#Maya Python Script

##DAG Hirarchy Directed Acyclic Graphs is a directed graph that contains no cycles.

In Maya, The DAG is composed of 2 types of DAG nodes

• transforms Maintain transformation and parenting information. Transform node can have multiple child nodes.
• shapes Reference geometry. Do not provide parenting or transformation infomation Any piece of geometry requires two DAG nodes above it, a shape node immediately above it, and a transform node above the shape node.

DAG describes how an instance of an object is constructed from a piece of geometry

• MFnDagNode - has methods for determining the number of parents and thr parents of a node
• MFnTransform - is the function set for operating on transform node (derived from MFnDagNode) and has methods to get and set transformation.
• MFnMesh - is one of many types of function sets which operate on the many types of shape nodes. (derived from MFnDagNode, but not derived from MFnTransform)

##Instancing Whenever a transform or shape node has multiple parent nodes, the node is considered to be instanced. Instancing can be useful to reduce the amount of geometry for a model.

The two valid DAG paths in this graph :

• Transform1 - Transform3 - Leaf
• Transform2 - Transform3 - Leaf

##DAG paths A path through the DAG is a set of nodes which uniquely identifies the location of the particular node or instance of a node in the graph. The path represents a graph ancestry beginning with the root node of the graph and containing, in succession, a particular child of the root node followed by particular child of this child,etc., down to the node identified by the path. For instanced nodes, there are multiple paths which lead from the root node to the instanced node, one path for each instance.

<<<<<<< HEAD In Maya, selection at the object level results in the selection of the transform node that is the parent of the shpae actually selected. When querying the selection using MGlobal::getActiveSelectionList(), the MDagPath returned to the caller only specifies the path to this transform and not down to the actual shape that was picked on the screen. A convenience method on MDagPath called extendToShape() can be called to add the shape node below the last transform to the path.

The valid function sets applicable to a particular MDagPath are determined by the last node on the path. If the last node is a transform node, then the function sets that can operate on transform nodes can be applied to the MDagPath instance. If a shape node is the last node of the path, then the applicable function sets for the MDagPath instance are those sets which can operate on the shape.

###Inclusive and exclusive matirices Since the nodes in a path exist at different levels of the DAG hierarchy. there is a different transformation that may have accumulated at each node in the path. the MDagPath class allows these transformations to be returned using the inclusiveMatrix() and exclusiveMatrix() classed.

-inclusive matrix - represents the accumulated transformation to the last node stored in the DAG path taking into account the last node. -exclusive matrix - represents the same accumulation with the exception that it does not take into account any transformation form the last node.

|RootTransform|Transform1|Transform2|Shape

• inclusive matrix down to Transform2
• exclusive matrix down to Transform1

##Generalized instancing I can't understand it yet

##Transforms with multiple shapes. A transform node can have any number of transform nodes as children. In general, a transform node can only have a single shape node as a child, and when viewing the DAG through an interactive window this will always be the case. However when examining the DAG through the API you will find that transforms may have multiple shape nodes as children. the happens when the original shape under the transform has been modified by the dependency graph. To maintain the transformations on the result of the dependency graph, the result is placed under the same transform asthe original node. The new node would have the same DAG transforms applied as original, but would be modified in some way. When this happens, only the final product is visible in an interactive window, and the original nodes are historical. I can't understand it yet

======= ###why add the shape node to a DAG path In Maya, selection at the object lebel results in the selection od the transform node that is the parent of the shpae actually selected. When querying the selection using MGlobal::getActiveSelectionList(), the MDagPath returned to the caller only specifies the path to this transform and not down to the actual shape that was picked on the screen. A convenience method on MDagPath called extendToShape() can be called to add the shape node below the last transform to the path.

The valid function sets applicable to a particular MDagPath are determined by the last node on the path. If the last node is a transform node, then the function sets that can operate on transform nodes can be applied to the MDagPath instance. If a shape node is the last node of the path, then the applicable function sets for the MDagPath instance are those sets which can operate on the shape.

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##Examples

import maya.api.OpenMaya as om

def getDag(name):
    selectionList = om.MSelectionList()
    try:
        selectionList.add(name)
    except:
        return None
        
    dagPath = selectionList.getDagPath(0)
    return dagPath
    
name = #DAG name
dag = om.MFnDagNode(getDag(name))
print name + ' has %s parents' %name, dag.parentCount()
print '[PATH] %s' % dag.getPath()

pCube1 has 1 parents
[PATH] pCube1

pCube1Shape has 2 parents
[PATH] pCube1|pCube1Shape
-> how to get 2 paths?

pCube1_instance has 1 parents
[PATH] pCube1_instance

group1 has 1 parents
[PATH] group1

pCube1_duplicated has 2 parents
[PATH] group1|pCube1_duplicated

pCube1_duplicatedShape has 1 parents
[PATH] group1|pCube1_duplicated|pCube1_duplicatedShape

group1_instance has 1 parents
[PATH] group1_instance