PARAVIS Module - Architecture and conception

This documentation is intended for SALOME’s developpers or anyone wishing to modify the module itself. If you are looking for user documentation, please launch SALOME, activate the PARAVIS module, and refer to the Help menu there.

PARAVIS is the visualization module of SALOME. The module is a tight integration of the functionalities offered by ParaView in the SALOME architecture. The architecture of the PARAVIS module has been revised end of 2014 to offer a smoother integration with ParaView.

If you are looking for the Doxygen of the C++ code, it can be found here: Doxygen documentation

Overview - Executive summary

The PARAVIS module represents the integration of ParaView inside SALOME.

SALOME uses by default the detached server mode of ParaView: the pvserver is launched outside the main Salome process and the ParaVis module, or the PVViewer view (Window -> ParaView view) connects to it.

Following this logic, the PVSERVER CORBA service has a very restrained role. Its only purpose is to:

  • control the start and stop of the pvserver process
  • provide the URL of the pvserver, so that a client can connect to it.

Hence, we emphazise the fact that the CORBA engine does not provide any access to the objects or the visualisation results themselves. It only serves to establish the link with the pvserver. The latter can then be queried (with the standard ParaView mechanisms) to retrieve those objects.

A typical session looks like this:

  • start SALOME’s GUI

  • request activation of PARAVIS (or request activation of a Paraview’s View)
    • activation of the PVSERVER CORBA service
    • invokation of the method FindOrStartPVServer(): launches the pvserver process and returns its URL (in the standard ParaView’s format, e.g. cs://localhost:11111)
    • invokation of the standard ParaView’s API to connect to the pvserver (e.g. Connect() method in the Python module paraview.simple)
  • use the standard ParaView’s API to interact with the server (either from the C++ side, within SALOME’s GUI or from a Python script, using for example the methods provided in the Python module pvsimple).

The picture below summarizes the architecture:

_images/archi.jpg

In terms of code structure, the main, all the initialization logic of ParaView is attached to the PVViewer (ParaView’s viewer) located in the GUI module in the src/PVViewer folder. The CORBA engine and the graphical interface of the ParaVis module are located in the ParaVis module of SALOME.

Functionalities

The following functionalities are offered by the PVSERVER and the ParaVis module:

  • full embedding of ParaView’s functionalities inside SALOME environment
  • manage ParaVis GUI and ParaView server data from Python scripts in synchronized mode.
  • compatibility of the Python scripting interface with the paraview.simple and the paraview.servermanager modules.

Folder structure

ParaVis module

In the ParaVis module, here is the list of code folders:

  • idl: contains the IDL for the PVSERVER CORBA service
  • src/ENGINE: implementation of the IDL’s functionalities in Python. Mainly deal with the start/stop of the pvserver
  • src/Plugins: SALOME’s specific plugins for ParaView: MEDReader, etc …
  • src/PVGUI: graphical elements constituing the ParaVis client in the SALOME GUI. Management of the menus, the toolbars, etc … seen in PARAVIS interface.
  • src/PV_SWIG: Python modules to be able to invoke visualization functionalities from a script

At the time of writing the PVSERVER CORBA service is sitll hosted by the ParaVis module, but it should move to GUI to be able to compile GUI without any dependency to PARAVIS. At present, this is only a weak dependency in the sense that nothing is needed at link time, but only at run-time.

GUI module

One can request a ParaView view without activating the ParaVis module itself. For example the MED module now integrates a control visualization which is in fact a ParaView view.

To make this work, a specific type of viewer (PVViewer, short for ParaView viewer) has been created in the GUI module itself. The code is located in src/PVViewer.

This folder contains the following classes:

  • PVViewer_Behaviors: re-instanciates the desired ParaView behaviors (a behavior defines for example the fact that ParaView should automatically reconnect to the server if a disconnection occurs)
  • PVViewer_EngineWrapper: encapsulates the calls to the PVSERVER CORBA service in a dynamic fashion, so that GUI can be compiled without having a link dependency to the ParaVis module
  • PVViewer_GUIElements: see Viewer part (in GUI module)
  • PVViewer_LogWindowAdapter: an adapter to redirect VTK and ParaView’s output messages to the SALOME’s message window (not working?)

