { -1 };
    MVector velocity { 0, 0, 0 };
    MMatrix parentInverseMatrix {};
    MMatrix restMatrix {};
    MMatrix worldMatrix {};
    bool    worldMatrixChanged { true };
};

using RigidMap = std::unordered_map<int, RigidBody>;

// For purposes of demonstration, rather
// than passing data between nodes, we'll
// use a global variable and pass an index
// into this map between nodes instead.
static RigidMap gRigids;
static bool has(int uid) { return gRigids.find(uid) != gRigids.end(); }


// Helper functions

inline MDataHandle InputValue(MDataBlock& datablock, MObject attr) {
    MStatus status;
    MDataHandle handle = datablock.inputValue(attr, &status);
    assert(status == MS::kSuccess);
    return handle;
}

inline MDataHandle OutputValue(MDataBlock& datablock, MObject attr) {
    MStatus status;
    MDataHandle handle = datablock.outputValue(attr, &status);
    assert(status == MS::kSuccess);
    return handle;
}


#include "solver.hpp"
#include "rigid.hpp"


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

    status = plugin.registerNode(SolverNode::name,
                                 SolverNode::id,
                                 SolverNode::creator,
                                 SolverNode::initialize);

    status = plugin.registerNode(RigidNode::name,
                                 RigidNode::id,
                                 RigidNode::creator,
                                 RigidNode::initialize);

    Print() << "==============\n";
    Print() << "Initialising\n";

    return status;
}


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

    status = plugin.deregisterNode(RigidNode::id);
    status = plugin.deregisterNode(SolverNode::id);

    Print() << "Deinitialising.\n";
    Print() << "==============\n";

    return status;
}

rigid.hpp

/**
 * Individual rigid body
 *
 */
class RigidNode : public MPxNode {
public:
    RigidNode() {};
    ~RigidNode() override;

    MStatus compute(const MPlug&, MDataBlock&) override;
    MStatus computeUid(const MPlug&, MDataBlock&);
    MStatus computeStartState(const MPlug&, MDataBlock&);
    MStatus computeCurrentState(const MPlug&, MDataBlock&);
    MStatus computeOutputTranslate(const MPlug&, MDataBlock&);

    static  void* creator() { return new RigidNode(); }
    static  MStatus initialize();
    void    postConstructor();
    void    reset(MDataBlock&);

    MStatus setDependentsDirty(const MPlug&, MPlugArray&) override;
    bool isPassiveOutput(const MPlug&) const override;

public:
    static  MString       name;
    static  MTypeId       id;
    static const MString  drawDbClassification;
    static const MString  drawRegistrantId;

    // Attributes
    static  MObject   aUid;
    static  MObject   aStartTime;
    static  MObject   aCurrentTime;
    static  MObject   aInputMatrix;
    static  MObject   aInputParentInverseMatrix;
    static  MObject   aOutputTranslate;
    static  MObject   aOutputTranslateX;
    static  MObject   aOutputTranslateY;
    static  MObject   aOutputTranslateZ;

    // <-- Pull on this to fetch latest goods from the solver
    static MObject aNextState;

    // --> Deliver initial state of this rigid to the solver
    static MObject aStartState;

    // --> Deliver animated values, like damping, to the solver
    static MObject aCurrentState;

private:
    int               _uid { -1 };
    bool              _firstFrame { false };
    MCallbackIdArray  _callbackIds;
};


MString       RigidNode::name("rigidNode");
MTypeId       RigidNode::id(0x80012);
const MString RigidNode::drawDbClassification("drawdb/geometry/RigidNode");
const MString RigidNode::drawRegistrantId("RigidNodePlugin");
MObject       RigidNode::aUid;
MObject       RigidNode::aInputMatrix;
MObject       RigidNode::aInputParentInverseMatrix;
MObject       RigidNode::aOutputTranslate;
MObject       RigidNode::aOutputTranslateX;
MObject       RigidNode::aOutputTranslateY;
MObject       RigidNode::aOutputTranslateZ;
MObject       RigidNode::aStartTime;
MObject       RigidNode::aCurrentTime;
MObject       RigidNode::aNextState;
MObject       RigidNode::aStartState;
MObject       RigidNode::aCurrentState;



