Module System#

VTK 9.0 introduces a new build system compared to previous versions. This version uses CMake’s built-in functionality for behaviors that were performed manually in the previous iteration of the build system.


  • module: A unit of API provided by a project. This is the core of the system and there are lots of features available through this mechanism that are not provided by CMake’s library or other usage requirements.

  • group: A configure-time collection of modules. These may be used to control whether member modules will be built or not with a single flag.

  • kit: A collection of modules for which all the compiled code is placed in a single library.

  • property: An attribute of a module. Only of real interest to developers of the module system and its extensions.

  • autoinit: A mechanism for triggering registration to global registries based on the complete set of linked-to libraries.

  • third party: A module representing an external dependency.

  • enable status: A 4-way state to allow for “weak” and “strong” selection or deselection of a module or group for building.


The module system was designed with a number of principles in mind. These should be followed as much as possible when developing extensions as well.

  • The minimum CMake version required by the module system should be as low as possible to get the required features. For example, if a new feature is available in 3.15 that improves core module functionality, that’d be a reasonable reason to require it. But a bugfix in 3.10 that can be worked around should not bump the minimum version. Currently CMake 3.8 is expected to work, though various features (such as kits) are only available with newer CMake versions.

  • Build tree looks like the install tree. The layout of the build tree is set up to mirror the layout of the install tree. This allows more code content to be shared between build and install time.

  • Convention over configuration. CMake conventions should be followed. Of note, projects are assumed to be “well-behaved” including, but not limited to:

  • Configuration through API. Where configuration is provided, instead of using global state or “magic” variables, configuration should be provided through parameters to the API functions provided. Concessions are made for rarely-used functionality or where the API would be complicated to plumb through the required information. These variables (which are typically parameterized) are documented at the end of this document. Such variables should be named so that it is unambiguous that they are for the module system.

  • Don’t pollute the environment. Variables should be cleaned up at the end of macros and functions should use variable names that don’t conflict with the caller environment (usually by prefixing with _function_name_ or the like).

  • Relocatable installs. Install trees should not bake-in paths from the build tree or build machine (at least by default). This makes it easier to create packages from install trees instead of having to run a post-processing step over it before it may be used for distributable packages.

Build process#

Building modules involves two phases. The first phase is called “scanning” and involves collecting all the information necessary for the second phase, “building”. Scanning uses the vtk_module_scan() function to search the vtk.module files for metadata, gathers the set of modules to build and returns them to the caller. That list of modules is eventually passed to vtk_module_build() which sorts the modules for their build order and then builds each module in turn. This separation allows for scanning and building modules in different groups. For example, the main set of modules may be scanned to determine which of some internal set of modules are required by those which is then scanned separately with different options.

Scanning should occur from the leaf-most module set and work its way inward to the lower levels. This is done so that modules in the lower level that are required higher up can be enabled gracefully. Builds should start at the lower level and move up the tree so that targets required by the higher groups exist when they are built.


Modules are described by vtk.module files. These files are “scanned” using the vtk_module_scan() function. They provide all the information necessary for the module system to:

  • provide cache variables for selecting the module (e.g., VTK_MODULE_ENABLE_ModuleName);

  • construct the dependency tree to automatically enable or disable modules based on whether it is built or not;

  • provide module-level metadata (such as exclusion from any wrapping and marking modules as third party)

The vtk.module files are read and “parsed”, but not executed directly. This ensures that the module files do not contain any procedural CMake code. The files may contain comments starting with # like CMake code. They may either be passed manually to vtk_module_scan() or discovered by using the vtk_module_find_modules() convenience function.

The most important (and only required) parameter is the NAME of a module. This is used as the target name in CMake and is how the module’s target should be referred to in all CMake code, inside the build and from the find_package which provides the module. To change the name of the compiled artifact (library or executable), the LIBRARY_NAME argument may be used.

It is highly recommended to provide a DESCRIPTION for the module. This is added to the documentation for the cache variable so that the user has more than just the module name to know what the module’s purpose is.

Modules may also belong to groups which are created implicitly by adding modules to the same-named group. Groups are listed under the GROUPS argument and are checked in order for a non-default setting to use.

A module may be hidden by using the CONDITION argument. The values passed to this field is added into a CMake if statement and checked for validity (all quoting is passed along verbatim). If the condition evaluates to FALSE, the module is treated as if it did not exist at all.

Module metadata#

A number of pieces of metadata are considered important enough to indicate them at the module level. These are used for managing slightly different workflows for modules which have these properties.

