scripted/pyVertexBufferMutator.py

# Copyright 2015 Autodesk, Inc. All rights reserved.
# 
# Use of this software is subject to the terms of the Autodesk
# license agreement provided at the time of installation or download,
# or which otherwise accompanies this software in either electronic
# or hard copy form.

from ctypes import *
import maya.api.OpenMayaRender as omr
import maya.api.OpenMaya as om

# Example plugin: vertexBufferMutator.cpp
#
# This plug-in is an example of a custom MPxVertexBufferMutator.
# It provides custom vertex streams based on shader requirements coming from 
# an MPxShaderOverride.  The semanticName() in the MVertexBufferDescriptor is used 
# to signify a unique identifier for a custom stream.

# This plugin is meant to be used in conjunction with the hwPhongShader
# where the position geometry is defined with the semantic "swizzlePosition"

def maya_useNewAPI():
    """
    The presence of this function tells Maya that the plugin produces, and
    expects to be passed, objects created using the Maya Python API 2.0.
    """
    pass

class MyCustomBufferMutator(omr.MPxVertexBufferMutator):
    def __init__(self):
        omr.MPxVertexBufferMutator.__init__(self)

    def modifyVertexStream(self, object, vertexBuffer, targetIndexing):
        # get the descriptor from the vertex buffer.  
        # It describes the format and layout of the stream.
        descriptor = vertexBuffer.descriptor()
        
        # we are expecting a float stream.
        if descriptor.dataType != omr.MGeometry.kFloat:
            return

        # we are expecting a float3
        if descriptor.dimension != 3:
            return

        # we are expecting a position channel
        if descriptor.semantic != omr.MGeometry.kPosition:
            return

        # get the mesh from the current path
        # if it is not a mesh we do nothing.
        mesh = om.MFnMesh(object)

        vertexCount = vertexBuffer.vertexCount()
        if vertexCount <= 0:
            return

        # acquire the buffer to fill with data.
        buffer = vertexBuffer.acquire(vertexCount, False)   # writeOnly = False : we want the current buffer values

        inc = sizeof(c_float)
        address = buffer

        for i in range(0, vertexCount):
            # Here we swap the x, y and z values
            xaddr = address
            yaddr = address+inc
            zaddr = address+2*inc
            address += 3*inc

            x = c_float.from_address(xaddr).value
            c_float.from_address(xaddr).value = c_float.from_address(yaddr).value   # y --> x
            c_float.from_address(yaddr).value = c_float.from_address(zaddr).value   # z --> y
            c_float.from_address(zaddr).value = x                                   # x --> z

        # commit the buffer to signal completion.
        vertexBuffer.commit(buffer)

# This is the buffer generator creation function registered with the DrawRegistry.
# Used to initialize the generator.
def createMyCustomBufferMutator():
    return MyCustomBufferMutator()

# The following routines are used to register/unregister
# the vertex mutators with Maya
def initializePlugin(obj):

    omr.MDrawRegistry.registerVertexBufferMutator("swizzlePosition", createMyCustomBufferMutator)

def uninitializePlugin(obj):

    omr.MDrawRegistry.deregisterVertexBufferMutator("swizzlePosition")