RigidNode::~RigidNode() {
    // Any node created will have an entry in this map
    gRigids.erase(_uid);
}


MStatus RigidNode::initialize() {
    MStatus status;
    MFnNumericAttribute numFn;
    MFnMatrixAttribute matFn;
    MFnUnitAttribute uniFn;

    aUid = numFn.create("uid", "uid", MFnNumericData::kInt, -1, &status);
    numFn.setWritable(false);
    numFn.setStorable(false);
    numFn.setKeyable(true);

    aNextState = numFn.create("nextState", "nest", MFnNumericData::kInt, -1, &status);
    numFn.setStorable(false);
    numFn.setReadable(false);

    aStartState = numFn.create("startState", "stst", MFnNumericData::kInt, -1, &status);
    numFn.setStorable(false);
    numFn.setWritable(false);

    aCurrentState = numFn.create("currentState", "cust", MFnNumericData::kInt, -1, &status);
    numFn.setStorable(false);
    numFn.setWritable(false);

    aCurrentTime = uniFn.create("currentTime", "cuti", MFnUnitAttribute::kTime, 0.0, &status);
    uniFn.setKeyable(true);

    aInputMatrix = matFn.create("inMatrix", "inma");
    matFn.setStorable(false);
    matFn.setReadable(false);
    aInputParentInverseMatrix = matFn.create("inParentInverseMatrix", "ipim");
    matFn.setStorable(false);
    matFn.setReadable(false);

    aOutputTranslateX = uniFn.create("outputTranslateX", "outx", MFnUnitAttribute::kDistance, 0.0, &status);
    uniFn.setWritable(false);
    uniFn.setStorable(false);
    uniFn.setChannelBox(true);
    aOutputTranslateY = uniFn.create("outputTranslateY", "outy", MFnUnitAttribute::kDistance, 0.0, &status);
    uniFn.setWritable(false);
    uniFn.setStorable(false);
    uniFn.setChannelBox(true);
    aOutputTranslateZ = uniFn.create("outputTranslateZ", "outz", MFnUnitAttribute::kDistance, 0.0, &status);
    uniFn.setWritable(false);
    uniFn.setStorable(false);
    uniFn.setChannelBox(true);
    aOutputTranslate = numFn.create("outputTranslate", "outr", aOutputTranslateX, aOutputTranslateY, aOutputTranslateZ);

    AddAttribute(aUid);
    AddAttribute(aNextState);
    AddAttribute(aStartState);
    AddAttribute(aCurrentState);
    AddAttribute(aCurrentTime);
    AddAttribute(aInputMatrix);
    AddAttribute(aInputParentInverseMatrix);
    AddAttribute(aOutputTranslate);

    // Affects initial state
    AttributeAffects(aInputMatrix, aStartState);
    AttributeAffects(aInputParentInverseMatrix, aStartState);

    // Affects simulation
    AttributeAffects(aCurrentTime, aStartState);
    AttributeAffects(aCurrentTime, aCurrentState);
    AttributeAffects(aCurrentTime, aOutputTranslate);
    AttributeAffects(aNextState, aOutputTranslate);

    return status;
}


bool RigidNode::isPassiveOutput(const MPlug& plug) const {
    /**
     * Let anything connected to these still be editable
     *
     */
    return plug == aOutputTranslateX ||
           plug == aOutputTranslateY ||
           plug == aOutputTranslateZ;
}


MStatus RigidNode::setDependentsDirty(const MPlug& plugBeingDirtied, MPlugArray& affectedPlugs) {
    return MPxNode::setDependentsDirty(plugBeingDirtied, affectedPlugs);
}


void RigidNode::postConstructor() {
    MStatus status { MS::kSuccess };