  • EXCLUDE_WRAP: This marks the module with a flag that all language wrapping facilities should use to know that this module is not meant for wrapping in any language. Usually this is for modules containing user interface classes, low-level functionality, or logic that is language specific.

  • IMPLEMENTABLE and IMPLEMENTS: These are used by the autoinit functionality to trigger the static factory registration calls. A module which is listed under an IMPLEMENTS list must be marked as IMPLEMENTABLE itself.

  • THIRD_PARTY: Indicates that the module represents a third party dependency. It may be internal or external to the source tree, but may be used as an additional configuration point if necessary. These modules are implicitly EXCLUDE_WRAP, not IMPLEMENTABLE and do not IMPLEMENTS any module.

Enabling modules for build#

Modules are enabled in a number of ways. These ways allow for project control and user control of which modules should be built or not. There are 4 states for controlling a module’s enable status as well as a DEFAULT setting which is used to allow for other mechanisms to select the enable status:

  • YES: The module must be built.

  • NO: The module must not be built. If a YES module has a NO module in its dependency tree, an error is raised.

  • WANT: The module should be built. It will not be built, however, if it depends on a NO module.

  • DONT_WANT: The module doesn’t need to be built. It will be built if a YES or WANT module depends on it.

  • DEFAULT: Look at other metadata to determine the status.

The first check for modules are via the REQUEST_MODULES and REJECT_MODULES arguments to the vtk_module_scan function. Modules passed to REQUEST_MODULES are treated as if they use YES and REJECT_MODULES as if they use NO. A module may not be passed to both arguments. Modules selected in this way do not have CMake cache variables exposed for them (since it is assumed they are selected via some other mechanism outside the module system).

The next selector is the VTK_MODULE_ENABLE_ variable for the module. This is added to the cache and defaults to DEFAULT. Assuming HIDE_MODULES_FROM_CACHE is not set to ON, this setting is exposed in the cache and allows users to change the status of modules not handled via the REQUEST_MODULES and REJECT_MODULES mechanism.

If a module is still selected as DEFAULT, the list of GROUPS it is a member of is used. In order, each group is looked at for a non-DEFAULT value. If so, its value is used for the module. Groups also default to using DEFAULT for their setting, but a project may set the _vtk_module_group_default_${group} variable to change this default value.

After all of the above logic, if a module is still marked as DEFAULT, the WANT_BY_DEFAULT argument to vtk_module_scan() is used to determine whether it is treated as a WANT or DONT_WANT request.

Now that all modules have a non-DEFAULT enable setting, the set of modules and kits that are available may be determined by traversing the dependency tree of the modules.


Modules have three types of dependencies:

  • DEPENDS: These are dependencies which must be available and are transitively provided to modules depending on this module. The API of the module may be affected by changes in these modules. This includes, but is not limited to, classes in this module inherit or expose classes from the dependent modules.

  • PRIVATE_DEPENDS: Dependencies which are only used in the implementation details of the module. The API of the module is not affected by changes in these modules.

  • OPTIONAL_DEPENDS: Dependencies which will be used if available, but the implementation can cope with their absence. These are always treated as PRIVATE_DEPENDS if they are available.

Modules which are listed in DEPENDS or PRIVATE_DEPENDS are always available to the module and can be assumed to exist if the module is being built. Modules listed in OPTIONAL_DEPENDS cannot be assumed to exist. In CMake code, a TARGET optional_depend condition may be used to detect whether it is available or not. The module system will add a VTK_MODULE_ENABLE_${module} compilation definition set to either 0 or 1 if it is available for use in the module’s code. This flag is made preprocessor-safe by replacing any :: in the module name with _. So an optional dependency on Namespace::Target will use a flag named VTK_MODULE_ENABLE_Namespace_Target.

At this stage, the dependency tree for all scanned modules is traversed, marking dependencies of YES modules as those that should be built, marking modules depending on NO modules as not to be built (and triggering an error if a conflict is found). Any WANT modules that have not been found in the trees of YES or NO modules are then enabled with their dependencies.

There is a script to help figuring out dependencies when building your own modules or VTK-dependant code (*.cxx, *.h) in order to generate a find_package command. The required json argument is only available in a build tree though.

Utilities/Maintenance/ -s /path/to/sources -j path/to/vtk_build/modules.json


There is some support for testing in the module system, but it is not as comprehensive as the build side. This is because testing infrastructure and strategies vary wildly between projects. Rather than trying to handle the minimum baseline of any plausible testing infrastructure or framework, the module system merely handles dependency management for testing and entering a subdirectory with the tests.

