Migration Guides

These migration guides explain how to update major versions of ITK, whic may contain breaking changes to the API.

The guides are generated from migration records, one YAML file per change.

Fine-grained versioning between release tags

ITK’s fourth (tweak) version component is the YYYYMMDD date of the most recent commit, computed automatically at configure time. It exists so downstream projects building against untagged ITK commits — for example during a git bisect — can make configure-time decisions at day granularity between release tags:

find_package(ITK REQUIRED)
if(ITK_VERSION_FULL VERSION_GREATER_EQUAL 6.0.0.20260722)
  set(BASELINE_DIR ${CMAKE_CURRENT_SOURCE_DIR}/Baseline/post-change)
else()
  set(BASELINE_DIR ${CMAKE_CURRENT_SOURCE_DIR}/Baseline/legacy)
endif()

ITK_VERSION_FULL and ITK_VERSION_TWEAK are exported through ITKConfig.cmake. Against an older ITK that predates the tweak component, ITK_VERSION_FULL is unset and the comparison is false, so the legacy branch is taken.

The date comes from git log in a working tree — including a shallow or tagless clone — and from the git archive-substituted .git_archival.txt in an exported source tarball; it falls back to 0 only when neither git history nor that substitution is available. No manual edits are needed — the tweak advances with every commit.