    // Find ourselves amongst rigids hosted by the solver
    _uid = ++gUniqueIdCounter;

    // Initialise our data, in a place accessible to our solver
    gRigids[_uid] = { _uid };
}


void RigidNode::reset(MDataBlock& datablock) {
    RigidBody& rigid = gRigids.at(_uid);
    rigid.restMatrix = InputValue(datablock, aInputMatrix).asMatrix();
    rigid.parentInverseMatrix = InputValue(datablock, aInputParentInverseMatrix).asMatrix();
    rigid.worldMatrix = rigid.restMatrix * rigid.parentInverseMatrix.inverse();
    rigid.velocity = {0, 0, 0};

    Print() << "RigidNode: Rigid reset @ " << Translate(rigid.restMatrix) << "\n";
}


/**
 * Restore rest matrix
 *
 */
MStatus RigidNode::computeStartState(const MPlug& plug, MDataBlock& datablock) {
    Print() << "RigidNode::computeStartState() ===============\n";
    MStatus status { MS::kSuccess };

    RigidBody& rigid = gRigids.at(_uid);
    rigid.worldMatrix = rigid.restMatrix * rigid.parentInverseMatrix.inverse();
    rigid.velocity = {0, 0, 0};

    OutputValue(datablock, aStartState).set(_uid);

    datablock.setClean(plug);

    _firstFrame = true;

    return status;
}


/**
 * Read values that change over time
 *
 * We don't really have any, but for example linearDamping
 *
 */
MStatus RigidNode::computeCurrentState(const MPlug& plug, MDataBlock& datablock) {
    Print() << "RigidNode::computeCurrentState()\n";
    MStatus status { MS::kSuccess };

    if (_firstFrame) {
        reset(datablock);
        _firstFrame = false;
    }

    datablock.setClean(plug);
    return status;
}


/**
 * Following the first frame
 *
 */
MStatus RigidNode::computeOutputTranslate(const MPlug& plug, MDataBlock& datablock) {
    Print() << "RigidNode::computeOutputTranslate()\n";

    MStatus status { MS::kSuccess };

    // Pull and let the SolverNode go to work..
    int changed = InputValue(datablock, aNextState).asInt();

    // .
    // ..
    // ...
    // aaaaand we're back.

    // E.g. start frame hasn't yet been reached, time was dirty but not different, etc.
    if (changed < 0) {
        Print() << "RigidNode: Solver unchanged\n";
        return MS::kFailure;
    }

    // At this point, the solver will have pulled on either..
    //
    // - RigidNode::computeStartState
    // - RigidNode::computeCurrentState
    //
    // ..depending on what time it was. They will in turn have populated
    // our translate values with the latest up-to-date simulated values
    // 

    RigidBody& rigid = gRigids.at(_uid);

    // Preserve hierarchy when manipulating an object during standstill
    // NOTE: Only relevant during parallel, because..
    //       ..DG doesn't trigger evaluation during manipulation..??
    if (rigid.worldMatrixChanged) {
        rigid.parentInverseMatrix = InputValue(datablock, aInputParentInverseMatrix).asMatrix();
        rigid.worldMatrixChanged = false;
    }

    MMatrix localMatrix = rigid.worldMatrix * rigid.parentInverseMatrix;

    MTransformationMatrix tm { localMatrix };
    MVector translate = tm.translation(MSpace::kTransform);
    MDataHandle tx = OutputValue(datablock, aOutputTranslateX),
                ty = OutputValue(datablock, aOutputTranslateY),
                tz = OutputValue(datablock, aOutputTranslateZ);

    tx.set(translate.x);
    ty.set(translate.y);
    tz.set(translate.z);

    datablock.setClean(plug);

    Print() << "RigidNode: " << translate << " written successfully!\n";

    return status;
}


MStatus RigidNode::computeUid(const MPlug& plug, MDataBlock& datablock) {
    Print() << "RigidNode::computeUid()\n";
    MStatus status { MS::kSuccess };

