View source for Documentation/Labs/VTK8

This page documents the update of Slicer to use VTK 8.0.

==Overview==

It is planned to update the version of VTK that Slicer uses from 7.1 to 8.0.

==Status==

* Builds on Linux and Windows.
* Fails to build on Mac due to linker error in VTK introduced in https://gitlab.kitware.com/vtk/vtk/merge_requests/1713. Potential fix is in progress: https://gitlab.kitware.com/vtk/vtk/merge_requests/3014.
* Windows shows OpenGL errors at exit with the OpenGL backend. Likely a regression from https://github.com/Kitware/VTK/commit/49802a3d15fac9dd64feb6e86fb8d0dfc2d31a05.
<pre>
   Generic Warning: In C:\path\to\S\VTKv7\Rendering\OpenGL\vtkOpenGLDisplayListPainter.cxx, line 52
   failed after ReleaseAllLists 16 OpenGL errors detected
   0 : (1282) Invalid operation
   1 : (1282) Invalid operation
   ...
   15 : (1282) Invalid operation
</pre>
* Likely will need to build Slicer with C++11 support.
* On Mac, if targeting 10.7 or 10.8, likely will need to set CMAKE_CXX_FLAGS=-stdlib=libc++ for C++11 support. Note that this would require Qt build with libc++, which is officially supported by Qt5. Note: Qt 5.7.1 minimum officially supported deployment target is 10.8.
* There may be picking issues with widgets. For example, the slice view ROI handles don't work correctly.
* <del>* Windows (VS2013) may have a dependency issue related to Python wrapping. At first, wrapping seems to not use the hierarchy files or runs before they are available. Building again generates the wrapped classes correctly.</del> (resolved)
* <del>Update "VTKv7" project name to "VTKv8".</del>
* <del>Add <tt>Slicer_VTK_VERSION_MAJOR</tt> option to support building with VTK7 and VTK8, as was done for VTK5/6 in https://github.com/Slicer/Slicer/commit/50281153c57c683106498295ea82472eaa20eee4. VTK7 wrapping should not build the hierarchy files.</del>
* <del>EMSegment remote module will require updates to fix vtkDebugLeaks output.</del> (updated in r17131)

===Branches/Merge Requests===

<b>VTK:</b>

* https://github.com/Slicer/VTK/commits/slicer-v8.0.0-2017-06-26-782d7ed

<b>Slicer:</b>

* https://github.com/msmolens/Slicer/commits/support-vtk8

====Outstanding VTK issues====

* <del>Respect access specifier of using statements in wrapping: https://gitlab.kitware.com/vtk/vtk/merge_requests/2988</del> {{done}}
* OpenGL errors occur when destroying vtkWin32OpenGLRenderWindow: https://gitlab.kitware.com/vtk/vtk/issues/17076
** Would be great to get a proper fix from the VTK team for this regression.
* Can't build VTK on Mac with <tt>-DVTK_WRAP_PYTHON:BOOL=ON -DVTK_ENABLE_KITS:BOOL=ON</tt> because libvtkWrapping-8.1.1.dylib doesn't link.
* Update Slicer/VTK branch to include upstreamed fixes

====Slicer-specific VTK commits====

Slicer's VTK 8.0 branch includes fixes/changes that aren't in upstream:
* Fix linking libvtkWrapping with Python wrapping and kits enabled on Mac
** https://github.com/Slicer/VTK/commit/cbb1269509cb32d3827adfce1795e51209132e73
* <del>Respect access specifier of using statements in wrapping</del> (Merged upstream)
** <del>https://github.com/Slicer/VTK/commit/93a7c677481259cf23121a053d82ba4b7c2286e6</del>
* ENH: Allow selection of seed points using vtkSeedWidget
** https://github.com/Slicer/VTK/commit/734709ee38a0a25c92a9a0c7477cfddec5e12cf3
* Ensure vtkVariant stream associated with << operator is set back to "dec".
** https://github.com/Slicer/VTK/commit/dbf5de60c0809ca6233fe428761ef19e561ef542
* BUG: WIP: fix vtkPickingManager interaction with widgets
** https://github.com/Slicer/VTK/commit/2c55e994475b6568a5b3e7363ab70c4dd722cc5d

===Test results===

<b>Windows:</b>

Test results:
* http://slicer.cdash.org/viewTest.php?onlyfailed&buildid=1058006 (Release configuration, OpenGL backend)

<b>Linux:</b>

Test results:
* http://slicer.cdash.org/viewTest.php?onlyfailed&buildid=1053469 (Release configuration, OpenGL backend, Ubuntu 16.04)
** py_StandaloneEditorWidgetTest is vtkDebugLeaks; also occurs when built with VTK7.
* http://slicer.cdash.org/viewTest.php?onlyfailed&buildid=1053471 (Release configuration, OpenGL2 backend, Ubuntu 16.04)
<b>OS X:</b>

Test results:
* TBD

===Rendering backend===

Although VTK 7 changed the default setting of <tt>VTK_RENDERING_BACKEND</tt> to <tt>OpenGL2</tt>, Slicer still explicitly sets it to <tt>OpenGL</tt>. The rendering backend in Slicer is set using <tt>Slicer_VTK_RENDERING_BACKEND</tt>.

