scripted/simpleSpring.py

#-
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########################################################################
# DESCRIPTION:
#
# Produces the dependency graph node "simpleSpring".
#
# This node is an example of a spring node that calculates the spring behavior,
# which Maya uses in a simulation.    
#
########################################################################

import maya.OpenMaya as OpenMaya
import maya.OpenMayaMPx as OpenMayaMPx

#
# To test out this plugin example:
# Run from a Python tab of the script editor.
#
#import maya.cmds as cmds
#
#cmds.file(force=True, new=True)
#cmds.currentUnit(linear="centimeter", angle="degree", time="film")   
#
## Load the plug-in.
##
#cmds.loadPlugin("simpleSpring.py")
#
## Create the spring node.
#cmds.createNode("spSimpleSpring", name="simpleSpring")
#
#cmds.curve(d=3, p=[(-9, 6, 0), (-4, 6, 0), (4, 6, 0), (9, 6, 0)], k=[(0), (0), (0), (1), (1), (1)])
#
#cmds.particle(n="fourParticles", p=[(11, -2, 0), (4, -2, 0), (-3, -2, 0), (-7, 4, 0)], c=1)
#
#cmds.gravity(pos=(0, 0, 0), m=9.8, dx=0, dy=-1, dz=0)
#cmds.connectDynamic("fourParticles", f="gravityField1")
#
#cmds.select("fourParticles", "curve1", "simpleSpring", r=True)
#
#cmds.spring(add=True, noDuplicate=False, minMax=True, mnd=0, mxd=0, useRestLengthPS=True, s=1, d=0.2, sfw=1, efw=1)
#
#cmds.playbackOptions(e=True, min=0.00, max=600.0)
#cmds.currentTime(0, e=True)
#cmds.play(wait=True, forward=True)
#


import sys

kPluginNodeTypeName = "spSimpleSpring"
spSimpleSpringNodeId = OpenMaya.MTypeId(0x87004)


class simpleSpring(OpenMayaMPx.MPxSpringNode):
    aSpringFactor = OpenMaya.MObject()

    def __init__(self):
        OpenMayaMPx.MPxSpringNode.__init__(self)
        self.__myFactor = 0.0


    def compute(self, plug, block):
        """
         In this simple example, do nothing in this method. But get the
         spring factor here for "applySpringLaw" to compute output force.

         Note: always let this method return "kUnknownParameter" so that 
         "applySpringLaw" can be called when Maya needs to compute spring force.

         It is recommended to only override compute() to get user defined
         attributes.
        """
        # Get spring factor,
        self.__myFactor = self.springFactor(block)

        # Note: return "kUnknownParameter" so that Maya spring node can
        # compute spring force for this plug-in simple spring node.
        return OpenMaya.kUnknownParameter


    def applySpringLaw(self, stiffness, damping, restLength, endMass1, endMass2,
                        endP1, endP2, endV1, endV2, forceV1, forceV2):
        """
         In this overridden method, the attribute, aSpringFactor, is used
         to compute output force with a simple spring law.

            F = - factor * (L - restLength) * Vector of (endP1 - endP2).
        """
        distV = endP1 - endP2
        L = distV.length()
        distV.normalize()

        F = self.__myFactor * (L - restLength)

        v1 = distV * -F
        forceV1.x = v1.x
        forceV1.y = v1.y
        forceV1.z = v1.z

        v2 = -forceV1
        forceV2.x = v2.x
        forceV2.y = v2.y
        forceV2.z = v2.z


    def springFactor(self, block):
        hValue = block.inputValue(simpleSpring.aSpringFactor)
        value = hValue.asDouble()
        return value


    def end1WeightValue(self, block):
        hValue = block.inputValue(simpleSpring.mEnd1Weight)
        value = hValue.asDouble()
        return value


    def end2WeightValue(self, block):
        hValue = block.inputValue(simpleSpring.mEnd2Weight)
        value = hValue.asDouble()
        return value


#####################################################################


def creator():
    return OpenMayaMPx.asMPxPtr(simpleSpring())


def initializer():
    numAttr = OpenMaya.MFnNumericAttribute()
    
    simpleSpring.aSpringFactor = numAttr.create("springFactor", "sf", OpenMaya.MFnNumericData.kDouble, 1.0)
    numAttr.setKeyable(True)
    simpleSpring.addAttribute(simpleSpring.aSpringFactor)


# initialize the script plug-in
def initializePlugin(mobject):
    mplugin = OpenMayaMPx.MFnPlugin(mobject, "Autodesk", "3.0", "Any")
    try:
        mplugin.registerNode(kPluginNodeTypeName, spSimpleSpringNodeId,
                                creator, initializer, OpenMayaMPx.MPxNode.kSpringNode)
    except:
        sys.stderr.write( "Failed to register node: %s" % kPluginNodeTypeName )
        raise


# uninitialize the script plug-in
def uninitializePlugin(mobject):
    mplugin = OpenMayaMPx.MFnPlugin(mobject)
    try:
        mplugin.deregisterNode( spSimpleSpringNodeId )
    except:
        sys.stderr.write( "Failed to deregister node: %s" % kPluginNodeTypeName )
        raise