    // We'll use this from the solver mostly to sanity-check
    // that we're talking to the rigid we think we're talking to.
    MDataHandle outputHandle = OutputValue(datablock, aUid);
    outputHandle.setInt(_uid);

    datablock.setClean(plug);

    return status;
}


MStatus RigidNode::compute(const MPlug& plug, MDataBlock& datablock) {
    MStatus status { MS::kUnknownParameter };

    // For some reason, we need to pull on time, even though we don't need it
    // in order for its dependents to continue to be dirtied when it changes.
    // The alternative is having it drawn in the channel box or AE.
    InputValue(datablock, aCurrentTime).asTime();

    if      (plug == aStartState) {
        status = computeStartState(plug, datablock);
    }

    else if (plug == aCurrentState) {
        status = computeCurrentState(plug, datablock);
    }

    else if (plug == aOutputTranslate ||
             plug == aOutputTranslateX ||
             plug == aOutputTranslateY ||
             plug == aOutputTranslateZ) {
        status = computeOutputTranslate(plug, datablock);
    }

    else if (plug == aUid) {
        status = computeUid(plug, datablock);
    }

    return status;
}

solver.hpp

/**
 * Pull on all rigid at once, and solve them in one big batch.
 * 
 */

class SolverNode : public MPxNode {
public:
    SolverNode() {};
    ~SolverNode() override {};

    static  void* creator() { return new SolverNode(); }
    static  MStatus initialize();
    MStatus compute(const MPlug& plug, MDataBlock& data) override;
    MStatus computeOutputRigids(const MPlug& plug, MDataBlock& data);
    MStatus addRigids(const MPlug& plug, MDataBlock& datablock);
    MStatus updateRigids(const MPlug& plug, MDataBlock& datablock);
    MStatus setDependentsDirty(const MPlug&, MPlugArray&) override;

    MStatus stepSimulation(MDataBlock& datablock);

public:
    static MString        name;
    static MTypeId        id;
    static const MString  drawDbClassification;
    static const MString  drawRegistrantId;

    // Attributes
    static MObject   aStartTime;
    static MObject   aCurrentTime;

    static MObject   aGravity;
    static MObject   aGravityX;
    static MObject   aGravityY;
    static MObject   aGravityZ;

    // Initial state of each rigid, e.g. position and orientation
    static MObject   aStartState;

    // Current values of rigid plugs, e.g. damping
    static MObject   aCurrentState;

    // Resulting transforms after simulation
    static MObject   aNextState;

private:
    bool    _isStartTime { false };
    bool    _beforeStartTime { false };
    bool    _isFirstTime { false };
    MTime   _lastTime { };
    MTime   _currentTime { };
    double  _deltaTime { 0.0 };

};


MString       SolverNode::name("solverNode");
MTypeId       SolverNode::id(0x80013);
const MString SolverNode::drawDbClassification("drawdb/geometry/SolverNode");
const MString SolverNode::drawRegistrantId("SolverNodePlugin");
MObject       SolverNode::aCurrentState;
MObject       SolverNode::aStartState;
MObject       SolverNode::aNextState;
MObject       SolverNode::aStartTime;
MObject       SolverNode::aCurrentTime;
MObject       SolverNode::aGravity;
MObject       SolverNode::aGravityX;
MObject       SolverNode::aGravityY;
MObject       SolverNode::aGravityZ;


MStatus SolverNode::initialize() {
    MStatus status;
    MFnNumericAttribute numFn;
    MFnUnitAttribute uniFn;

    aStartTime = uniFn.create("startTime", "stti", MFnUnitAttribute::kTime, 0.0, &status);
    uniFn.setKeyable(true);

    aCurrentTime = uniFn.create("currentTime", "cuti", MFnUnitAttribute::kTime, 0.0, &status);
    uniFn.setKeyable(true);

    aStartState = numFn.create("startState", "stst", MFnNumericData::kInt, -1, &status);
    numFn.setStorable(false);
    numFn.setReadable(true);
    numFn.setWritable(true);
    numFn.setArray(true);
    numFn.setUsesArrayDataBuilder(true);

