The Maya IK Solver is a rigging staple but it does have a few shortcomings. Among them is the angle between the two bones appearing to snap when getting close to zero. This can negatively affect animation and it's not easy to manage.
In comes Soft IK.
It's not a new concept but one worth some investigation especially as far as I am aware it is still not part of core Maya. This node piggy backs on the standard IK Solver basically tracking the position of a driving transform but applying a function curve over the top that gradually attenuates the movement based on an input value.
When looking into this I found a great site that showed a solution that had been worked on for XSI. Check this out here.
As ever I have included a python implementation of the node to try out with some support code.
Soft_IK_Constraint from SBGrover on Vimeo.
Python plugin
import maya.cmds as cmds
import maya.OpenMaya as OpenMaya
import maya.OpenMayaMPx as omMPx
import math
kPluginNodeTypeName = "SoftIKConstraint"
kPluginNodeClassify = 'utility/general'
kPluginNodeId = OpenMaya.MTypeId(0x81081)
class SoftIKConstraint(omMPx.MPxConstraint):
matA = OpenMaya.MObject()
matB = OpenMaya.MObject()
matC = OpenMaya.MObject()
matInv = OpenMaya.MObject()
soft = OpenMaya.MObject()
output = OpenMaya.MObject()
pointX = OpenMaya.MObject()
pointY = OpenMaya.MObject()
pointZ = OpenMaya.MObject()
magRetrieved=0
chainLength=0
#e=2.71828
def __init__(self):
omMPx.MPxConstraint.__init__(self)
def compute(self,plug,data):
if plug == SoftIKConstraint.output:
softVal = data.inputValue(SoftIKConstraint.soft).asFloat()
inverseWorldMatrix = data.inputValue(SoftIKConstraint.matInv).asMatrix()
transformAWorldMatrix = data.inputValue(SoftIKConstraint.matA).asMatrix()
transformBWorldMatrix = data.inputValue(SoftIKConstraint.matB).asMatrix()
vecAwm = OpenMaya.MPoint() * transformAWorldMatrix
vecBwm = OpenMaya.MPoint() * transformBWorldMatrix
vecAB = vecAwm-vecBwm
currentLength=getMagnitude(vecAB)
if self.magRetrieved == 0:
transformCWorldMatrix = data.inputValue(SoftIKConstraint.matC).asMatrix()
vecCwm = OpenMaya.MPoint() * transformCWorldMatrix
vecAC = vecAwm-vecCwm
vecCB = vecCwm-vecBwm
self.chainLength = getMagnitude(vecAC) + getMagnitude(vecCB)
self.magRetrieved = 1
if softVal == 0:
ratioOfLength=1
else:
affectedLength=self.chainLength*softVal
unnafectedLength=self.chainLength-affectedLength
#create a falloff based on where the unnafected length ends. The function creates a curve all the way from 0 to max length but the result is forced to become linear where the unnafected length applies.
if currentLength <= unnafectedLength:
fractionOfLength = currentLength
else:
fractionOfLength = affectedLength*(1-math.exp((unnafectedLength - currentLength)/affectedLength))+unnafectedLength
ratioOfLength = fractionOfLength / currentLength
worldPosition = ((vecAwm * (1.0 - ratioOfLength)) + OpenMaya.MVector(vecBwm * ratioOfLength))
localPosition = worldPosition * inverseWorldMatrix
outHandle = data.outputValue(SoftIKConstraint.output)
outHandleX = outHandle.child(SoftIKConstraint.pointX)
outHandleY = outHandle.child(SoftIKConstraint.pointY)
outHandleZ = outHandle.child(SoftIKConstraint.pointZ)
outHandleX.setMDistance(OpenMaya.MDistance(localPosition.x))
outHandleY.setMDistance(OpenMaya.MDistance(localPosition.y))
outHandleZ.setMDistance(OpenMaya.MDistance(localPosition.z))
data.setClean(plug)
else:
return OpenMaya.kUnknownParameter
def getMagnitude(vector):
magnitude=math.sqrt((vector.x*vector.x) + (vector.y*vector.y) + (vector.z*vector.z))
return magnitude
def nodeCreator():
