Major improvements in ITK 5.0 include a switch to C++11 language standard, improved performance, and cleanup of legacy code. More information about the C++11 transition, spearheaded by Hans Johnson, can be read in the [ITK 5.0 Alpha 1 announcement](https://discourse.itk.org/t/itk-5-0-alpha-1-modern-c/843).
ITK 5.0 Alpha 2 emphasizes performance improvements: 1) A refactored multithreading infrastructure, 2) in ITK 5, a thread pool is available by default, 3) an Intel Threading Building Blocks (TBB) threading backend with dynamic region splitting, 4) improved image iterator performance, 5) simple APIs to execute functions in parallel, 6) simple construction of unary and binary generator image filters filters, and 7) new, experimental C++11 range classes for iteration in range-based for loops.
1. There are now two multi-threaded "GenerateData" method signatures. The classic one, *ThreadedGenerateData*, is kept when a filter needs to know in advance the number of pieces into which the requested region will be split. This information was previously used by some filters to allocate intermediate, thread-local data. The new dynamic signature, *DynamicThreadedGenerateData*, allows the region to be split dynamically in response to data content and multi-processor load to achieve load balancing. Presently, only the TBB backend performs dynamic splitting. More information can be found in the Multi-Threading section of in The [ITK Software Guide](https://itk.org/ItkSoftwareGuide.pdf). Instructions that describe changes required to leverage dynamic parallelism in external filters can be found in the [migration guide](https://github.com/InsightSoftwareConsortium/ITK/blob/master/Documentation/ITK5MigrationGuide.md#multithreading-refactored). Most existing ITK-based codes will build with minimal to no modifications required.
2. An *itk::PoolMultiThreader* is the default in ITK 5, which provides a thread pool for platform native threads. The *itk::PlatformMultiThreader* is also available, which behaves like the previous *itk::MultiThreader* class.
3. Our multi-threading refactoring enables better load balancing for filters with variable per-pixel computation cost. As [TBB](https://www.threadingbuildingblocks.org/) is one of the best open source libraries for load-balanced threading, it was an easy choice. The *itk::TBBMultiThreader* can be enabled by setting `Module_ITKTBB` to `ON` in ITK's CMake configuration.
4. Most methods in neighborhood iterators were virtual, but this was a rarely-used feature. That virtualization is now a [configure-time](https://github.com/InsightSoftwareConsortium/ITK/blob/7a7d88b85f258c3162001ccfa4e5a89540b4c411/Modules/Core/Common/include/itkMacro.h#L1276-L1288) parameter, and it is off by default. This significantly improves performance of filters which utilize neighborhood iterators. More information can be found in the Discourse [discussion](https://discourse.itk.org/t/removal-of-virtual-keywords-from-constneighborhooditerator/814/).
5. A [ParallelizeImageRegion](https://github.com/InsightSoftwareConsortium/ITK/blob/9382f7bdd89f08a1122d57f8144702ac9341caf3/Modules/Core/Common/include/itkMultiThreaderBase.h#L185) method allows multi-threaded processing without writing a full-blown filter class. As evident in [this example](https://itk.org/ITKExamples/src/Core/Common/FilterAndParallelizeImageRegion/Documentation.html), multiple parallelized operations of arbitrary complexity can now be concisely specified and executed with [C++11 lamda](http://en.cppreference.com/w/cpp/language/lambda)'s or [std::function](http://en.cppreference.com/w/cpp/utility/functional/function)'s.
6. The new [itk::UnaryGeneratorImageFilter](https://itk.org/Doxygen/html/classitk_1_1UnaryGeneratorImageFilter.html) and [itk::BinaryGeneratorImageFilter](https://itk.org/Doxygen/html/classitk_1_1BinaryGeneratorImageFilter.html)'s allow per-pixel operations to be specified in several ways: 1) traditional "Functor", with `operator()`, 2) C++11 lambda functions, with closures, 3) C++11 std::function 4) C-style function pointers.
7. New, experimental classes for C++11 iteration in [range-based for loops](https://en.cppreference.com/w/cpp/language/range-for), e.g. [itk::Experimental::ShapedImageNeighborhoodRange](https://itk.org/Doxygen/html/classitk_1_1Experimental_1_1ShapedImageNeighborhoodRange.html), provide a modern, convenient syntax for image pixel iteration, and dramatic performance improvements.
Furthmore, ITK 5 improves performance by leveraging [Rvalue references](http://thbecker.net/articles/rvalue_references/section_01.html) to implement move semantics and perfect forwarding.
