def add_channel(tagname, tagchannel):

    buffer = c4d.BaseList2D(c4d.Zmultipass)
    buffer.GetDataInstance()[c4d.MULTIPASSOBJECT_TYPE] = c4d.MULTIPASSOBJECT_OBJECTBUFFER
    # check nach vorhandenen MPs
    rd = doc.GetActiveRenderData()
    vp = rd.GetFirstMultipass()
    exist_buffer = 0

    while vp:
        if vp.GetTypeName() == "Object Buffer":

           if vp[c4d.MULTIPASSOBJECT_OBJECTBUFFER] == tagchannel:
               print "channel "+ str(tagchannel) + " vorhanden"
               exist_buffer = 1

        vp = vp.GetNext()

    if exist_buffer == 0:
        buffer.SetName ( tagname )
        buffer[c4d.MULTIPASSOBJECT_OBJECTBUFFER] = tagchannel
        rd.InsertMultipass (buffer)