COMP: Detect the VTK rendering backend from imported targets

[hjmjohnson]

Fixes #6846. ITKVtkGlue cannot configure against a VTK built without rendering, even though the module contains explicit logic intended to support that case. The logic is unreachable because it tests VTK_RENDERING_BACKEND, a VTK 8 variable that VTK 9 does not define.

Detect the backend from the imported targets instead, so the existing STREQUAL "None" guards work as written.

Why the existing guards never fire

itk-module-init.cmake defaults the backend when the variable is empty:

if(NOT VTK_RENDERING_BACKEND)
  set(VTK_RENDERING_BACKEND OpenGL2)
endif()

VTK 9 never sets VTK_RENDERING_BACKEND — confirmed against a 9.6.2 install:

$ grep -rl VTK_RENDERING_BACKEND <vtk-build>/lib/cmake/vtk-9.6/
(no matches)

So the variable is always empty, always becomes OpenGL2, and every
if(NOT VTK_RENDERING_BACKEND STREQUAL "None") guard is unconditionally true.
The rendering libraries are appended in all cases, and
vtk_module_autoinit is then called with VTK::RenderingOpenGL2 on a VTK that
may not have it.

The module was written to support a rendering-free VTK: src/CMakeLists.txt
already guards QuickView.cxx, and test/CMakeLists.txt already guards the
viewer tests, both on the same STREQUAL "None" condition. Only the line that
computes the value is wrong, so an intended-optional dependency has been
mandatory for the whole VTK 9 era.

Four sites

The detection block appears three times, and two of those run at the
consumer's find_package(ITK) rather than in ITK's own configure:

Site	Evaluated at	Affects
itk-module-init.cmake	ITK configure	ITK's own build
CMakeLists.txt, ITKVtkGlue_EXPORT_CODE_INSTALL	consumer find_package(ITK)	installed ITK
CMakeLists.txt, ITKVtkGlue_EXPORT_CODE_BUILD	consumer find_package(ITK)	build-tree ITK

So the effect is not limited to building ITK — every downstream project that
consumes an installed ITK re-runs the same broken detection.

A fourth site is in the wrapping layer and contains no reference to the
variable at all. itk_auto_load_and_end_wrap_submodules() discovers every
.wrap file, so itkViewImage was wrapped regardless of the backend, and
itkViewImage.hxx includes vtkRenderWindow.h. With the three CMake sites
fixed, configure succeeded and the build then failed:

itkViewImage.hxx:21: fatal error: 'vtkRenderWindow.h' file not found

wrapping/itkViewImage.wrap now carries the same STREQUAL "None" guard the
sources and tests already use.

The two export-code copies live inside quoted strings whose escaping is
documented at CMakeLists.txt:70: \${VTK_RENDERING_BACKEND} must stay
escaped so it expands at consumer time, and the comment warns that mixing
escaped and unescaped forms is a latent bug (see PR #6144). The replacement
introduces no new variable references — only an if(TARGET ...) test — so it
does not interact with that hazard.

Verification

macOS arm64, two purpose-built VTK 9.6.2 trees differing only in rendering.

With rendering (VTK_GROUP_ENABLE_Rendering=WANT) — no regression:

• configure succeeds
• build completes with 0 errors; 12/12 VtkGlue tests pass, unchanged from before

Without rendering (VTK_GROUP_ENABLE_Rendering=NO) — the case that
previously could not configure at all:

• configure now succeeds; on main it fails with

  CMake Error at Modules/Bridge/VtkGlue/src/CMakeLists.txt:5 (target_link_libraries):
    Target "ITKVtkGlue" links to:
      VTK::RenderingOpenGL2
    but the target was not found.
  

• the build completes, 0 errors

• 8 tests register and 8/8 pass: the three non-rendering C++ tests, the
  three Python tests and the two style checks

• the three viewer tests (itkVtkMedianImageFilterTest, QuickViewTest,
  itkVtkConnectedComponentImageFilterTest) are correctly excluded

Note for reviewers merging alongside #6848

#6848 also touches Modules/Bridge/VtkGlue/itk-module-init.cmake, adding
configure-time checks for unusable VTK configurations. The two changes are in
different regions of the file and are logically independent, but a textual
conflict is possible depending on merge order. Happy to rebase whichever lands
second.

[greptile-apps]

Greptile Summary

Updates ITKVtkGlue configuration to detect VTK rendering support from the imported OpenGL2 target, retain rendering-enabled behavior when available, and avoid enabling rendering-dependent integration where VTK does not provide it.

Confidence Score: 5/5

Safe to merge.

No outstanding findings remain. The previously raised source-comment requirement was addressed by hjmjohnson, who confirmed the comment was condensed in commit b1f8fc7; the current code contains the condensed single-line comment.

Files Needing Attention: None.

_{Reviews (2): Last reviewed commit: "COMP: Detect the VTK rendering backend f..." | Re-trigger Greptile}

[dzenanz]

Good even as-is.