return omMPx.asMPxPtr(SoftIKConstraint())
def nodeInitializer():
inputMatrix = OpenMaya.MFnMatrixAttribute()
softAttr = OpenMaya.MFnNumericAttribute()
outputAttr = OpenMaya.MFnUnitAttribute()
compoundOutputAttr = OpenMaya.MFnNumericAttribute()
SoftIKConstraint.matB = inputMatrix.create("HandleDriverWorldMatrix","hcwm")
inputMatrix.setHidden(1)
SoftIKConstraint.matA = inputMatrix.create("ChainDriverWorldMatrix","cdwm")
inputMatrix.setHidden(1)
SoftIKConstraint.matC = inputMatrix.create("MiddleJointWorldMatrix","mjwm")
inputMatrix.setHidden(1)
SoftIKConstraint.matInv = inputMatrix.create("HandleParentInverseMatrix","hpim")
inputMatrix.setHidden(1)
SoftIKConstraint.soft = softAttr.create("SoftIKValue","sik",OpenMaya.MFnNumericData.kFloat,0)
softAttr.setMin(0)
softAttr.setMax(1)
softAttr.setChannelBox(1)
#scene scale independent output
SoftIKConstraint.pointX = outputAttr.create("outputX", "outx",OpenMaya.MFnUnitAttribute.kDistance,0)
outputAttr.setWritable(0)
SoftIKConstraint.pointY = outputAttr.create("outputY", "outy",OpenMaya.MFnUnitAttribute.kDistance,0)
outputAttr.setWritable(0)
SoftIKConstraint.pointZ = outputAttr.create("outputZ", "outz",OpenMaya.MFnUnitAttribute.kDistance,0)
outputAttr.setWritable(0)
SoftIKConstraint.output = compoundOutputAttr.create("Output", "out",SoftIKConstraint.pointX,SoftIKConstraint.pointY,SoftIKConstraint.pointZ)
compoundOutputAttr.setWritable(0)
compoundOutputAttr.setHidden(1)
SoftIKConstraint.addAttribute(SoftIKConstraint.matA)
SoftIKConstraint.addAttribute(SoftIKConstraint.matB)
SoftIKConstraint.addAttribute(SoftIKConstraint.matC)
SoftIKConstraint.addAttribute(SoftIKConstraint.matInv)
SoftIKConstraint.addAttribute(SoftIKConstraint.soft)
SoftIKConstraint.addAttribute(SoftIKConstraint.output)
SoftIKConstraint.attributeAffects(SoftIKConstraint.matA,SoftIKConstraint.output)
SoftIKConstraint.attributeAffects(SoftIKConstraint.matB,SoftIKConstraint.output)
SoftIKConstraint.attributeAffects(SoftIKConstraint.matC,SoftIKConstraint.output)
SoftIKConstraint.attributeAffects(SoftIKConstraint.matInv,SoftIKConstraint.output)
SoftIKConstraint.attributeAffects(SoftIKConstraint.soft,SoftIKConstraint.output)
def initializePlugin(mobject):
print "> Initialising SoftIK Plugin"
fnPlugin = omMPx.MFnPlugin(mobject)
fnPlugin.registerNode(kPluginNodeTypeName,kPluginNodeId,nodeCreator,nodeInitializer,omMPx.MPxNode.kDependNode,kPluginNodeClassify)
def uninitializePlugin(mobject):
print "> Uninitialising SoftIK Plugin"
fnPlugin = omMPx.MFnPlugin(mobject)
fnPlugin.deregisterNode(kPluginNodeId)
Python helper code
# working on a scene in cm
import maya.cmds as mc
shoulder = mc.joint(n="shoulder", p=(-3,0,0))
elbow = mc.joint(n="elbow", p=(0,0,1))
wrist = mc.joint(n="wrist", p=(3,0,0))
mc.xform(mc.group(mc.circle(n="shoulder_ctrl"),w=True, n="shoulder_ctrl_grp"), ws=True, t=(-3,0,0))
mc.xform(mc.group(mc.circle(n="wrist_ctrl"),w=True, n="wrist_ctrl_grp"), ws=True, t=(3,0,0))
handle = mc.ikHandle(sj=shoulder, ee=wrist)[0]
mc.delete(mc.ls(type="softIKConstraint"))
mc.flushUndo()
mc.unloadPlugin("softIKConstraint")
mc.loadPlugin(r"softIKConstraint")
sik = mc.createNode("SoftIKConstraint")
mc.connectAttr("shoulder_ctrl.worldMatrix", sik + ".ChainDriverWorldMatrix")
mc.connectAttr(elbow + ".worldMatrix", sik + ".MiddleJointWorldMatrix")
mc.connectAttr("wrist_ctrl.worldMatrix", sik + ".HandleDriverWorldMatrix")
mc.connectAttr(handle + ".parentInverseMatrix", sik + ".HandleParentInverseMatrix")
mc.pointConstraint("shoulder_ctrl", shoulder)
mc.connectAttr(sik + ".Output", handle + ".translate")
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