Modules may have TEST_DEPENDS and TEST_OPTIONAL_DEPENDS lists provided as well. These modules are required or optionally used by the testing code for the module.

When scanning, the ENABLE_TESTS argument may be set to ON, OFF, WANT (the default), or DEFAULT. Modules which appear in TEST_DEPENDS for the module are affected by this setting.

  • ON: Modules required for testing are treated as required. Tests will be enabled.

  • OFF: Tests will not be enabled.

  • WANT: If possible, TEST_DEPENDS modules will also be enabled if they are not disabled in some other way.

  • DEFAULT: Check when tests are checked whether all of TEST_DEPENDS are available. If they are, enable testing for the module, otherwise skip it.

The only guarantee for testing provided is that all modules in the TEST_DEPENDS will be available before the testing is added and TEST_OPTIONAL_DEPENDS are available if they’d be available at all (i.e., they won’t be made available later).

Modules may also have TEST_LABELS set to ease labeling all tests for the module. The module system itself does nothing with this other than set a global property with the value. It is up to any test infrastructure used within the module’s CMake code to make use of the value.

The tests for a module are expected to live in a subdirectory of the module code itself. The name of this directory is given by the TEST_DIRECTORY_NAME argument to the vtk_module_build() function. If the directory is available and the module’s testing is enabled, the module system will add_subdirectory this directory at the appropriate time. This is decoupled so that testing code can depend on modules that depend on the module that is being tested and the same TARGET ${dependency} check can be used for optional module dependencies.

Building modules#

After scanning is complete, vtk_module_scan() returns a list of modules and kits to build in the variables given by the PROVIDES_MODULES and PROVIDES_KITS arguments to it. It also provides lists of modules that were found during scanning that were not scanned by that call. These are given back in the variables passed to the UNRECOGNIZED_MODULES and REQUIRES_MODULES variables.

The UNRECOGNIZED_MODULES list contains modules passed to REQUIRES_MODULES and REJECT_MODULES that were not found during the scan. This typically indicates that the values passed to those arguments were not constructed properly. However, it may also mean that they should be passed on to further scans if they may be found elsewhere. Callers should handle the variable as necessary for their use case.

The REQUIRES_MODULES are modules that were named as dependencies of the scanned modules and need to be provided in some way before building the provided modules (the build step will require that they exist when it tries to build the modules which required them). These can be passed on to future REQUIRES_MODULES arguments in future scans or used to error out depending on the use case of the caller.

When using vtk_module_build(), the PROVIDES_MODULES and PROVIDES_KITS from a single scan should be passed together. Multiple scans may be built together as well if they all use the same build parameters as each other.

Build-time parameters#

The vtk_module_build() function is where the decision to build with or without kits is decided through the BUILD_WITH_KITS option. Only if this is set will kits be built for this set of modules.

The decision to default third party modules to using an external or internal copy (where such a decision is possible) is done using the USE_EXTERNAL argument.

Where build artifacts end up in the build tree are left to CMake’s typical variables for controlling these locations:

The defaults for these place outputs into the binary directory where the targets were added. The module system will set these to be sensible for itself if they are not already set, but it is recommended to set these at the top-level so that targets not built under vtk_module_build() also end up at a sensible location.

Library parameters#

When building libraries, it is sometimes useful to have top-level control of library metadata. For example, VTK suffixes its library filenames with a version number. The variables that control this include:

  • LIBRARY_NAME_SUFFIX: If non-empty, all libraries and executable names will be suffixed with this value prefixed with a hyphen (e.g., a suffix of foo will make Namespace::Target’s library be named Target-foo or, if the module sets its LIBRARY_NAME to nsTarget, nsTarget-foo).

  • VERSION: Controls the VERSION property for all library modules.

  • SOVERSION: Controls the SOVERSION property for all library modules.

Installation support#

vtk_module_build() also offers arguments to aid in installing module artifacts. These include destinations for pieces that are installed, CMake packaging controls, and components to use for the installations.

A number of destinations control arguments are provided:








See the API documentation for default values for each which are based on GNUInstallDirs variables. Note that all installation destinations are expected to be relative paths. This is because the conveniences provided by the module system are all assumed to be installed to a single prefix (CMAKE_INSTALL_PREFIX) and placed underneath it.