    aCurrentState = numFn.create("currentState", "cust", MFnNumericData::kInt, -1, &status);
    numFn.setStorable(false);
    numFn.setReadable(true);
    numFn.setWritable(true);
    numFn.setArray(true);
    numFn.setUsesArrayDataBuilder(true);

    aNextState = numFn.create("nextState", "nest", MFnNumericData::kInt, -1, &status);
    numFn.setStorable(false);
    numFn.setWritable(false);
    numFn.setArray(true);
    numFn.setUsesArrayDataBuilder(true);

    aGravityX = uniFn.create("gravityX", "grvx", MFnUnitAttribute::kDistance, 0.0, &status);
    aGravityY = uniFn.create("gravityY", "grvy", MFnUnitAttribute::kDistance, -981.0, &status);
    aGravityZ = uniFn.create("gravityZ", "grvz", MFnUnitAttribute::kDistance, 0.0, &status);
    aGravity = numFn.create("gravity", "grav", aGravityX, aGravityY, aGravityZ);
    numFn.setKeyable(true);

    AddAttribute(aStartTime);
    AddAttribute(aCurrentTime);
    AddAttribute(aGravity);
    AddAttribute(aCurrentState);
    AddAttribute(aStartState);
    AddAttribute(aNextState);

    AttributeAffects(aStartTime, aNextState);
    AttributeAffects(aCurrentTime, aNextState);
    AttributeAffects(aGravity, aNextState);
    AttributeAffects(aStartState, aNextState);
    AttributeAffects(aCurrentState, aNextState);

    return status;
}


MStatus SolverNode::setDependentsDirty(const MPlug& plugBeingDirtied, MPlugArray& affectedPlugs) {
    return MPxNode::setDependentsDirty(plugBeingDirtied, affectedPlugs);
}


MStatus SolverNode::compute(const MPlug& plug, MDataBlock& datablock) {
    MStatus status { MS::kUnknownParameter };

    if (plug == aNextState) {

        // Sometimes the compound plug is dirtied, but when and why I do not know.
        // Either way, we don't care for it.
        if (plug.isElement()) {
            const MTime currentTime = InputValue(datablock, aCurrentTime).asTime();
            const MTime startTime = InputValue(datablock, aStartTime).asTime();

            Print() << "- time: " << currentTime << " -----------------------------------------------\n";

            _currentTime = currentTime;
            _deltaTime = (_currentTime - _lastTime).as(MTime::kSeconds);
            _isFirstTime = _isStartTime && _deltaTime > 0.0;
            _isStartTime = currentTime == startTime;
            _beforeStartTime = currentTime < startTime;

            status = computeOutputRigids(plug, datablock);

            _lastTime = currentTime;
        }
    }

    return status;
}


MStatus SolverNode::computeOutputRigids(const MPlug& plug, MDataBlock& datablock) {
    Print() << "SolverNode::computeOutputRigids()\n";
    MStatus status { MS::kSuccess };

    if (_beforeStartTime) {
        Print() << "SolverNode: Doing nothing..\n";
    }

    else if (_isStartTime) {
        status = addRigids(plug, datablock);
    }

    else if (_isFirstTime) {
        status = updateRigids(plug, datablock);
    }

    else if (_deltaTime > 0.0) {
        status = updateRigids(plug, datablock);
    }

    else {
        // Unchanged, the rigid will expect a value 0 or
        // higher when the solver has actually changed
        MArrayDataHandle outputHandle = datablock.outputArrayValue(aNextState, &status);
        for (unsigned int i = 0; i < outputHandle.elementCount(); i++) {
            outputHandle.jumpToElement(i);
            outputHandle.outputValue().setInt(-1);
        }
    }

    assert(status == MS::kSuccess);

