How to select hard edges from a mesh in Maya?

A hard edge (aka crease edge or feature edge) is:

1. shared by two polygons.
2. and the dihedral angle (i.e the angle between two polygons) is above user-specified threshold.

To find these edges in Maya, the intuitive approach is to traverse all the edges by MItMeshEdge and examining the dot product of two adjacent faces’ normals.

Suppose we have MObject(s) of target mesh and edge components (optional) selected by user, and we want to replace current selection with the hard edges from these input. We could first create a MItMeshEdge with the mesh and edge components, then for each visited edge:

1. Get ids of adjacent faces by MItMeshEdge.getConnectedFaces.
2. Use these ids to get face normals with MFnMesh.getPolygonNormals.
3. Determine if the dot product of two face normals is less than the threshold given by user.
4. Append these edges to a selection list.
5. Finally, set the selection list as an active one with MGlobal.setActiveSelectionList.

def selectHardEdge(angleInDegree, dpItem, moComp=om.cvar.MObject_kNullObj):
    """
    \param dpItem The dag Path of given object.
    \param moComp The MObject of the components.
    """

    angle = math.radians(angleInDegree)
    cosThreshold = math.cos(angle)

    fnMesh = om.MFnMesh(dpItem)
    itEdge = om.MItMeshEdge(dpItem, moComp)

    faceIds = om.MIntArray()
    faceNormal1 = om.MVector()
    faceNormal2 = om.MVector()

    selResult = om.MSelectionList()

    while not itEdge.isDone():

        itEdge.getConnectedFaces(faceIds)
        assert(len(faceIds) == 2)

        fnMesh.getPolygonNormal(faceIds[0], faceNormal1)
        fnMesh.getPolygonNormal(faceIds[1], faceNormal2)

        if cosThreshold > faceNormal1 * faceNormal2:
            selResult.add(dpItem, itEdge.currentItem(), True)

        itEdge.next()

    om.MGlobal.setActiveSelectionList(selResult)

Note: If we use OpenMaya.MObject.kNullObj as the default value of moComp, we would get following error message: # Error: NotImplementedError: Wrong number of arguments for overloaded function ‘new_MItMeshEdge’. Possible C/C++ prototypes are: MItMeshEdge(MObject &,MStatus *) MItMeshEdge(MObject &,MObject &,MStatus *) MItMeshEdge(MDagPath const &,MObject &,MStatus *) #

It is because Maya binds these static variables with cvar in each module, thus we should use OpenMaya.cvar.MObject_kNullObj instead!