Suppression of header installation is provided via the INSTALL_HEADERS argument to vtk_module_build(). Setting this to OFF will suppress the installation of:

  • headers

  • CMake package files

  • hierarchy files (since their use requires headers)

Basically, suppression of headers means that SDK components for the built modules are not available in the install tree.

Components for the installation are provided via the HEADERS_COMPONENT and TARGETS_COMPONENT arguments. The former is used for SDK bits and the latter for runtime bits (libraries, executables, etc.).

For CMake package installation, the PACKAGE and INSTALL_EXPORT arguments are available. The former controls the names used by the CMake files created by the module system while the former is the export set to use for the member modules when creating those CMake files. Non-module targets may also exist in this export set when vtk_module_build() is called, but the export set is considered “closed” afterwards since it has already been exported (if INSTALL_HEADERS is true).

Test data information#

The directory that is looked for in each module is specified by using the TEST_DIRECTORY_NAME argument. If it is set to the value of NONE, no testing directories will be searched for. It defaults to Testing due to VTK’s conventions.

The module system, due to VTK’s usage of it, has convenience parameters for controlling the ExternalData module that is available to testing infrastructure. These include:

  • TEST_DATA_TARGET: The data target to use for tests.

  • TEST_INPUT_DATA_DIRECTORY: Where ExternalData should look for data files.

  • TEST_OUTPUT_DATA_DIRECTORY: Where ExternalData should place the downloaded data files.

  • TEST_OUTPUT_DIRECTORY: Where tests should place output files.

Each is provided in the testing subdirectory as _vtk_build_${name}, so the TEST_DATA_TARGET argument is available as _vtk_build_TEST_DATA_TARGET.

Building a module#

Building a module is basically the same as a normal CMake library or executable, but is wrapped to use arguments to facilitate wrapping, exporting, and installation of the tools as well.

There are two main functions provided for this:

The former creates a library for the module being built while the latter can create an executable for the module itself or create utility executable associated with the module. The module system requires that the CMakeLists.txt for a module create a target with the name of the module. In the case of INTERFACE modules, it suffices to create the module manually in many cases.


Most modules end up being libraries that can be linked against by other libraries. Due to cross-platform support generally being a good thing, the EXPORT_MACRO_PREFIX argument is provided to specify the prefix for macro names to be used by GenerateExportHeader. By default, the LIBRARY_NAME for the module is transformed to uppercase to make the prefix.

Some modules may need to add additional information to the library name that will be used that is not statically know and depends on other environmental settings. The LIBRARY_NAME_SUFFIX may be specified to add an additional suffix to the LIBRARY_NAME for the module. The vtk_module_build() LIBRARY_NAME_SUFFIX argument value will be appended to this name as well.

Normally, libraries are built according to the BUILD_SHARED_LIBS variable, however, some modules may need to be built statically all the time. The FORCE_STATIC parameter exists for this purpose. This is generally only necessary if the module is in some other must-be-static library’s dependency tree (which may happen for a number of reasons). It is not an escape hatch for general usage; it is there because use cases which only support static libraries (even in a shared build) exist.

If a library module is part of a kit and it is being built via the vtk_module_build() BUILD_WITH_KITS argument, it will be built as an OBJECT library and the kit machinery in vtk_module_build() will create the resulting kit library artifact.

Header-only modules must pass HEADER_ONLY to create an INTERFACE library instead of expecting a linkable artifact.


HEADER_ONLY modules which are part of kits is currently untested. This should be supported, but might not work at the moment.

Source listing#

Instead of using CMake’s “all sources in a single list” pattern for add_library, vtk_module_add_module() classifies its source files explicitly:




The HEADERS and TEMPLATES are installed into the HEADERS_DESTINATION specified to vtk_module_build() and may be added to a subdirectory of this destination by using the HEADERS_SUBDIR argument. Note that the structure of the header paths passed is ignored. If more structure is required from the installed header layout, vtk_module_install_headers() should be used.

Files passed via HEADERS are treated as the API interface to the code of the module and are added to properties so that language wrappers can discover the API of the module.


Only headers passed via HEADERS are eligible for wrapping; those installed via vtk_module_install_headers() are not. This is a known limitation at the moment.

There are also private variations for HEADERS and TEMPLATES named PRIVATE_HEADERS and PRIVATE_TEMPLATES respectively. These are never installed nor exposed to wrapping mechanisms.