The folder also contain the adaptor classes needed to make the ParaView native 3D view (a pqTabbedMultiViewWidget) fit into the SUIT model (i.e. the model imposed by SALOME’s GUI architecture to define a new type of view):

  • PVViewer_ViewManager: this class centralizes all the initialization logic (see method ParaviewInitApp) of the ParaView application (pqCoreApplication).
  • PVViewer_ViewModel
  • PVViewer_ViewWindow

Reminder about ParaView’s architecture

ParaView works in a client/server mode. In two words, a server part (the pvserver) takes care of the ‘intensive’ computations (filter, etc …) and a client part serves to control this server, and obviously visualize the final rendering.

The pvserver represents the main visualisation server, and can be either:

  • built-in, in which case, launching ParaView suffices to activate it automatically;
  • detached, in which case, one has to launch the server first (possibly on another host) and then connect to it from a client.

The various types of clients are:

  • either the standard ParaView GUI (where the name and type of the current server can be seen by looking at the top element in the pipeline widget)
  • or a Python script, using for example the module paraview.simple and the Connect() method.

Historically the pvserver was not able to receive the connections from multiple clients, but this has been changed from ParaView 4.0 (or was it 3.98?). Salome now exploits this feature.

Viewer part (in GUI module)

In the GUI module of SALOME, the folder src/PVViewer contains all the code needed to activate a minimal ParaView 3D view, without activating the ParaVis module itself. This folder hence deals with:

  • the initialization of the ParaView application (pqApplicationCore)
  • the initialization of ParaView’s desired behaviors (class PVViewer_GUIElements)
  • the initialization of all the GUI elements needed for a later activation of the ParaVis interface: at the time of writing the pipeline, some menus, and other elements are very hard to connect after having set up a 3D view. They are however not wanted when the user just requested a 3D view, outside the ParaVis interface. We hence create those elements any way, but hide them, so that we can later show them again, once the ParaVis module is activated. The class PVViewer_GUIElements is in charge of this.

The PVViewer follows otherwise the standard structure of a Salome’s view (SUIT model).

A special trick is used to make PVGUI_ViewWindow the parent of the pqViewManager widget. It is created initally by pqMainWindowCore with the desktop as a parent, so when it is shown, a PVGUI_ViewWindow instance is passed to its setParent() method. In the destructor PVGUI_ViewWindow::~PVGUI_ViewWindow()``the parent is nullified to avoid deletion of the ``pqViewManager widget (that would break the pqMainWindowCore class).

ParaVis graphical interface

The initialization of the viewer (see previous section) takes part of instantiating the most important widgets, notably:

  • the pipeline
  • the dynamic menus (filters and sources)
  • the macros
  • the Properties panel
  • and finally the toolbars

All those menus are dynamic in the sense that they are automatically populated when a plugin/a configuration is loaded (this is also they need to be connected so early by the PVViewer_GUIElements class seen before).

In the ParaVis module, the class PVGUI_Module represents the GUI client compliant with the usual architecture of a SALOME GUI module. The implementation is split in three cxx files:

  • PVGUI_Module.cxx: core stuff: module initialization and activation, management of the Python trace, etc …
  • PVGUI_Module_actions.cxx: creation of the Qt actions and menus
  • PVGUI_Module_widgets.cxx: hide/show various widgets and save/restore their positions in the main window.

Embedded Python interpreter - Multi-threading

ParaView is a mono-threaded application. It also provides an embedded Python interpreter to make the Python shell work. SALOME on the other hand is multi-threaded, and also provides an embedded Python’s interpreter.

Making the two work together has often been (and still is) a painful job! If you run into this sort of problems, take a look at what the GIL is: Global Interpreter Lock

In Salome, the current setup is to:

  • patch ParaView itself so that all calls to the Python C API are GIL safe (using PyGILState_Ensure, PyGILState_Release)
  • have Salome’s embedded Python console work in mono-threaded mode (although it is fully capable of being asynchronous). This is achieved in src/PyConsole/PyConsole_Editor.cxx and the initialization of the myIsSync boolean member to True.

