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21Aug/121

APyTA_16 : OBJECT Part II, How to inherit a skeleton

APyTA_16 : OBJECT Part II, How to inherit a skeleton

I promise you that object-oriented coding will be meaningful.
For a more accurate example, I have a biggest snippet to show you , dedicaced to rig team.

First we have to introduce a new concept between objects : the inheritance.
You can also create a hierarchy between objects, indicating explicitely in a class a super-class.
Not like parenting, telling a class is issued from another object (could be your classes or another built-in type) is about describing an object which is a more complex version of this class.
The super-class is a generalisation of your class.
Here is a quick example of hierarchy into classes:

class Animal:
  pass
class Mamal(Animal):
  pass
class Cat(Mamal):
  pass

Well, so you can create objects with inheritance of other classes
and you can create members (self-owned variables, which are objects too).
Sometimes it is very hard to DESIGN when contain, and when inherit.
This will help you in most of the case:
"inherits" means " is a sort of "
"contains (members)" means "have some "

Not as before, I will comment the code directly inside the snippet, as there is a lot to say!
So, as I want to create a skeleton, how object-orientation can change my way of coding ?
This is my design:
I propose to create an Avatar object. It contains Limbs.
There are two sort of limbs, arms and legs.
Each of those are basically a list of joints.
Here's the snippet (do not care for lame-algorithms, a more completed version will follow in next post)

import math
import maya.OpenMaya
import pymel.core


class Avatar(object):  # Also when desribing your mother-of-all class, always inherit from the python class-object (you will benefits of some built-ins)
    """ This object is a rigged-skeleton representation of a character """

    def __init__(self, _Size=1.70, _NbArm=2, _NbLeg=2, _hasTail=False):
        self.Size = _Size  # How tall it is. Float,  in meters

        self.LimbList = []   # Limb Lists
        self.ArticulationList = []   # All Joints Lists
        self.Hips = None  # Quick Access shortcut
        self.NeckBase = None  # Quick Access shortcut

        self._createTrunk()  # We can call an method into the inialisation process of our Avatar instance.

        RelativePosition = maya.OpenMaya.MVector(math.cos(0), 0, math.sin(0))
        currentArm = Arm("Left", self.NeckBase,  RelativePosition * 0.2)  # Here we create a new instance of class Arm, and Arm-Object.
        self.LimbList.append(currentArm)

        RelativePosition = maya.OpenMaya.MVector(math.cos(math.pi), 0, math.sin(math.pi))
        currentArm = Arm("Right", self.NeckBase,  RelativePosition * 0.2)
        self.LimbList.append(currentArm)

        RelativePosition = maya.OpenMaya.MVector(math.cos(0), 0, math.sin(0))
        currentLeg = Leg("Left", self.Hips,  RelativePosition * 0.2)
        self.LimbList.append(currentLeg)

        RelativePosition = maya.OpenMaya.MVector(math.cos(math.pi), 0, math.sin(math.pi))
        currentLeg = Leg("Right", self.Hips,  RelativePosition * 0.2)
        self.LimbList.append(currentLeg)

    def _createTrunk(self):
        """ Will create the trunk of the body """
        HipsPos = maya.OpenMaya.MVector(0, self.Size * 0.4, 0)
        SpineOffset = (self.Size - HipsPos.y) / float(8)

        for increment in range(8):
            pymel.core.general.select(clear=True)   # We do not want Maya to create fuzzy stuff with active selection. So before each creation we clear the selection.
            currrentArticulation = pymel.core.nodetypes.Joint(name="Spine_" + str(increment + 1).zfill(2))
            currrentArticulation.setTranslation(maya.OpenMaya.MVector(0,  HipsPos.y + increment * SpineOffset, 0))

            if self.ArticulationList:  # Not the first joint, we have to connect it to the previous on...
                pymel.core.connectJoint(currrentArticulation,  self.ArticulationList[-1],  parentMode=True)  # Remember,  -1 is the index of the last member of a list.
            else:
                self.Hips = currrentArticulation

            if increment == 6:  # The last two bones are Neck and Head, so Arms takes their root from bellow.
                self.NeckBase = currrentArticulation

            self.ArticulationList.append(currrentArticulation)


class Limb(object):
    def __init__(self, _Name):
        self.ArticulationList = []  # Same name as in Avatar member, but 'self.' indicates the ownership of this class instance. We're deadling with two very different variables so.
        self.Name = _Name


class Arm(Limb):
    def __init__(self, _Name, _NeckBase,  _Offset):
        super(Arm, self).__init__("Arm_" + _Name)  # Inheritance in python is not automatic. Super (Mother) class of Arm is Limb, and its methods are callable form Arm instance, but it needs the keywords super to do that.
        BasePosition = _NeckBase.getTranslation(space='world')

        for i in range(3):
            pymel.core.general.select(clear=True)
            currentArticulation = pymel.core.nodetypes.Joint(name=self.Name + "_" + str(i + 1).zfill(2))
            currentArticulation.setTranslation(BasePosition + _Offset * (i + 1))
            if not self.ArticulationList:  # The first arm joint must be connected to the Base.
                pymel.core.connectJoint(currentArticulation,  _NeckBase,  parentMode=True)
            else:
                pymel.core.connectJoint(currentArticulation,  self.ArticulationList[-1],  parentMode=True)
            self.ArticulationList.append(currentArticulation)


class Leg(Limb):
    NbArticulation = 3

    def __init__(self, _Name, _HipsBase,  _Offset):
        super(Leg, self).__init__("Leg_" + _Name)
        BasePosition = _HipsBase.getTranslation(space='world')
        previouslyCreated = _HipsBase
        for i in range(3):
            pymel.core.general.select(clear=True)
            currentArticulation = pymel.core.nodetypes.Joint(name=self.Name + "_" + str(i + 1).zfill(2))

            if(previouslyCreated == _HipsBase):  # First joint
                currentArticulation.setTranslation(BasePosition + _Offset, space='world')
            else:
                InverseLerp = (1 - (i / float(2)))   # Lerp is the transpose of a range FROM [min = >max] TO [0 = >1]   # Arm.NbArticulation-1 because we do not care about the first Articulation
                currentArticulation.setTranslation(maya.OpenMaya.MVector(previouslyCreated.getTranslation(space='world').x, previouslyCreated.getTranslation(space='world').y * InverseLerp, previouslyCreated.getTranslation(space='world').z), space='world')

            pymel.core.connectJoint(currentArticulation,  previouslyCreated,  parentMode=True)
            previouslyCreated = currentArticulation
            self.ArticulationList.append(currentArticulation)

I hope you had catch it. Now let's manipulate that:


JohnDoe = Avatar()  # This variable aim to a newly created object, typed from class Avatar.

#As in pymel, you no longer need to find Hips by name and fit a naming-convention.

JohnDoe.Hips.setTranslation(maya.OpenMaya.MVector(1,2,3)) #Move hips

#Now, let's select both hands:
pymel.core.general.select(clear=True)  
for currentLimb in JohnDoe.LimbList:
    if type(currentLimb) is Arm:
        pymel.core.general.select( currentLimb.ArticulationList[-1],add=True )

See how Object-Orientation is more about giving sense to a bunch of integers and strings ?
But this backside of the medal is this design thing. Sometimes you don't need to deeply create a very precise hierarchy of objects.
Worse, you can have a big change in your framework (like a new Software in the pipeline), which needs you to re-think your all classes...
As this concise example lacks of real interest, the next post will go (a little) further into Avatar creation.

Comments (1) Trackbacks (0)
  1. Wow,
    This is an awesome example for OOP and inheritance.

    Thanks very much.


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