    // nextState was pulled, let's clean it
    MArrayDataHandle outputArrayHandle = datablock.outputArrayValue(aNextState, &status);
    assert(status == MS::kSuccess);

    outputArrayHandle.setAllClean();
    datablock.setClean(plug);

    return status;
}


MStatus SolverNode::addRigids(const MPlug& plug, MDataBlock& datablock) {
    MStatus status { MS::kUnknownParameter };
    Print() << "SolverNode::addRigids()\n";

    // In DG mode, evaluation never happens on frames previously
    // rendered until this is called. Madness. Why oh why?
    Print() << "ogs -reset\n";
    MGlobal::executeCommand("ogs -reset");

    MArrayDataHandle outputArrayHandle = datablock.outputArrayValue(aNextState, &status);
    assert(status == MS::kSuccess);

    MArrayDataHandle inputRigidsHandle = datablock.inputArrayValue(aStartState, &status);
    assert(status == MS::kSuccess);

    MDataHandle outputHandle = outputArrayHandle.outputValue(&status);
    assert(status == MS::kSuccess);

    MPlug outputRigidsPlug { thisMObject(), aNextState };

    // Iterate over the entire array, since we can't
    // know when the *last* rigid is added. We'll treat
    // every rigid as a newly added rigid.
    outputArrayHandle.jumpToElement(0);

    // Since we use indexes into a global array for Rigid data,
    // the indexes we use into that array get left behind when
    // disconnected. So we need to ensure we only deal with
    // indexes that are both valid and *still connected*.
    do {
        int index = outputArrayHandle.elementIndex(&status);
        assert(status == MS::kSuccess);

        // Only consider rigids that are still connected
        MPlug elementPlug = outputRigidsPlug.elementByLogicalIndex(index, &status);
        assert(status == MS::kSuccess);
        if (!elementPlug.isConnected()) { Print() << "SolverNode: Disconnected index: " << index << "\n"; return MS::kFailure; }

        // Pull on rigid.outputStartState
        int uid = inputRigidsHandle.inputValue(&status).asInt();
        assert(status == MS::kSuccess || uid >= 0);

        Print() << "SolverNode: Solver.output[" << index << "] --> Rigid.next[" << uid << "]\n";

        outputHandle.set(index);

    } while (outputArrayHandle.next() == MS::kSuccess);

    return status;
}


MStatus SolverNode::updateRigids(const MPlug& plug, MDataBlock& datablock) {
    Print() << "SolverNode::updateRigids()\n";
    MStatus status { MS::kSuccess };

    // Pull on RigidNode::aCurrentState
    MArrayDataHandle inputHandle = datablock.inputArrayValue(aCurrentState, &status);
    assert(status == MS::kSuccess);

    for (unsigned int i = 0; i < inputHandle.elementCount(); i++) {
        inputHandle.jumpToElement(i);
        inputHandle.inputValue().asInt();
    }

    stepSimulation(datablock);

    // Output a "solver-changed" message (1) to Rigid
    MArrayDataHandle outputHandle = datablock.outputArrayValue(aNextState, &status);
    assert(status == MS::kSuccess);

    for (unsigned int i = 0; i < outputHandle.elementCount(); i++) {
        outputHandle.jumpToElement(i);
        outputHandle.outputValue().setInt(1);
    }

    return status;
}


/**
 * Apply gravity and update position
 *
 */
MStatus SolverNode::stepSimulation(MDataBlock& datablock) {
    Print() << "SolverNode::stepSimulation()\n";
    MStatus status { MS::kSuccess };
    MVector gravity = InputValue(datablock, aGravity).asDouble3();

    for (auto& pair : gRigids) {
        RigidBody& rigid = pair.second;
        MTransformationMatrix tm { rigid.worldMatrix };

        rigid.velocity += gravity * _deltaTime;
        tm.addTranslation(rigid.velocity, MSpace::kTransform);
        Print() << "SolverNode: rigid[" << rigid.uid << "].translate " << tm.translation(MSpace::kTransform) << "\n";

        rigid.worldMatrix = tm.asMatrix();
        rigid.worldMatrixChanged = true;
    }

    return status;
}