There are also a couple of convenience parameters that use VTK’s file naming conventions to ease usage. These include:

  • CLASSES: For each value <class>, adds <class>.cxx to SOURCES and <class>.h to HEADERS.

  • TEMPLATE_CLASSES: For each value <class>, adds <class>.txx to TEMPLATES and <class>.h to HEADERS.

  • PRIVATE_CLASSES: For each value <class>, adds <class>.cxx to SOURCES and <class>.h to PRIVATE_HEADERS.

  • PRIVATE_TEMPLATE_CLASSES: For each value <class>, adds <class>.txx to PRIVATE_TEMPLATES and <class>.h to PRIVATE_HEADERS.


Executables may be created using vtk_module_add_executable(). The first argument is the name of the executable to build. Since the scanning phase does not know what kind of target will be created for each module (and it may change based on other configuration values), an executable module which claims it is part of a kit raises an error since this is not possible to do.

For modules that are executables using this function, the metadata from the module information is used to set the relevant properties. The module dependencies are also automatically linked in the same way as a library module would do so.

For utility executables, NO_INSTALL may be passed to keep it within the build tree. It will not be available to consumers of the project. If the name of the executable is different from the target name, BASENAME may be used to change the executable’s name.

Module APIs#

All of CMake’s target_ function calls have analogues for modules. This is primarily due to the kits feature which causes the target name created by the module system that is required to use the target_ functions dependent on whether the module is a member of a kit and kits are being built. The CMake version of the function and the module API analogue (as well as differences, if any) is:

Packaging support#

Getting installed packages to work for CMake is, unfortunately, not trivial. The module system provides some support for helping with this, but it does place some extra constraints on the project so that some assumptions that vastly simplify the process can be made.


The main assumption is that all modules passed to a single vtk_module_build() have the same CMake namespace (the part up to and including the ::, if any, in a module name. For exporting dependencies, that namespace matches the PACKAGE argument for vtk_module_build(). These are done so that the generated code can use CMAKE_FIND_PACKAGE_NAME variable can be used to discover information about the package that is being found.

The package support also assumes that all modules may be queried using COMPONENTS and OPTIONAL_COMPONENTS and that the component name for a module corresponds to the name of a module without the namespace.

These rules basically mean that a module named Namespace::Target will be found using find_package(Namespace), that COMPONENTS Target may be passed to ensure that that module exists, and OPTIONAL_COMPONENTS Target may be passed to allow the component to not exist while not failing the main find_package call.

Creating a full package#

The module system provides no support for the top-level file that is used by find_package. This is because this logic is highly project-specific and hard to generalize in a useful way. Instead, files are generated which should be included from the main file.

Here, the list of files generated are based on the PACKAGE argument passed to vtk_module_build():

  • <PACKAGE>-targets.cmake: The CMake-generated export file for the targets in the INSTALL_EXPORT.

  • <PACKAGE>-vtk-module-properties.cmake: Properties for the targets exported into the build.

The module properties file must be included after the targets file so that they exist when it tries to add properties to the imported targets.

External dependencies#

Since the module system is heavily skewed towards using imported targets, these targets show up by name in the find_package of the project as well. This means that these external projects need to be found to recreate their imported targets at that time. To this end, there is the vtk_module_export_find_packages() function. This function writes a file named according to its FILE_NAME argument and place it in the build and install trees according to its CMAKE_DESTINATION argument.

This file will be populated with logic to determine whether third party packages found using vtk_module_find_package() are required during the find_package of the package or not. It will forward REQUIRED and QUIET parameters to other find_package calls as necessary based on the REQUIRED and QUIET flags for the package and whether that call is involved in a non-optional COMPONENT (a component-less find_package call is assumed to mean “all components”).

This file should be included after the <PACKAGE>-vtk-module-properties.cmake file generated by the vtk_module_build() call so that it can use the module dependency information set via that file.

After this file is included, for each component that it checks, it will set ${CMAKE_FIND_PACKAGE_NAME}_<component>_FOUND to 0 if it is not valid and append a reason to ${CMAKE_FIND_PACKAGE_NAME}_<component>_NOT_FOUND_MESSAGE so that the package can collate the reason why things are not available.

Setting the _FOUND variable#

The module system does not currently help in determining the top-level ${CMAKE_FIND_PACKAGE_NAME}_FOUND variable based on the results of the components that were requested and the status of dependent packages. This may be provided at some point, but there has not currently been enough experience to determine what patterns are available for factoring it out as a utility function.

