VTK/Module Development

A new modular build system was developed for VTK 6.0, which makes it much simpler to add new modules to VTK and to express the dependencies between modules and other libraries. Before VTK 6 there were kits arranged in one directory level, such as Common, Charts, Rendering, Filtering, etc, in VTK 6 due to the growth of the toolkit the decision was taken to arrange modules in two levels. The top level can only be one work, camel-cased, and acts to organize modules into high level themes such as Common, Rendering, Filters, etc. The second level contains the actual classes, along with the relevant CMake code.

In order to facilitate a more dynamic toolkit, where source can be added or removed a more loosely coupled system was developed inspired by ITK and the Boost CMake build systems. VTK is configured in two passes, the first pass globs all directories two levels deep for a file called module.cmake. This file must be present for the module to be evaluated, once found each of these files is included by CMake calling the vtk_module macro and evaluating the module dependencies. Once all modules have been evaluated their dependencies are used to determine a valid order for the modules to be added dynamically at configure time.

module.cmake

The typical module.cmake file must declare the module name. Beyond the module name all other parameters are considered optional. A typical module.cmake for vtkCommonDataModel is shown below,

<source lang="cmake"> vtk_module(vtkCommonDataModel

 DEPENDS
   vtkCommonSystem
   vtkCommonMath
   vtkCommonMisc
   vtkCommonTransforms
 TEST_DEPENDS
   vtkTestingCore
   vtkCommonExecutionModel
   vtkIOGeometry
   vtkRenderingCore
 )

</source>

The first argument is the module name, this is the name of the C++ library that will be compiled. Next is the DEPENDS argument which allows the developer to express the module's dependencies. The build system will ensure that each of those libraries is added before this module, and it will take care of linking the C++ library to these dependencies. The TEST_DEPENDS argument adds extra dependencies that the tests require, these will not be linked to the C++ library and by default the testing for the module will only be enabled if these dependencies are being built.

The vtkRenderingOpenGL module introduces a few more optional arguments that can be supplied to the vtk_module macro.

<source lang="cmake"> vtk_module(vtkRenderingOpenGL

 GROUPS
   Rendering
 IMPLEMENTS
   vtkRenderingCore
 DEPENDS
   # These are likely to be removed soon - split Rendering/OpenGL further.
   vtkImagingHybrid # For vtkSampleFunction
 COMPILE_DEPENDS
   vtkParseOGLExt
   vtkUtilitiesEncodeString
 TEST_DEPENDS
   vtkInteractionStyle
   vtkTestingRendering
   vtkIOExport
   vtkRenderingFreeTypeOpenGL
   vtkRenderingImage
   vtkRenderingLOD
   vtkRenderingLabel
   vtkImagingGeneral
   vtkImagingSources
   vtkFiltersProgrammable
   vtkRenderingAnnotation
 )

</source>

The GROUPS parameter adds the module to a higher level group option that can be turned on or off to enable multiple modules. The IMPLEMENTS keyword informs us that the module implements some of the API classes in the vtkRenderingCore module, this means that the object factory override mechanism will be used by this module to override some of the interface classes in that module. COMPILE_DEPENDS informs us that this module needs the vtkParseOGLExt and vtkUtilitiesEncodeString modules to be available at build time, but that we do not want to link to them (although we could do so manually).