The last point is of crucial importance: it basically means that all the GUI events are in a single thread. Even without considering

All the calls to the Python API in the rest of SALOME are (should be!) GIL safe.

The ParaView Python’s trace mechanism has long been a problem, but has fortunately been rationalized thanks to the API of ParaView providing clear methods to control the start/stop (and other options of the trace). This is grouped in the ParaViewCore/ServerManager/Core/vtkSMTrace class and used in the method PVGUI_Module::startTrace().

Python modules

The modules found in src/PV_SWIG are mostly simple namespace forwards from the original ParaView’s modules (i.e. they redirect to the original modules):

  • pvsimple is a forward of paraview.simple with little extra functionalities to make sure:
    • the connection to the correct PVSERVER is automatically established
    • that a ParaView’s view is available when importing the module from the embedded Python console.
  • paravisSM is a forward of paraview.servermanager. It is left mostly for backward compatibility (it used to be full of nasty overrides).

Those forward/similarities are naturally intended so that a script written for pure ParaView can easily be ported to ParaVis. The conversion boils down to replacing import paraview.simple by impory pvsimple (with a few other extra details of lesser importance).

Updating to a newer ParaView version

The following items should be revised each time an upgrade to a newer ParaView version is done. They are often a copy/paste of ParaView’s source code with a slight amendment to fit SALOME’s requirements.

  • initialization sequence: currently located in GUI module, PVViewer_ViewManager::ParaViewInitApp(): the following classes should be inspected, and compared with their equivalent in ParaView source code to see if an update is necessary:

    • PVViewer_ViewManager (GUI module): method ParaviewInitApp(), ParaviewInitBehaviors(), ParaviewLoadConfigurations() and finally ConnectToExternalPVServer() should be re-read. Their precise ordering is used in PVGUI_Module::initialize() and the whole sequence should be compared with what can be found in:

      • Applications/ParaView/ParaViewMainWindow.cxx
      • CMake/branded_paraview_initializer.cxx.in, and the method Initialize()
    • PVViewer_Behaviors (GUI module): compare with Qt/ApplicationComponents/pqParaViewBehaviors.cxx

  • menus and actions: PVGUI_Module_widgets.cxx (ParaVis module) should be compared with Applications/ParaView/ParaViewMainWindow.cxx

  • toolbars: PVViewer_GUIElements::myBuildToolbars() (GUI module): compare with pqParaViewMenuBuilders::buildToolbars()

  • dock widgets placement: PVGUI_Module::setupDockWidgets() (ParaVis module): compare with Application/Paraview/ParaviewMainWindow.ui

  • settings dialog box: PVGUI_ParaViewSettingsPane (ParaVis module) should be compared with Qt/Components/pqSettingsDialog.h

  • trace mechanism: method PVGUI_Module::startTrace() should be compared with pqTraceReaction::start() in file Qt/ApplicationComponents/pqTraceReaction.h

Miscellaneous

Trace management

Contrary to ParaView, which can start/stop its trace at any moment, in PARAVIS the trace is activated or deactivated for the whole session.

The trace functionality can be switched on/off in SALOME preferences dialog box, in the PARAVIS tab (main menu | Preferences…). It contains a check box “Deactivate Trace”. By default the trace is activated. Change of check box state makes effect only for next session.

Also, the trace is used for the “Dump Study” functionality. But if the tracing is switched off then the “Dump Study” doesn’t save PARAVIS module trace.

Application options

If it is necessary to define a spcific command line parameter for ParaView application, then it can be defined with the help of the PARAVIEW_OPTIONS environment variable. For example:

export PARAVIEW_OPTIONS=--server=myServer

If it is necessary to define several command line parameters, these parameters have to be separated by the “:” character.

Various TODO

  • make the PVSERVER a true CORBA service not linked to the PARAVIS module
  • the PARAVIS module should be a light module (TODO check again why this is blocking).

Doxygen documentation

If you are looking for the Doxygen of the C++ code, it can be found here: Doxygen documentation