**ITK 5 Performance Benchmark Improvements** *[ITK Performance Benchmarks](https://github.com/InsightSoftwareConsortium/ITKPerformanceBenchmarking), lower is better, for ITK 4.13.0, ITK 5.0 Alpha 2 with the platform thread pool, and ITK 5.0 Alpha 2 with TBB. ITK 5.0 advances performance in general, especially with the TBB threading backend.*
Thanks to everyone who contributed to the major advancements made in the ITK 5 performance refactoring. Special recognition goes to Dženan Zukić, Kitware, who lead the refactoring effort. Contributions were also made by Jared Vicory, Robert Maynard, Francois Budin, Matt McCormick, Pablo Hernandez-Cerdan, Kitware, Jonathan Lefman, formerly of Intel, Bradley Lowekamp, associated with the National Library of Medicine, Hans Johnson, the University of Iowa, Simon Warfield, Benoit Scherrer, Rene-Paul Debroize, Amir Jaberzadeh, and Etienne St-Onge, Boston's Children's Hospital. The TBB-refactoring was inspired by the [TBBImageToImageFilter](https://github.com/InsightSoftwareConsortium/ITKTBBImageToImageFilter), which was supported in part by the Intel Parallel Computing Center at the Computational Radiology Laboratory, Boston Children's Hospital. Bradley Lowekamp contributed the *itk::UnaryGeneratorImageFilter* and *itk::BinaryGeneratorImageFilter* filters. Neils Dekker, Leiden University Medical Center, contributed the new range classes.
There were many other improvements and bug fixes since 5.0 Alpha 1. For example, addition of [.editorconfig](https://github.com/InsightSoftwareConsortium/ITK/blob/master/.editorconfig), prominent placement of [.clang-format](https://github.com/InsightSoftwareConsortium/ITK/blob/master/.clang-format), addition of ITK-specific debug [visualizers](https://github.com/InsightSoftwareConsortium/ITK/tree/master/Utilities/Debugger) for Visual Studio versions 2012-2017 improve editing and debugging experience in Visual Studio.
To test the ITK 5.0 Alpha 2 Python packages, run
bash
python -m pip install --upgrade pip
python -m pip install --upgrade --pre itk
Note: there are a few warnings that appear when using the wrapping. These will be addressed with the next release.
The next planned release, ITK 5.0 RC1, will focus on the transition of code review process to GitHub.
Changes from v5.0 Alpha 1 to v5.0 Alpha 2
-----------------------------------------------------------
Bradley Lowekamp (22):
PERF: remove virtual methods in neighborhood iterators
ENH: Add ITK_ITERATOR_FINAL
ENH: Use named inputs for the PasteImageFilter
PERF: Remove iterators stored as member variables
BUG: Add specification of OutputImage Type for TobogganImageFilter
BUG: Add specification of OutputImage Type for TobogganImageFilter
ENH: Improve numerics of resampling for linear transform
COMP: Fix gcc 4.8 initialize warning
ENH: Add baseline image to sporadically failing test
ENH: Adding Unary and Binary GeneratorImageFilter
ENH: Convert functor based filters to generator
ENH: Update internal filters to use BinaryGereratorImageFilter
ENH: Updating Functor filter with parameters to the new Generator
COMP: Address Generator filters destructors warning
DOC: Fix BSplineDecompositionImageFilter Doxygen see also link
ENH: reuse default splitter
BUG: revert thread update to BSplineScatteredDataPointSetToImageFilter
ENH: Update CircleCI CMake version to 3.11.2
BUG: Correct detection of most recent data file
ENH: Improve numeric computation of linear interpolation for Resample
ENH: Update baseline image for ResampleImageFilter3Test1
BUG: Add 4 pixel tolerance to ResampleImageFilter3Test1
Dženan Zukić (37):
ENH: start migration guide document
STYLE: increasing initial size of the debug window
BUG: fixing null pointer dereference at the end of iteration
COMP: reducing direct usage of MultiThreader
BUG: remove observations in FilterWatcher's destructor
ENH: support Unicode characters in class documentation
BUG: fixing index computation in resample filter
ENH: reducing code duplication in FilterWatcher
ENH: refactoring MultiThreader infrastructure
ENH: deciding which filters can use the new threading model
ENH: reporting progress
ENH: simplifying Canny filter by using new threading infrastructure
STYLE: Renaming MultiThreader to PlatformMultiThreader
ENH: use C++11 thread primitives instead of SpawnThread
COMP: improve backwards compatibility by retaining the renamed header
ENH: Add TBB-based multi-threader
ENH: refactoring filters to avoid use of memory barrier
ENH: reworking logic for determining default multi-threader
COMP: fixing compile error on GCC 4.8.5 20150623 (Red Hat 4.8.5-28)
STYLE: remove outdated and useless comments from VectorExpandImageFilter
ENH: add a baseline for QuickViewTest.png (Win7x64+VS2017+VTK8.1OpenGL2)
STYLE: reduce number of lines by removing unnecessary empty lines
DOC: document FilterWatcher removal
ENH: adding an .editorconfig file to aid IDE's code formatting options
BUG: forgotten ifdefs for ITK_LEGACY_REMOVE
ENH: adding debugger help files for Visual Studio 2017
ENH: moving .clang-format file into the root folder,
ENH: using the new threading mechanism for FFT module
ENH: explicitly setting threading model in each multi-threaded filter
ENH: support classic ThreadedGenerateData via ITK_CLASSIC_THREADING_MODEL
BUG: fixing a test which has been failing since multi-threading refactoring
ENH: updating baselines for itkBSplineScatteredDataPointSetToImageFilterTest*
COMP: fixing 2 compile warnings
STYLE: renaming migration guide document
ENH: extending migration guide with threading refactoring instructions
ENH: reducing redundancy in itkLandmarkBasedTransformInitializerTest
ENH: eliminate CMake option ITK_CLASSIC_THREADING_MODEL
Hans Johnson (5):
STYLE: Use C++11 override keyword directly
STYLE: Use C++11 override keyword directly
ENH: Add compression option for Transform files
COMP: Address type mis-match compiler warning
COMP: Remove unused variable warning
Jon Haitz Legarreta Gorroño (10):
COMP: Use and move ITK_DISALLOW_COPY_AND_ASSIGN calls to public section.