====OpenGL2 backend known issues====

* Need to move OS X nightly build to new factory machine to be able to set <tt>CMAKE_OSX_DEPLOYMENT_TARGET:STRING=10.7</tt>. This will ensure that the required OpenGL version is found. See https://github.com/Slicer/Slicer/pull/595.
** WIP PR: https://github.com/Slicer/DashboardScripts/pull/12. Will need to be updated for Qt5.
* [http://www.na-mic.org/Bug/view.php?id=4252 4252: Slicer crashes on start when started through Windows Remote Desktop]

===Discussion===

The transition plan was announced on the Slicer forum: https://discourse.slicer.org/t/transition-to-vtk-8-0/379.

==Migration guide==

This section lists categories of code changes necessary to build and run Slicer with VTK 8.0. Each category has a short description, a suggested upgrade path, and references to relevant commits (TBD once merged).

===Use hierarchy files for VTK Python wrapping===

In VTK8 it's necessary to generate hierarchy files for proper wrapping VTK classes in Python. Without the information provided by the hierarchy files, the Python wrapping tool lacks complete information about classes and types. In this case, the generated classes contain methods that shouldn't be wrapped and fail to compile, and include references to types such as <tt>vtkTypeBool</tt>. Once the hierarchy files are generated and provided to the Python wrapping tool, the generated classes compile and typedefs like <tt>vtkTypeBool</tt> are correctly resolved.

Once the VTK8 changes are merged, generating hierarchy files is handled by https://github.com/Slicer/Slicer/blob/master/CMake/vtkMacroKitPythonWrap.cmake.

<b>References:</b>
* http://www.vtk.org/Wiki/VTK/WrapHierarchy

===Call InitializeObjectBase() in vtkObject New() methods===

In VTK8 it's necessary for <tt>vtkObject</tt> <tt>New()</tt> methods to call <tt>InitializeObjectBase()</tt> on the new object for proper tracking with <tt>vtkDebugLeaks</tt>. The standard macros (<tt>vtkStandardNewMacro</tt>, <tt>vtkObjectFactoryNewMacro</tt>) handle this. For those classes that don't use the macros, add a call to <tt>InitializeObjectBase()</tt> immediately after constructing the object by <tt>new vtkXXX()</tt>.

Additionally, <tt>vtkObjectFactory::CreateInstance()</tt> now doesn't register the class name with vtkDebugLeaks if the factory fails to create the object. Therefore, it's no longer necessary to unregister the class name with vtkDebugLeaks. Remove calls to <tt>vtkDebugLeaks::DestructClass(className)</tt> following <tt>vtkObjectFactory::CreateInstance()</tt>.

To support both VTK8 and earlier versions of VTK, wrap these changes in preprocessor checks for whether <tt>VTK_HAS_INITIALIZE_OBJECT_BASE</tt> is defined.

<b>References:</b>

* https://github.com/Kitware/VTK/commit/e5c793dbdf87e838bb2b60c6a5905ced0e5548f9
* http://public.kitware.com/pipermail/vtk-developers/2016-September/034332.html

===Add C++11 keywords===

VTK8 requires C++11. Subclasses of VTK classes must mark overridden methods with <tt>VTK_OVERRIDE</tt>.

===Qt5: QVTKOpenGLWidget===

When using Qt5, [https://github.com/Kitware/VTK/blob/9393774ff9b3d735a40eb445ff6c045e55c90eae/GUISupport/Qt/QVTKOpenGLWidget.h QVTKOpenGLWidget] should be used in place of QVTKGLWidget. To ensure that QVTKOpenGLWidget receives a properly configured OpenGL context it's necessary to call QSurfaceFormat::setDefaultFormat() before constructing the QApplication instance. QVTKOpenGLWidget::defaultFormat() supplies a suitable format. See http://doc.qt.io/qt-5/qopenglwidget.html.

==Future work==

Future work in Slicer related to VTK 8 could include:
* Remove BTX/ETX markers.
* Check for and resolve build warnings in Slicer VTK classes.
* Check that settings in <tt>External_VTKv8.cmake</tt> are all still necessary, specifically <tt>VTK_USE_PARALLEL</tt>, and anything related to Qt.