The general pattern should be to go through the list of components requested, determine whether targets for those components exist. Then for each found component, use the module dependency information to ensure that all targets in the dependency trees are found (propagating not-found statuses through the dependency tree). The ${CMAKE_FIND_PACKAGE_NAME}_NOT_FOUND_MESSAGE should be built up based on the reasons the find_package call did not work based on these discoveries.

This is the process for modules in a package, but packages may contain non-module components, and it is hard for the module system to provide support for them, so they are not attempted. See the CMake documentation for more details about creating a package configuration.

Advanced topics#

There are a number of advanced features provided by the module system that are not normally required in a simple project.


Kits are described in vtk.kit files which act much like vtk.module files. However, they only have NAME, LIBRARY_NAME, and DESCRIPTION fields. These all act just like they do in the vtk.module context. These files may either be passed manually to vtk_module_scan() or discovered by using the vtk_module_find_kits() convenience function.

Before a module may be a member of a kit, a vtk.kit must declare it and be scanned at the same time. This means that kits may only contain modules that are scanned with them and cannot be extended later nor may kits be made of modules that they do not know about.


In order to actually use kits, CMake 3.12 is necessary in order to do the OBJECT library manipulations done behind the scenes to make it Just Work. 3.8 is still the minimum version for using a project that is built with kits however. This is only checked when kits are actually in use, so projects requiring older CMake versions as their minimum version may still provide kits so that users with newer CMake versions can use them.

Kits create a single library on disk, but the usage requirements of the modules should still be the same (except for that which is inherently required to be different by combining libraries). So include directories, compile definitions, and other usage requirements should not leak from other modules that are members of the same kit.


The module system supports a mechanism for triggering static code construction for modules which require it. This cannot be done through normal CMake usage requirements because the requirements are intersectional. For example, a module F having a factory where module I provides an implementation for it means that a target linking to both F and I needs to ensure that I registers its implementation to the factory code. There is no such support in CMake and due to the complexities and code generation involved with this support, it is unlikely to exist.

Code which uses modules may call the vtk_module_autoinit() function to use this functionality. The list of modules passed to the function are used to compute the defines necessary to trigger the registration to factories when necessary.

For details on the implementation of the autoinit system, please see the relevant section in the API documentation.


VTK comes with support for wrapping its classes into other languages. Currently, VTK supports wrapping its classes for use in the Python and Java languages. In order to wrap a set of modules for a language, a separate function is used for each language.

All languages read the headers of classes with a __VTK_WRAP__ preprocessor definition defined. This may be used to hide methods or other details from the wrapping code if wanted.


For Python, the vtk_module_wrap_python() function must be used. This function takes a list of modules in its MODULES argument and creates Python modules for use under the PYTHON_PACKAGE package. No for this package is created automatically and must be provided in some other way.

A target named by the TARGET argument is created and installed. This target may be linked to in order to be able to import static Python modules. In this case, a header and function named according to the basename of TARGET (e.g., VTK::PythonWrapped has a basename of PythonWrapped) must be used. The header is named <TARGET_BASENAME>.h and the function which adds the wrapped modules to the static import table is <void TARGET_BASENAME>_load(). This function is also created in shared builds, but does nothing so that it may always be called in static or shared builds.

The modules will be installed under the MODULE_DESTINATION given to the function into the PYTHON_PACKAGE directory needed for it. The vtk_module_python_default_destination() function is used to determine a default if one is not passed.

The Python wrappers define a __VTK_WRAP_PYTHON__ preprocessor definition when reading code which may be used to hide methods or other details from the Python wrapping code.


For Java, the vtk_module_wrap_java() function must be used. This function creates Java sources for classes in the modules passed in its MODULES argument. The sources are written to a JAVA_OUTPUT directory. These then can be compiled by CMake normally.

For this purpose, there are <MODULE>Java targets which contain a _vtk_module_java_files properties containing a list of .java sources generated for the given module. There is also a <MODULE>Java-java-sources target which may be depended upon if just the source generation needs to used in an add_dependencies call.

The Java wrappers define a __VTK_WRAP_JAVA__ preprocessor definition when reading code which may be used to hide methods or other details from the Java wrapping code.

Hierarchy files#

Hierarchy files are used by the language wrapper tools to know the class inheritance for classes within a module. Each module has a hierarchy file associated with it. The path to a module’s hierarchy file is stored in its hierarchy module property.

Third party#

The module system has support for representing third party modules in its build. These may be built as part of the project or represented using other mechanisms (usually find_package and a set of imported targets from it).

