Getting the Input Geometry MObject

Often when we are writing a deformer, we want access to the input geometry MObject in order to get information such as vertex normals, uvs, etc. In the MPxDeformerNode::deform function, all we have available is the MItGeometry iterator which only gives us positional information. We can get the MObject from the datablock but we have to be careful not to trigger any unnecessary dependency graph calculations.

MStatus SomeDeformer::deform( MDataBlock& data, MItGeometry& itGeo, const MMatrix &localToWorldMatrix, unsigned int geomIndex )
{
    MStatus status;
    MArrayDataHandle hInput = data.outputArrayValue( input, &status );
    CHECK_MSTATUS_AND_RETURN_IT( status )
    status = hInput.jumpToElement( geomIndex );
    CHECK_MSTATUS_AND_RETURN_IT( status )
    MObject oInputGeom = hInput.outputValue().child( inputGeom ).asMesh();
    MFnMesh fnInputMesh( oInputGeom );
}

def deform(self, data, itGeo, localToWorldMatrix, geomIndex ):
    inputAttribute = OpenMayaMPx.cvar.MPxDeformerNode_input
    inputGeom = OpenMayaMPx.cvar.MPxDeformerNode_inputGeom
    hInput = data.outputArrayValue( inputAttribute )
    hInput.jumpToElement( geomIndex )
    oInputGeom = hInput.outputValue().child( inputGeom ).asMesh()
    fnInputMesh = OpenMaya.MFnMesh( oInputGeom )

In MPxDeformerNode, the compute method is already implemented for us. The compute method gets the input geometry for us, creates the geometry interator, and calls the deform method, which is what we implement. Notice when I get the data handles, I use outputArrayValue and outputValue. This prevents Maya from triggering a dirty propagation. If I were to use inputArrayValue and inputValue, Maya would recalculate the input geometry, causing an unnecessary graph evaluation since this was already done in the compute method.