ENH: Update the WikiExamples remote module commit hash.
COMP: Bump the remote modules' versions.
DOC: Fix miscellaneous typos.
DOC: Update the ITK logo files in `Documentation` folder.
DOC: Remove PSD files containing old ITK logo.
BUG: Address use of CMake variables in CMakeLists.txt.
COMP: Set the minimum required CMake version to 3.10.2.
STYLE: Stick to the ITK style guidelines.
COMP: Bump the remote modules' versions after the CMake ver update.
Lucas Gandel (2):
ENH: include WrapITKConfig for external module
ENH: Allow for itk::ImageSource wrapping override
Matthew McCormick (17):
ENH: Support Python snake case keyword arguments in object initializion
PERF: Avoid extra condition check in itkBase.py
COMP: Detect Linux in itkMemoryUsageObserver.h on Alpine Linux
COMP: Define ITK_DELETE_FUNCTION for backwards compatibility
BUG: Fix bugs in ITKV4_COMPATIBILITY definition
COMP: Wrap MultiResolutionPDEDeformableRegistration for Pyramid filter types
COMP: Do not disallow constructors in Python classes
DOC: Add Mathieu Malaterre to GDCM copyright holders in NOTICE
COMP: Address array initialization in itkShapedImageNeighborhoodRangeGTest
COMP: ShapeImageNeighborhoodRange signed unsigned comparison
COMP: Add missing itkViewImage.h include to itkViewImage.hxx
COMP: Initialize CovariantVector in DeformableSimplexMesh3DFilter
COMP: Update Unary and Binary filters for new generator class base
COMP: Initialize DeformableSimpleMesh3DFilter displacement
COMP: Suppress maybe-uninitialized in SimpleDataObjectDecorator Component
COMP: Do not include itkMultiThreader.h in the header tests
COMP: Fix Python wrapping of MultiThreaderBase
Niels Dekker (11):
ENH: Added Compute member functions to NeighborhoodInnerProduct
STYLE: Added DISALLOW_COPY_AND_ASSIGN(GaussianDerivativeImageFunction), etc.
ENH: Added ShapedImageNeighborhoodRange for modern C++ style iteration
STYLE: Removed '&' from pixelPointer param of NeighborhoodAccessorFunctor::Set
PERF: Removed virtual ~NeighborhoodAccessorFunctor() and made class final
COMP: Fixed ShapedImageNeighborhoodRange doxygen + signed/unsigned warning
BUG: Dropped OffsetTable assumptions from ShapedImageNeighborhoodRange
PERF: Made HoughTransform2DCircles GaussianDerivative call non-virtual
COMP: Workaround Clang range-loop-analysis warning NeigborhoodRange test
ENH: Added class to create offsets for a hyperrectangular neighborhood shape
ENH: Random access support for ShapedImageNeighborhoodRange iterators
Pablo Hernandez-Cerdan (4):
ENH: Add stateless viewer ViewImage with Python wrap.
DOC: Add docs to ViewImage.
COMP: Fix catch by value warning in SpatialObject.
COMP: Fix VNL to compile with gcc8.
Rashad Kanavath (1):
COMP: fix linking on OpenBSD
Sean McBride (4):
COMP: -Wunused-template & -Winconsistent-missing-destructor-override warnings
BUG: removed illegal int to enum conversion
COMP: fix warning about implicit double to bool conversion
COMP: fix warning about implicit double to bool conversion
Simon Rit (1):
COMP: missing include path for FFTW
Taylor Braun-Jones (1):
COMP: Fix clang diagnostic: case value cannot be narrowed
VXL Maintainers (1):
VNL 2018-05-16 (26628604)
Enjoy ITK!