The primary API is vtk_module_third_party() which creates a VTK_MODULE_USE_EXTERNAL_Namespace_Target option for the module to switch between an internal and external source for the third party code. This value defaults to the setting of the USE_EXTERNAL argument for the calling vtk_module_build() function. Arguments passed under the INTERNAL and EXTERNAL arguments to this command are then passed on to vtk_module_third_party_internal() or vtk_module_third_party_external(), respectively, depending on the VTK_MODULE_USE_EXTERNAL_Namespace_Target option.

Note that third party modules (marked as such by adding the THIRD_PARTY keyword to a vtk.module file) may not be part of a kit, be wrapped, or participate in autoinit.

External third party modules#

External modules are found using CMake’s find_package mechanism. In addition to the arguments supported by vtk_module_find_package() (except PRIVATE and PRIVATE_IF_SHARED), information about the found package is used to construct a module target which represents the third party package. The preferred mechanism is to give a list of imported targets to the LIBRARIES argument. These will be added to the INTERFACE of the module and provide the third party package for use within the module system.

If imported targets are not available (they really should be created if not), variable names may be passed to INCLUDE_DIRS, LIBRARIES, and DEFINITIONS to create the module interface.

In addition, any variables which should be forwarded from the package to the rest of the build may be specified using the USE_VARIABLES argument.

The STANDARD_INCLUDE_DIRS argument creates an include interface for the module target which includes the “standard” module include directories to. Basically, the source and binary directories of the module.

Internal third party modules#

Internal modules are those that may be built as part of the build. These should ideally specify a set of LICENSE_FILES indicating the license status of the third party code. These files will be installed along with the third party package to aid in any licensing requirements of the code. It is also recommended to set the VERSION argument so that it is known what version of the code is provided at a glance.

By default, the LIBRARY_NAME of the module is used as the name of the subdirectory to include, but this may be changed by using the SUBDIRECTORY argument.

Header-only third party modules may be indicated by using the HEADER_ONLY argument. Modules which represent multiple libraries at once from a project may use the INTERFACE argument.

The STANDARD_INCLUDE_DIRS argument creates an include interface for the module target which includes the “standard” module include directories to. Basically, the source and binary directories of the module. A subdirectory may be used by setting the HEADERS_SUBDIR option. It is implied for HEADERS_ONLY third party modules.

After the subdirectory is added a target with the module’s name must exist. However, a target is automatically created if it is HEADERS_ONLY.

Properly shipping internal third party code#

There are many things that really should be done to ship internal third party code (also known as vendoring) properly. The issue is mainly that the internal code may conflict with other code bringing in another copy of the same package into a process. Most platforms do not behave well in this situation.

In order to avoid conflicts at every level possible, a process called “name mangling” should be performed. A non-exhaustive list of name manglings that must be done to fully handle this case includes:

  • moving headers to a subdirectory (to avoid compilations from finding incompatible headers);

  • changing the library name (to avoid DLL lookups from finding incompatible copies); and

  • mangling symbols (to avoid symbol lookup from confusing two copies in the same process).

Some projects may need further work like editing CMake APIs or the like to be mangled as well.

Moving headers and changing library names is fairly straightforward by editing CMake code. Mangling symbols usually involves creating a header which has a #define for each public symbol to change its name at runtime to be distinct from another copy that may end up existing in the same process from another project.

Typically, a header needs to be created at the module level which hides the differences between third party code which may or may not be provided by an external package. In this case, it is recommended that code using the third party module use unmangled names and let the module interface and mangling headers handle the mangling at that level.


The module system can output debugging information about its inner workings by using the _vtk_module_log variable. This variable is a list of “domains” to log about, or the special ALL value causes all domains to log output. The following domains are used in the internals of the module system:

  • kit: discovery and membership of kits

  • module: discovery and CONDITION results of modules

  • enable: resolution of the enable status of modules

  • provide: determination of module provision

  • building: when building a module occurs

  • testing: missing test dependencies

It is encouraged that projects expose user-friendly flags to control logging rather than exposing _vtk_module_log directly.

Control variables#

These variables do not follow the API convention and are used if set:

  • _vtk_module_warnings: If enabled, “strict” warnings are generated. These are not strictly problems, but may be used as linting for improving usage of the module system.

  • _vtk_module_log: A list of “domains” to output debugging information.

  • _vtk_module_group_default_${group}: used to set a non-DEFAULT default for group settings.

Some mechanisms use global properties instead:

  • _vtk_module_autoinit_include: The file that needs to be included in order to make the VTK_MODULE_AUTOINIT symbol available for use in the autoinit support.

SPDX files generation#

The generation of VTK module SPDX files relies on three components:

SPDX files are named after <ModuleName>.spdx and are generated for all VTK modules.

Generated SPDX files are based on the SPDX 2.2 specification.

If some information is missing, VTK will warn during configuration or during build but the SPDX file will still be generated with unknown fields being attributed a NOASSERTION or other default value.

The collected license identifiers are joined together using AND keyword.

Similarly all collected copyright texts are joined using a new line.

SPDX arguments in vtk_module_build#

Support for SPDX file generation requires to specify the following vtk_module_build() arguments:




GENERATE_SPDX is used to enable the generation and install of SPDX file for each modules. Set this to ON to enable it.

SPDX_DOCUMENT_NAMESPACE is used as a basename for the DocumentNamespace SPDX field. The name of the module will simply be appended to the basename. If not provided, will be used. This is the value VTK project uses as well. Note that the namespace does not need to be an actual website URL, but just a unique Uniform Resource Identifier (URI).


If VTK decide to host SPDX files in the future, the namespace in use for the VTK SPDX files may change accordingly.

SPDX_DOWNLOAD_LOCATION is used as a basename for the PackageDownloadLocation when not provided at module level. The relative path to the module will simply be appended in order to generate the actual PackageDownloadLocation SPDX field. If not provided at module or in vtk_module_build(), NOASSERTION will be used.

SPDX arguments in vtk.module#

Defining these three arguments in vtk.module is required:




SPDX_LICENSE_IDENTIFIER is an expected field corresponding to the PackageLicenseDeclared SPDX field that is considered as the global license for all files of the module that are not parsed during generation. This field is used to set the PackageLicenseConcluded SPDX field.


The SPDX generation system do not and cannot replace the LICENSE_FILES mechanism. Indeed, certains license (e.g Apache 2.0) requires additional files (e.g NOTICE) to also be distributed.

SPDX_COPYRIGHT_TEXT is an expected field that correspond to the copyright applying to all files that are not parsed during generation, it is used to generate PackageCopyrightText.

SPDX_DOWNLOAD_LOCATION is a optional field for modules (see above for setting this in vtk_module_build) and expected field for third parties. If provided, it is used as is for the PackageDownloadLocation SPDX field.

SPDX arguments in vtk_module_add_module#

It is possible to specify a SPDX_SKIP_REGEX when adding a module in order to skip specific file during SPDX tags parsing. It is a python regex which is used to match with the filename of the source files.

Custom license support#

If the VTK module contains a custom license that is not part of the SPDX license list then adding a custom license may be needed.

The SPDX generation system support to specify exactly one custom license by module, supplemental to standard licenses. The text of this license should be made available in a file and added to the module definition using SPDX_CUSTOM_LICENSE_FILE , the name of the license should be specified using SPDX_CUSTOM_LICENSE_NAME (eg: LicenseName and the SPDX_LICENSE_IDENTIFIER for this license should be LicenseRef- followed by the name (eg: LicenseRef-licenseName). See this entry for more info.


If this custom license is to be added to VTK proper, it must be compatible with the BSD-3-Clause license of VTK and not add more restriction to the code.

SPDX Tags in the sources files#

For VTK modules (except the one declared as THIRD_PARTY), sources files are parsed for specific SPDX tags in a specific order.

First N lines of with the the SPDX-FileCopyrightText tag, then one line with the SPDX-License-Identifier tag. Like this:

// SPDX-FileCopyrightText: Copyright (c) Ken Martin, Will Schroeder, Bill Lorensen
// SPDX-FileCopyrightText: Copyright (c) Awesome contributor
// SPDX-License-Identifier: BSD-3-Clause

If a source file does not contain both SPDX-FileCopyrightText and SPDX-License-Identifier tags, a warning at build time is reported.


  • Correctness of the SPDX-FileCopyrightText and SPDX-License-Identifier tags is not ensured. The value will be used as is.

  • The generated SPDX files only include the Package information section. This means that there are no File information sections describing source files or build artifacts.

  • Third party source files are not parsed for SPDX tags.

  • Adding empty lines between // SPDX-FileCopyrightText and // SPDX-License-Identifier tags is not supported.

  • Certain files are not parsed at all, eg: cmake files, python files, test files, …